Clinical Approach to Fever

Comprehensive Practical Framework

1
Symptom Overview
Overview
2
Pathophysiology
Mechanism
3
History Taking
History
4
Physical Examination
Examination
5
Differential Diagnosis
Differential
6
Investigations
Workup
7
Clinical Decision-Making
Algorithm
8
Pearls and Pitfalls
Summary

1. Symptom Overview

Understanding the clinical significance and classification of Fever

Fever is one of the most common presenting complaints in clinical medicine, accounting for approximately 5% of all emergency department visits and up to 30% of pediatric outpatient consultations. In hospitalized patients, fever develops in 20-30% of admissions and is associated with increased morbidity and mortality. Despite being a ubiquitous symptom, fever remains a diagnostic challenge—studies show that in approximately 5-15% of cases presenting with prolonged fever, no definitive cause is identified even after extensive investigation (fever of unknown origin).

Definition

Fever is defined as an elevation of body temperature above the normal daily variation, resulting from a change in the thermoregulatory set point of the hypothalamus. The most widely accepted threshold is a core body temperature of 38.0°C (100.4°F) or higher. However, normal body temperature varies with time of day (lowest in early morning, highest in late afternoon), site of measurement, age, and individual baseline. Fever represents a controlled increase in body temperature—distinguishing it from hyperthermia, where temperature rises uncontrollably due to failed thermoregulation.

Classification by Duration

Category	Duration	Common Causes	Clinical Significance
Acute Fever	Less than 7 days	Viral upper respiratory infections, influenza, acute bacterial infections, urinary tract infections	Most common presentation; majority are self-limiting viral infections; focus on identifying serious bacterial infections
Subacute Fever	7 to 14 days	Prolonged viral syndromes, localized bacterial infections (abscess, osteomyelitis), early tuberculosis	Warrants more thorough investigation; consider occult infections and early systemic diseases
Prolonged Fever	14 to 21 days	Tuberculosis, infective endocarditis, intra-abdominal abscess, malignancy, connective tissue diseases	Requires systematic diagnostic approach; infection, malignancy, and autoimmune conditions must all be considered
Fever of Unknown Origin (FUO)	Greater than 3 weeks with temperature above 38.3°C on multiple occasions and no diagnosis after 1 week of investigation	Infections (25-50%), malignancies (15-25%), non-infectious inflammatory diseases (15-25%), undiagnosed (10-25%)	Classic definition by Petersdorf and Beeson; requires structured investigation protocol; prognosis generally favorable if malignancy excluded

Classification by Height of Fever

Low-Grade Fever

Temperature: 38.0°C to 38.9°C (100.4°F to 102.0°F)

Common in viral infections, early bacterial infections, and inflammatory conditions. May be the only manifestation in elderly or immunocompromised patients with serious infections.

Moderate Fever

Temperature: 39.0°C to 39.9°C (102.2°F to 103.8°F)

Suggests more significant infectious or inflammatory process. Common in bacterial pneumonia, pyelonephritis, and systemic inflammatory conditions.

High-Grade Fever

Temperature: 40.0°C to 41.0°C (104.0°F to 105.8°F)

Indicates severe infection or inflammation. Associated with bacteremia, meningitis, severe malaria, and drug reactions. Requires urgent evaluation.

Hyperpyrexia

Temperature: Greater than 41.1°C (106.0°F)

Medical emergency. Consider central nervous system infection, malignant hyperthermia, neuroleptic malignant syndrome, serotonin syndrome, heat stroke, or severe drug reaction. Risk of end-organ damage increases significantly.

Classification by Fever Pattern

Pattern	Description	Classic Associations
Continuous (Sustained)	Temperature remains elevated with minimal fluctuation (less than 1°C variation over 24 hours)	Typhoid fever (during second week), lobar pneumonia, gram-negative bacteremia, central nervous system infections
Intermittent	Temperature returns to normal at least once every 24 hours; wide swings between peaks and troughs	Pyogenic abscess, infective endocarditis, tuberculosis, lymphoma; quotidian pattern (daily spikes) suggests systemic infection
Remittent	Temperature fluctuates more than 1°C but does not return to normal baseline	Most common pattern; seen in viral infections, bacterial pneumonia, infective endocarditis
Hectic (Septic)	Wide temperature swings (greater than 2°C) with dramatic peaks and troughs, often accompanied by rigors and sweating	Severe sepsis, pyogenic abscess, infective endocarditis, miliary tuberculosis
Relapsing (Recurrent)	Fever-free intervals of days to weeks between febrile episodes	Malaria (tertian, quartan patterns), brucellosis, relapsing fever (Borrelia), Hodgkin lymphoma (Pel-Ebstein fever), periodic fever syndromes
Biphasic (Saddleback)	Two distinct febrile phases separated by a period of defervescence	Dengue fever, leptospirosis, Colorado tick fever, poliomyelitis

Special Fever Patterns with Diagnostic Value

Pattern Name	Characteristics	Associated Condition
Tertian Fever	Fever spikes every 48 hours (days 1, 3, 5)	Plasmodium vivax, Plasmodium ovale malaria
Quartan Fever	Fever spikes every 72 hours (days 1, 4, 7)	Plasmodium malariae malaria
Pel-Ebstein Fever	Cycles of fever lasting 1-2 weeks alternating with afebrile periods of similar duration	Hodgkin lymphoma (rarely seen in modern practice)
Reverse Temperature Pattern	Higher temperature in morning than evening (reversal of normal diurnal variation)	Disseminated tuberculosis, typhoid fever
Drug Fever	Fever with relative bradycardia, patient appears well despite high temperature	Beta-lactam antibiotics, sulfonamides, phenytoin, allopurinol

Key Concept: The “Big Three” Categories of Fever Etiology

In systematic evaluation of fever, especially prolonged or unexplained fever, three major categories account for the vast majority of diagnoses:

Infections — account for 25-50% of fever of unknown origin cases; include occult abscesses, endocarditis, tuberculosis, osteomyelitis
Malignancies — account for 15-25%; lymphoma and leukemia are most common; solid tumors (renal cell carcinoma, hepatocellular carcinoma) may also present with fever
Non-infectious inflammatory diseases — account for 15-25%; include adult-onset Still disease, systemic lupus erythematosus, vasculitis, inflammatory bowel disease

In approximately 10-25% of classic fever of unknown origin cases, no diagnosis is established despite thorough investigation. These patients generally have favorable outcomes, suggesting benign, self-limited conditions.

2. Pathophysiology and Mechanisms

Understanding the underlying mechanisms of Fever

Fever is a complex, highly regulated physiological response that involves the coordinated interaction between the immune system and the central nervous system. Unlike hyperthermia (where thermoregulation fails), fever represents a controlled upward adjustment of the hypothalamic thermoregulatory set point. Understanding this mechanism explains why patients with fever actively generate heat (shivering, vasoconstriction) to achieve the new set point, and why antipyretics work by lowering the set point rather than directly cooling the body.

The Fever Pathway: From Stimulus to Response

Step	Component	Mechanism
1. Trigger	Exogenous Pyrogens	Microbial products (lipopolysaccharide, peptidoglycan, viral RNA, fungal components) or non-infectious stimuli (immune complexes, tissue damage, drugs) initiate the response
2. Immune Recognition	Pattern Recognition Receptors	Toll-like receptors and other pattern recognition receptors on monocytes, macrophages, and dendritic cells detect pathogen-associated molecular patterns and damage-associated molecular patterns
3. Cytokine Production	Endogenous Pyrogens	Activated immune cells release pyrogenic cytokines: interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and interferons
4. Central Signaling	Hypothalamic Communication	Cytokines reach the hypothalamus via: (a) circumventricular organs lacking blood-brain barrier, (b) active transport across blood-brain barrier, (c) vagal afferent signaling
5. Prostaglandin Synthesis	Cyclooxygenase-2 Activation	Cytokines induce cyclooxygenase-2 (COX-2) in hypothalamic vascular endothelium, producing prostaglandin E2 (PGE2)—the final common mediator
6. Set Point Elevation	Preoptic Anterior Hypothalamus	PGE2 acts on EP3 receptors on thermosensitive neurons, raising the thermoregulatory set point
7. Effector Response	Heat Conservation and Generation	Autonomic and behavioral responses increase body temperature to match new set point: vasoconstriction, shivering, seeking warmth, reduced sweating

Pyrogens: Exogenous and Endogenous

Exogenous Pyrogens

Definition: Substances originating outside the body that trigger the fever response

Examples:

Lipopolysaccharide (gram-negative bacteria)
Peptidoglycan and lipoteichoic acid (gram-positive bacteria)
Viral double-stranded RNA
Fungal cell wall components (beta-glucans, mannans)
Parasitic antigens
Drug molecules (acting as haptens)

Clinical relevance: The nature of the exogenous pyrogen does not determine fever pattern—all exogenous pyrogens ultimately act through endogenous cytokine pathways

Endogenous Pyrogens

Definition: Host-derived cytokines that directly act on the hypothalamus to induce fever

Key Cytokines:

Interleukin-1 (IL-1α and IL-1β): Most potent endogenous pyrogen; also induces acute phase response
Interleukin-6 (IL-6): Major inducer of hepatic acute phase proteins; correlates with fever magnitude
Tumor Necrosis Factor-alpha (TNF-α): Causes fever and contributes to systemic inflammatory response
Interferons (IFN-α, IFN-γ): Particularly important in viral infections

Clinical relevance: Elevated IL-6 levels often correlate with severity of illness and predict outcomes in sepsis

Prostaglandin E2: The Final Common Pathway

Why Antipyretics Work

Prostaglandin E2 (PGE2) is the critical mediator that directly elevates the hypothalamic set point. This explains the mechanism of antipyretic drugs:

Nonsteroidal anti-inflammatory drugs (NSAIDs) — inhibit cyclooxygenase enzymes (COX-1 and COX-2), blocking PGE2 synthesis
Acetaminophen (Paracetamol) — inhibits central COX enzymes, reducing hypothalamic PGE2 production
Corticosteroids — inhibit phospholipase A2 (reducing arachidonic acid availability), suppress cytokine production, and block COX-2 induction

Importantly, antipyretics lower the set point but do not directly cool the body. Once the set point is lowered, normal thermoregulatory mechanisms (sweating, vasodilation) reduce body temperature.

How Different Conditions Cause Fever

Category	Mechanism	Examples	Clinical Implication
Bacterial Infections	Toll-like receptor activation by bacterial components (lipopolysaccharide, peptidoglycan) triggers cytokine cascade	Pneumonia, urinary tract infection, cellulitis, bacteremia	Fever may correlate with bacterial burden; persistent fever despite antibiotics suggests inadequate source control or resistant organism
Viral Infections	Recognition of viral nucleic acids by intracellular pattern recognition receptors induces interferon and cytokine production	Influenza, COVID-19, mononucleosis, viral hepatitis	Interferon-mediated fever often accompanied by myalgias and fatigue; fever pattern may help distinguish from bacterial infection
Malignancy	Tumor cells directly produce pyrogenic cytokines (especially IL-6) or trigger immune response generating cytokines	Lymphoma, leukemia, renal cell carcinoma, hepatocellular carcinoma	Fever may be only manifestation of occult malignancy; responds to NSAIDs (Naproxen test); may indicate disease progression
Autoimmune/Inflammatory	Dysregulated immune activation produces sustained cytokine elevation independent of infection	Adult-onset Still disease, systemic lupus erythematosus, vasculitis, inflammatory bowel disease	Fever pattern often quotidian; associated with rash, arthritis, serositis; responds to corticosteroids
Drug Fever	Drugs act as haptens triggering immune response, or directly stimulate cytokine release, or alter thermoregulation	Beta-lactam antibiotics, sulfonamides, phenytoin, allopurinol	Fever may be only sign; relative bradycardia common; resolves 72-96 hours after drug discontinuation
Tissue Necrosis	Damaged cells release damage-associated molecular patterns (DAMPs) that activate inflammatory pathways	Myocardial infarction, pulmonary embolism, stroke, trauma, surgery	Low-grade fever is expected; high fever or fever beyond 48-72 hours suggests superimposed infection

Fever Versus Hyperthermia: Critical Distinction

Fever

Hypothalamic set point is elevated
Thermoregulation intact and functioning
Body actively works to raise temperature
Patient feels cold, seeks warmth, shivers
Responds to antipyretics
Rarely exceeds 41°C (106°F)
Treatment: antipyretics, treat underlying cause

Hyperthermia

Hypothalamic set point is normal
Thermoregulation overwhelmed or impaired
Heat gain exceeds heat dissipation capacity
Patient feels hot, seeks cooling
Does NOT respond to antipyretics
May exceed 41°C, often above 40°C
Treatment: external cooling, treat underlying cause

Hyperthermia Syndrome	Mechanism	Key Features
Heat Stroke	Environmental heat overwhelms thermoregulatory capacity	Temperature greater than 40°C, altered mental status, anhidrosis (classic) or sweating (exertional), multi-organ dysfunction
Malignant Hyperthermia	Genetic mutation causes uncontrolled calcium release from sarcoplasmic reticulum after exposure to volatile anesthetics or succinylcholine	Rapidly rising temperature, muscle rigidity, hypercarbia, metabolic acidosis, rhabdomyolysis
Neuroleptic Malignant Syndrome	Dopamine receptor blockade (antipsychotics) causes hypothalamic dysfunction and muscle rigidity	Fever, lead-pipe rigidity, altered mental status, autonomic instability; onset over days
Serotonin Syndrome	Excess serotonergic activity causes autonomic dysfunction and neuromuscular excitation	Fever, agitation, tremor, hyperreflexia, clonus, diarrhea; rapid onset after drug change
Thyroid Storm	Severe thyrotoxicosis causes hypermetabolic state	High fever, tachycardia out of proportion to fever, altered mental status, goiter, exophthalmos

The Adaptive Function of Fever

Is Fever Beneficial?

Fever is an evolutionarily conserved response present in nearly all vertebrates and even some invertebrates, suggesting significant survival advantage. Proposed benefits include:

Enhanced immune function: Increased neutrophil migration, enhanced phagocytosis, improved T-cell proliferation, increased interferon activity
Inhibited pathogen growth: Many bacteria and viruses replicate optimally at normal body temperature; elevated temperature impairs replication
Reduced iron availability: Fever triggers sequestration of iron, limiting its availability to iron-dependent pathogens

Clinical implication: Routine antipyretic treatment in stable patients with infection may not improve outcomes and could theoretically prolong illness. However, fever reduction remains appropriate for patient comfort and is essential when fever poses risk (cardiac disease, pregnancy, neurological compromise).

Often Overlooked: Relative Bradycardia

Normally, heart rate increases approximately 10-15 beats per minute for each 1°C rise in temperature. When heart rate fails to rise appropriately with fever (relative bradycardia or pulse-temperature dissociation), consider these diagnoses:

Typhoid fever (classic association)
Brucellosis
Legionella pneumonia
Psittacosis and other intracellular pathogens
Drug fever
Factitious fever
Central nervous system lesions
Lymphoma

Also consider beta-blocker or calcium channel blocker use, which can blunt the expected tachycardic response.

3. History Taking

A comprehensive approach to eliciting the Fever history

Red Flags — Require Urgent Evaluation

Hemodynamic instability — suggests sepsis or septic shock
Altered mental status or confusion — meningitis, encephalitis, severe sepsis
Severe headache with neck stiffness — bacterial meningitis
Petechial or purpuric rash — meningococcemia, disseminated intravascular coagulation
Temperature greater than 41°C (hyperpyrexia) — life-threatening infection or hyperthermia syndrome

Immunocompromised state with fever — neutropenic fever, opportunistic infection
Recent surgery or invasive procedure — surgical site infection, device-related infection
Severe localized pain — necrotizing fasciitis, pyogenic abscess, septic arthritis
Fever with new heart murmur — infective endocarditis
Rigors (shaking chills) — bacteremia, abscess, pyelonephritis, malaria

Systematic History: The “FEVER” Approach

Use the mnemonic “FEVER” to ensure comprehensive history taking:

F — Features of the Fever: Onset, duration, pattern, maximum temperature, response to antipyretics, associated symptoms (rigors, sweats, chills)
E — Exposures and Environment: Travel history, sick contacts, animal/insect exposures, water exposure, occupational hazards, food history, sexual history
V — Vulnerability Factors: Immunocompromised state (HIV, chemotherapy, transplant, steroids), diabetes, chronic disease, splenectomy, indwelling devices
E — Exact Localizing Symptoms: Respiratory (cough, dyspnea), urinary (dysuria, frequency), gastrointestinal (diarrhea, abdominal pain), neurological (headache, neck stiffness), musculoskeletal (joint pain, back pain)
R — Recent Events and Rx: Recent hospitalization, surgery, procedures, antibiotic use, new medications, vaccinations

Targeted Questions by Suspected Cause

Suspected Cause	Key Features	Ask This Question
Bacterial Pneumonia	Productive cough, pleuritic chest pain, dyspnea	“Are you coughing up sputum? What color is it? Do you have pain when you breathe deeply?”
Urinary Tract Infection / Pyelonephritis	Dysuria, frequency, urgency, flank pain	“Do you have burning with urination? Are you going more often? Any back or side pain?”
Infective Endocarditis	Prolonged fever, new murmur, embolic phenomena, risk factors	“Have you had any dental work recently? Do you inject drugs? Have you noticed any new spots on your skin or nails?”
Intra-abdominal Abscess	Abdominal pain, recent surgery, altered bowel habits	“Have you had any abdominal surgery recently? Where exactly is the pain? Has your bowel pattern changed?”
Meningitis / Encephalitis	Severe headache, neck stiffness, photophobia, altered consciousness	“Is this the worst headache of your life? Does bending your neck forward cause pain? Is bright light bothering you?”
Tuberculosis	Chronic cough, night sweats, weight loss, hemoptysis	“How long have you been coughing? Do you wake up drenched in sweat? Have you lost weight without trying? Have you coughed up blood?”
Malaria	Travel to endemic area, cyclical fever, rigors	“Have you traveled to Africa, Asia, or South America? When did the fever episodes occur—is there a pattern?”
Drug Fever	Fever with relative bradycardia, patient appears well, recent new medication	“Have you started any new medications in the past few weeks? Do you feel as sick as your temperature suggests?”
Lymphoma / Malignancy	B symptoms (fever, night sweats, weight loss), lymphadenopathy	“Have you noticed any lumps or swollen glands? Have you lost more than 10% of your body weight? Do you have drenching night sweats?”
Adult-Onset Still Disease	Quotidian fever, evanescent salmon-colored rash, arthralgia, sore throat	“Does your fever spike once or twice daily and then return to normal? Do you get a rash that comes and goes with the fever?”

Detailed Exposure History

Exposure Type	Questions to Ask	Associated Infections
Travel History	Where did you travel? When? Rural or urban? Activities? Prophylaxis taken?	Malaria, typhoid, dengue, chikungunya, hepatitis A/E, travelers’ diarrhea, rickettsial infections
Animal Contact	Pets? Farm animals? Wild animals? Bites or scratches?	Cat scratch disease, Q fever, brucellosis, leptospirosis, psittacosis, rabies
Insect/Tick Exposure	Outdoor activities? Tick bites? Mosquito exposure? Geographic location?	Lyme disease, Rocky Mountain spotted fever, ehrlichiosis, West Nile virus, dengue, malaria
Water Exposure	Swimming? Hot tubs? Freshwater lakes? Flooding exposure?	Leptospirosis, Legionella, Pseudomonas folliculitis, primary amebic meningoencephalitis
Food History	Unpasteurized dairy? Raw or undercooked meat? Seafood? Deli meats?	Listeriosis, Salmonella, Campylobacter, hepatitis A, Vibrio, brucellosis
Sexual History	New partners? Unprotected intercourse? Men who have sex with men?	HIV acute retroviral syndrome, syphilis, gonorrhea (disseminated), hepatitis B
Occupational Exposure	Healthcare worker? Veterinarian? Farmer? Laboratory worker?	Tuberculosis, hepatitis B/C, brucellosis, Q fever, anthrax
Sick Contacts	Anyone at home or work ill? Daycare or school contacts? Nursing home?	Influenza, COVID-19, tuberculosis, meningococcal disease, viral gastroenteritis

Medication and Past Medical History

Medications That Cause Fever

Antibiotics — Beta-lactams (penicillins, cephalosporins), sulfonamides, nitrofurantoin, minocycline
Anticonvulsants — Phenytoin, carbamazepine, lamotrigine (may cause DRESS syndrome)
Cardiovascular drugs — Procainamide, quinidine, hydralazine, methyldopa
Allopurinol — Can cause severe hypersensitivity syndrome
Antipsychotics — May cause neuroleptic malignant syndrome
Serotonergic drugs — SSRIs, SNRIs, MAOIs, tramadol (serotonin syndrome)
Immunotherapies — Checkpoint inhibitors, cytokine therapies
Biologics — Infliximab, rituximab (infusion reactions)

Key Past Medical History

HIV status: CD4 count determines risk for opportunistic infections
Malignancy: Chemotherapy timing, neutropenia risk
Transplant: Type, immunosuppression regimen, time since transplant
Diabetes: Increased infection risk, atypical presentations
Splenectomy: Risk for encapsulated organisms (pneumococcus, meningococcus, Haemophilus)
Valvular heart disease: Endocarditis risk
Indwelling devices: Central lines, pacemakers, prosthetic joints, urinary catheters
Recent procedures: Surgery, dental work, endoscopy

Characterizing the Fever Pattern

Essential Questions About the Fever Itself

Onset: “When did the fever start? Was it sudden or gradual?”
Duration: “How many days have you had fever?”
Pattern: “Is the fever constant or does it come and go? Does it follow a pattern?”
Maximum temperature: “What is the highest temperature you’ve recorded?”
Method of measurement: “How are you taking your temperature?” (oral, axillary, rectal, tympanic)
Rigors: “Do you have shaking chills where your teeth chatter?” (suggests bacteremia)
Night sweats: “Do you wake up with your sheets soaked?” (suggests tuberculosis, lymphoma, endocarditis)
Response to antipyretics: “Does paracetamol or ibuprofen bring the fever down?” (hyperthermia does not respond)

Social History Considerations

Substance Use

Injection drug use: Endocarditis (tricuspid valve), skin and soft tissue infections, HIV, hepatitis B/C
Alcohol use: Increased pneumonia risk, spontaneous bacterial peritonitis, impaired immunity
Smoking: Increased respiratory infection risk, Legionella risk

Living Situation

Homelessness: Tuberculosis, skin infections, body lice (trench fever)
Nursing home/Long-term care: Resistant organisms, aspiration pneumonia, urinary tract infections
Prison/Incarceration: Tuberculosis, HIV, hepatitis
Recent immigration: Tuberculosis, parasitic infections, hepatitis B

4. Physical Examination

A systematic head-to-toe approach for Fever

Systematic Framework: Use the “Head to Extremities” approach for complete examination of patients presenting with Fever. The goal is to identify the source of infection or inflammation while assessing for signs of severe illness or organ dysfunction.

General Inspection

Overall appearance: Toxic versus non-toxic appearance; a patient who “looks sick” warrants urgent evaluation regardless of vital signs
Level of consciousness: Alert, confused, lethargic, obtunded; altered mental status is a red flag
Respiratory effort: Tachypnea, use of accessory muscles, nasal flaring, ability to speak in full sentences
Skin color: Pallor (anemia, shock), flushing (fever), cyanosis (hypoxemia), jaundice (hepatobiliary infection, hemolysis)
Diaphoresis: Profuse sweating suggests defervescence, sepsis, or hypoglycemia
Body habitus: Cachexia suggests chronic illness, malignancy, or tuberculosis
Rigors: Visible shaking chills during examination strongly suggest bacteremia

Vital Signs

Vital Sign	What to Look For	Clinical Significance
Temperature	Height of fever; note measurement site (oral, axillary, rectal, tympanic); temporal pattern	Greater than 38.0°C confirms fever; greater than 41°C is hyperpyrexia (medical emergency); elderly and immunocompromised may have blunted response
Heart Rate	Tachycardia (expected: increase of 10-15 bpm per 1°C); relative bradycardia (pulse-temperature dissociation)	Tachycardia out of proportion suggests sepsis, hypovolemia, or hyperthyroidism; relative bradycardia suggests typhoid, drug fever, or intracellular pathogens
Blood Pressure	Hypotension (systolic less than 90 mmHg or mean arterial pressure less than 65 mmHg); wide pulse pressure	Hypotension indicates septic shock until proven otherwise; wide pulse pressure early in sepsis (vasodilation)
Respiratory Rate	Tachypnea (greater than 20 breaths per minute); Kussmaul breathing	Tachypnea is sensitive for pneumonia and sepsis; respiratory rate greater than 22 is a SOFA criterion for sepsis
Oxygen Saturation	Hypoxemia (SpO2 less than 94% on room air)	Suggests pneumonia, pulmonary embolism, or sepsis-induced acute respiratory distress syndrome

Head and Neck Examination

Eyes

Conjunctival pallor: Anemia
Conjunctival petechiae: Endocarditis
Icterus: Hepatobiliary infection, hemolysis, malaria
Roth spots (fundoscopy): Infective endocarditis
Uveitis: Autoimmune disease, tuberculosis, sarcoidosis

Ears, Nose, Sinuses

Otitis media: Bulging, erythematous tympanic membrane
Mastoid tenderness: Mastoiditis
Sinus tenderness: Acute sinusitis
Nasal discharge: Purulent suggests bacterial sinusitis
Septal perforation: Cocaine use, granulomatosis with polyangiitis

Oropharynx

Tonsillar exudate: Streptococcal pharyngitis, mononucleosis
Peritonsillar swelling: Peritonsillar abscess
Dental abscess: Source of bacteremia, endocarditis risk
Oral candidiasis: Immunocompromise, HIV
Oral ulcers: Herpes simplex, Behçet disease, neutropenia

Neck

Nuchal rigidity: Resistance to passive neck flexion; suggests meningitis (positive Brudzinski sign: hip flexion with neck flexion; positive Kernig sign: pain with knee extension when hip flexed)
Lymphadenopathy: Location, size, tenderness, consistency (tender suggests infection; firm, fixed suggests malignancy)
Thyroid enlargement: Thyroiditis (subacute thyroiditis causes fever with tender thyroid)
Jugular venous distension: Right heart failure, pericarditis, cardiac tamponade

Respiratory Examination

Inspection

Respiratory rate and pattern
Use of accessory muscles
Intercostal retractions
Chest wall asymmetry

Palpation

Tracheal position: Deviation suggests tension pneumothorax, large effusion, or collapse
Chest expansion: Reduced on affected side in consolidation or effusion
Tactile fremitus: Increased in consolidation; decreased in effusion or pneumothorax

Percussion

Dullness: Consolidation (pneumonia), pleural effusion (stony dull), hemothorax
Hyperresonance: Pneumothorax

Auscultation

Finding	Description	Associated Conditions
Bronchial breath sounds	Loud, high-pitched sounds heard over peripheral lung fields	Lobar pneumonia with consolidation
Crackles (rales)	Discontinuous sounds; fine (Velcro-like) or coarse	Pneumonia, pulmonary edema, interstitial lung disease
Wheezes	Continuous musical sounds, typically expiratory	Asthma exacerbation, bronchitis, chronic obstructive pulmonary disease exacerbation
Pleural friction rub	Grating sound, heard in both inspiration and expiration	Pleuritis, pulmonary embolism with infarction, pneumonia with pleural involvement
Absent breath sounds	No air movement heard over lung field	Large pleural effusion, pneumothorax, complete lung collapse
Egophony (“E to A” change)	Spoken “E” sounds like “A” on auscultation	Consolidation (pneumonia), pleural effusion at upper border

Cardiovascular Examination

Jugular venous pressure: Elevated in right heart failure, pericarditis, tricuspid endocarditis
Apex beat: Displaced in cardiomegaly
Heart sounds: New murmur is red flag for endocarditis; S3 gallop suggests heart failure; muffled sounds in pericardial effusion
Murmurs: Regurgitant murmurs (especially new) suggest endocarditis; document character, location, radiation, timing
Pericardial friction rub: Three-component scratchy sound; suggests pericarditis
Peripheral pulses: Bounding in early sepsis; weak and thready in shock
Capillary refill: Greater than 3 seconds suggests poor perfusion

Abdominal Examination

Inspection: Distension (ascites, bowel obstruction), surgical scars (source of intra-abdominal abscess), visible peristalsis
Auscultation: Absent bowel sounds (ileus, peritonitis); hyperactive sounds (gastroenteritis, early obstruction)
Palpation: Tenderness location guides diagnosis; guarding and rebound tenderness suggest peritonitis; hepatomegaly (hepatic abscess, hepatitis); splenomegaly (infectious mononucleosis, endocarditis, malaria, lymphoma)
Murphy sign: Inspiratory arrest with palpation of right upper quadrant — acute cholecystitis
McBurney point tenderness: Right lower quadrant — appendicitis
Rovsing sign: Right lower quadrant pain with left lower quadrant palpation — appendicitis
Costovertebral angle tenderness: Pyelonephritis

Skin Examination

Finding	Description	Associated Conditions
Petechiae / Purpura	Non-blanching red or purple spots; petechiae less than 2mm, purpura greater than 2mm	Meningococcemia (rapidly progressive), disseminated intravascular coagulation, thrombocytopenia, vasculitis, endocarditis
Erythema	Diffuse redness; assess for warmth, borders, spread	Cellulitis, erysipelas, necrotizing fasciitis (pain out of proportion)
Vesicles / Bullae	Fluid-filled lesions; vesicles less than 5mm, bullae greater than 5mm	Herpes simplex, varicella-zoster, bullous impetigo, necrotizing fasciitis
Maculopapular rash	Flat (macules) and raised (papules) lesions	Viral exanthems, drug eruption, secondary syphilis, rickettsial diseases
Erythema migrans	Expanding annular erythema with central clearing (“bull’s eye”)	Lyme disease
Splinter hemorrhages	Linear red-brown streaks under fingernails or toenails	Infective endocarditis (also trauma)
Janeway lesions	Painless erythematous lesions on palms and soles	Infective endocarditis (septic emboli)
Osler nodes	Painful, raised lesions on fingers and toes	Infective endocarditis (immune complex deposition)
Evanescent salmon-colored rash	Transient pink macular rash appearing with fever spikes	Adult-onset Still disease

Musculoskeletal Examination

Joint examination: Swelling, warmth, erythema, effusion, range of motion; monoarthritis with fever suggests septic arthritis until proven otherwise
Spine examination: Point tenderness over vertebrae suggests vertebral osteomyelitis or epidural abscess
Muscle tenderness: Diffuse myalgias common in viral infections; localized tenderness suggests pyomyositis or necrotizing myositis

Extremities

Clubbing: Chronic infection (lung abscess, empyema, bronchiectasis), endocarditis, inflammatory bowel disease, malignancy
Peripheral edema: Heart failure, nephrotic syndrome, hypoalbuminemia from chronic infection
Deep vein thrombosis signs: Unilateral leg swelling, calf tenderness; septic thrombophlebitis possible
Injection sites: Track marks suggest injection drug use; assess for abscesses, cellulitis
Indwelling catheter sites: Erythema, purulence, or tenderness at central line, PICC line, or peripheral IV sites

Neurological Examination

Level of consciousness: Glasgow Coma Scale; confusion or decreased level of consciousness with fever suggests meningitis, encephalitis, or sepsis-associated encephalopathy
Meningeal signs: Nuchal rigidity, Kernig sign, Brudzinski sign
Focal deficits: May suggest brain abscess, stroke (from endocarditis emboli), or encephalitis
Cranial nerve examination: Particularly important if suspecting skull base infection or Lyme disease

Expected Findings by Etiology

Condition	General Appearance	Key Examination Findings	Red Flags
Bacterial Pneumonia	Tachypneic, may appear toxic	Focal crackles, bronchial breath sounds, dullness to percussion, egophony	Hypoxemia, tachypnea greater than 30, hypotension
Urinary Tract Infection / Pyelonephritis	May appear well (cystitis) or toxic (pyelonephritis)	Suprapubic tenderness (cystitis), costovertebral angle tenderness (pyelonephritis)	Rigors, hypotension, altered mental status
Bacterial Meningitis	Toxic, altered consciousness	Nuchal rigidity, positive Kernig/Brudzinski signs, photophobia	Petechial rash, seizures, focal deficits
Infective Endocarditis	May appear chronically ill	New or changing murmur, splenomegaly, splinter hemorrhages, Janeway lesions, Osler nodes	Embolic phenomena, heart failure signs
Intra-abdominal Abscess	Toxic, guarding posture	Localized abdominal tenderness, guarding, rebound, palpable mass (occasionally)	Peritonitis signs, sepsis
Cellulitis / Necrotizing Fasciitis	May appear toxic in necrotizing fasciitis	Erythema, warmth, swelling; crepitus, bullae, necrosis in necrotizing fasciitis	Pain out of proportion, rapid spread, crepitus, systemic toxicity
Viral Syndrome	Non-toxic, mildly ill	Often minimal findings; may have pharyngitis, lymphadenopathy, rash	None (diagnosis of exclusion)
Drug Fever	Appears well despite high temperature	Relative bradycardia; may have rash; no localizing signs	None (resolves with drug cessation)

Important Teaching Point

A normal physical examination does not exclude serious infection! Several conditions commonly present with fever and minimal or absent physical findings:

Early bacteremia — may have no localizing signs before seeding of organs
Urinary tract infection in elderly — may present only with confusion, without urinary symptoms
Occult abscess — deep abscesses (psoas, hepatic, splenic) may have no external signs
Infective endocarditis — murmur may be absent, especially in right-sided disease
Viral infections — often have non-specific findings only
Drug fever — characteristically has no localizing findings
Fever of unknown origin — by definition, source is not apparent

When examination is unrevealing, clinical suspicion and targeted investigations become paramount. The patient’s immune status, comorbidities, and risk factors should guide the diagnostic approach.

5. Differential Diagnosis

Systematic approach organized by probability and clinical features

Acute Fever (Duration: Less than 7 days)

Probability	Condition	Key Features	Red Flags
COMMON (approximately 70%)	Viral Upper Respiratory Tract Infection	Rhinorrhea, sore throat, cough, myalgias; self-limiting	None (diagnosis of exclusion)
	Influenza	Sudden onset, high fever, severe myalgias, headache, dry cough; seasonal pattern	Dyspnea, confusion, persistent vomiting
	Acute Gastroenteritis	Diarrhea, vomiting, abdominal cramps; often viral (norovirus, rotavirus)	Bloody diarrhea, severe dehydration, high fever
	Urinary Tract Infection	Dysuria, frequency, urgency; suprapubic discomfort	Flank pain, rigors, high fever (suggests pyelonephritis)
	Skin and Soft Tissue Infection	Localized erythema, warmth, swelling, pain; clear borders in erysipelas	Rapid spread, crepitus, bullae, necrosis, pain out of proportion
LESS COMMON (approximately 20%)	Bacterial Pneumonia	Productive cough, dyspnea, pleuritic chest pain; focal examination findings	Hypoxemia, tachypnea greater than 30, hypotension, confusion
	Acute Pyelonephritis	Flank pain, costovertebral angle tenderness, rigors; often with lower urinary symptoms	Sepsis, obstruction (hydronephrosis)
	Acute Cholecystitis	Right upper quadrant pain, positive Murphy sign, nausea; pain after fatty meals	Jaundice, palpable gallbladder, peritonitis
	Acute Appendicitis	Periumbilical pain migrating to right lower quadrant, anorexia, nausea	Peritonitis, palpable mass, high fever (suggests perforation)
UNCOMMON BUT SERIOUS (approximately 10%)	Bacterial Meningitis	Severe headache, neck stiffness, photophobia, altered mental status	Petechial rash, seizures, focal neurological deficits, rapid deterioration
	Sepsis / Bacteremia	May have no localizing source; tachycardia, tachypnea, rigors	Hypotension, altered mental status, mottled skin, oliguria
	Necrotizing Fasciitis	Pain out of proportion to examination findings, rapid progression	Crepitus, bullae, skin necrosis, systemic toxicity
	Malaria (in travelers)	Travel to endemic area, cyclical fever, rigors, headache, splenomegaly	Altered consciousness, jaundice, severe anemia, respiratory distress

Prolonged Fever (Duration: Greater than 2-3 weeks)

Step-by-Step Approach to Prolonged Fever:

Step 1: Rule out common infections with unusual presentations — tuberculosis, endocarditis, occult abscess, osteomyelitis
Step 2: Consider the “Big Three” categories — Infection (25-50%), Malignancy (15-25%), Non-infectious inflammatory disease (15-25%)
Step 3: Review medications for drug fever — one of the most commonly missed diagnoses
Step 4: Investigate for less common causes if initial workup is negative

Category	Condition	Approximate Frequency in FUO	Key Distinguishing Features
INFECTIONS (25-50%)	Tuberculosis	10-15%	Chronic cough, night sweats, weight loss; may be extrapulmonary (lymph nodes, bone, meninges)
	Infective Endocarditis	5-10%	New or changing murmur, embolic phenomena, splenomegaly; risk factors (valve disease, injection drug use)
	Intra-abdominal Abscess	5-10%	Recent surgery or procedure, abdominal pain (may be vague), leukocytosis
	Osteomyelitis / Septic Arthritis	3-5%	Localized bone pain, vertebral tenderness; may follow trauma or surgery
	Cytomegalovirus / Epstein-Barr Virus	3-5%	Mononucleosis-like syndrome, atypical lymphocytes, mild transaminitis
MALIGNANCIES (15-25%)	Lymphoma (Hodgkin and Non-Hodgkin)	8-12%	B symptoms (fever, night sweats, weight loss greater than 10%), lymphadenopathy, hepatosplenomegaly
	Leukemia	3-5%	Cytopenias, hepatosplenomegaly, lymphadenopathy, bleeding, recurrent infections
	Renal Cell Carcinoma	2-3%	Hematuria, flank pain, palpable mass; paraneoplastic fever common
	Hepatocellular Carcinoma	1-2%	Known cirrhosis or hepatitis B/C, elevated alpha-fetoprotein, liver mass
NON-INFECTIOUS INFLAMMATORY (15-25%)	Adult-Onset Still Disease	5-8%	Quotidian fever, evanescent salmon-colored rash, arthralgia, sore throat, markedly elevated ferritin
	Systemic Lupus Erythematosus	3-5%	Malar rash, photosensitivity, arthritis, serositis, cytopenias, positive antinuclear antibody
	Vasculitis (Giant Cell Arteritis, Polyarteritis Nodosa)	3-5%	Headache, jaw claudication (giant cell arteritis); hypertension, neuropathy (polyarteritis nodosa)
	Inflammatory Bowel Disease	2-3%	Chronic diarrhea (may be bloody), abdominal pain, weight loss, extraintestinal manifestations

Anatomical Approach to Fever Source

Head and Neck

Sinusitis

Otitis media / Mastoiditis

Dental abscess

Pharyngitis / Peritonsillar abscess

Meningitis / Encephalitis

Brain abscess

Thorax

Pneumonia / Lung abscess

Empyema

Tuberculosis

Infective endocarditis

Pericarditis

Mediastinitis

Abdomen and Pelvis

Cholecystitis / Cholangitis

Hepatic abscess

Appendicitis / Diverticulitis

Intra-abdominal abscess

Pyelonephritis / Perinephric abscess

Pelvic inflammatory disease

Skin, Soft Tissue, and Musculoskeletal

Cellulitis / Erysipelas

Necrotizing fasciitis

Surgical site infection

Osteomyelitis

Septic arthritis

Device-related infection

Drug-Induced Fever

Key Points About Drug Fever

Drug fever is one of the most commonly missed diagnoses in the evaluation of prolonged fever. It should be considered in any patient with unexplained fever who is receiving medications, particularly antibiotics. Characteristic features include:

Patient appears well despite high temperature
Relative bradycardia (pulse-temperature dissociation)
No other explanation for fever despite thorough workup
Fever resolves within 72-96 hours of drug discontinuation

Drug or Drug Class	Mechanism	Characteristics	Time to Resolution After Stopping
Beta-lactam Antibiotics	Hypersensitivity reaction (hapten formation)	Often occurs 7-10 days after starting; may have eosinophilia, rash	48-72 hours
Sulfonamides	Hypersensitivity, may cause DRESS syndrome	Can be severe; watch for rash, hepatitis, cytopenias	72-96 hours (longer if DRESS)
Phenytoin	Hypersensitivity, anticonvulsant hypersensitivity syndrome	Rash, lymphadenopathy, hepatitis, eosinophilia	Days to weeks
Allopurinol	Severe hypersensitivity reaction	May cause severe cutaneous adverse reaction with fever, renal failure	Days to weeks
Antipsychotics	Dopamine receptor blockade (neuroleptic malignant syndrome)	Rigidity, altered mental status, autonomic instability; medical emergency	Days to 2 weeks; requires specific treatment
Serotonergic Drugs	Excess serotonergic activity (serotonin syndrome)	Hyperreflexia, clonus, agitation, diarrhea; rapid onset after drug change	24-72 hours
Amphotericin B	Direct pyrogenic effect	Infusion-related fever and rigors; predictable with each dose	Hours (after infusion ends)
Bleomycin	Direct pyrogenic effect	Fever with or without chills after administration	Hours to days
Checkpoint Inhibitors	Immune-related adverse events	May cause colitis, hepatitis, pneumonitis, or hypophysitis with fever	Variable; may require immunosuppression

Differential by Special Population

Population	Unique Considerations	Priority Diagnoses
Neutropenic Patients	Fever may be only sign of infection; physical findings often absent	Bacteremia (gram-negative and gram-positive), fungal infection (Aspergillus, Candida), typhlitis
HIV/AIDS (CD4 less than 200)	Opportunistic infections based on CD4 count	Pneumocystis pneumonia, Mycobacterium avium complex, cytomegalovirus, cryptococcal meningitis, toxoplasmosis
Transplant Recipients	Timeline since transplant affects differential; reject versus infection	Cytomegalovirus, Pneumocystis, Aspergillus, post-transplant lymphoproliferative disorder
Injection Drug Users	High risk for bloodstream infections	Right-sided endocarditis (Staphylococcus aureus), skin and soft tissue infections, septic thrombophlebitis
Post-operative Patients	Use the “5 W’s” — Wind, Water, Wound, Walking, Wonder drugs	Atelectasis/pneumonia, urinary tract infection, surgical site infection, deep vein thrombosis, drug fever
Returning Travelers	Geography determines differential	Malaria (always consider), typhoid, dengue, chikungunya, rickettsial infections, hepatitis A/E
Elderly Patients	Blunted febrile response; atypical presentations	Urinary tract infection, pneumonia, intra-abdominal infection, endocarditis; malignancy

Quick Reference: “If You See This, Think This”

Clinical Clue	Think This First	Next Step
Fever + petechial rash	Meningococcemia	Immediate blood cultures, lumbar puncture, empiric antibiotics
Fever + new heart murmur	Infective endocarditis	Multiple blood cultures, echocardiography
Fever + relative bradycardia	Drug fever, typhoid, intracellular pathogens	Review medications, blood cultures, specific serologies
Fever + rigors	Bacteremia, pyelonephritis, cholangitis, abscess	Blood cultures, imaging of suspected source
Fever + travel to endemic area	Malaria (always first)	Thick and thin blood smears, rapid diagnostic test
Fever + quotidian pattern + rash	Adult-onset Still disease	Ferritin level (often greater than 10,000), exclude infection
Fever + night sweats + weight loss	Tuberculosis, lymphoma, endocarditis	Chest imaging, blood cultures, consider CT scan
Fever + neutropenia	Neutropenic fever (medical emergency)	Blood cultures, empiric broad-spectrum antibiotics within 1 hour
Fever + back pain	Vertebral osteomyelitis, epidural abscess	MRI spine, blood cultures
Fever + monoarthritis	Septic arthritis	Urgent joint aspiration, blood cultures

6. Diagnostic Investigations

A stepwise, cost-effective approach guided by clinical suspicion

Baseline Investigations for All Patients with Fever

Investigation	Purpose	What to Look For	Practical Points
Complete Blood Count with Differential	Assess for infection, malignancy, bone marrow pathology	Leukocytosis with left shift (bacterial); lymphocytosis (viral); neutropenia (immunocompromise); atypical lymphocytes (mononucleosis); cytopenias (malignancy, overwhelming sepsis)	Normal white blood cell count does not exclude infection; elderly and immunocompromised may not mount leukocytosis
C-Reactive Protein (CRP)	Marker of inflammation; helps distinguish bacterial from viral	CRP greater than 100 mg/L suggests bacterial infection; serial measurements useful for monitoring response	Rises within 6-8 hours; peaks at 48 hours; more useful for trend than single value
Procalcitonin	More specific marker for bacterial infection	Greater than 0.5 ng/mL suggests bacterial infection; greater than 2 ng/mL high likelihood of sepsis	Useful for antibiotic stewardship; less elevated in localized infection; may be elevated in some non-infectious conditions
Blood Cultures (2-3 sets)	Identify bacteremia; essential before antibiotics	Organism identification and sensitivities; contaminants versus true pathogens	Draw from separate venipuncture sites; 20 mL total per set (10 mL each bottle); draw before antibiotics when possible
Urinalysis and Urine Culture	Detect urinary tract infection	Pyuria (greater than 10 white blood cells per high-power field), bacteriuria, nitrites, leukocyte esterase	Asymptomatic bacteriuria is common in elderly—clinical correlation needed; catheter specimens may be contaminated
Basic Metabolic Panel	Assess renal function, electrolytes, glucose	Acute kidney injury (sepsis, dehydration); hyponatremia (pneumonia); hyperglycemia (stress response, diabetes)	Creatinine rise may indicate sepsis; lactate often added for sepsis assessment
Liver Function Tests	Screen for hepatobiliary infection, hepatitis	Transaminitis (hepatitis, sepsis); cholestatic pattern (biliary obstruction, cholangitis); elevated bilirubin	Isolated elevated alkaline phosphatase may suggest granulomatous disease or infiltrative process
Chest Radiograph	Detect pneumonia, tuberculosis, malignancy, pleural effusion	Consolidation, infiltrates, cavitation, hilar lymphadenopathy, pleural effusion, masses	May be normal early in pneumonia; CT more sensitive for subtle findings; lateral view increases sensitivity
Lactate	Assess tissue perfusion; sepsis marker	Greater than 2 mmol/L suggests tissue hypoperfusion; greater than 4 mmol/L indicates severe sepsis/septic shock	Should be measured early in suspected sepsis; serial measurements guide resuscitation

Targeted Investigations by Suspected Etiology

If Suspecting Bacterial Pneumonia

First-Line Tests

Chest radiograph: Consolidation, air bronchograms, pleural effusion
Sputum Gram stain and culture: Before antibiotics if possible; acceptable specimen has greater than 25 neutrophils and less than 10 epithelial cells per low-power field
Blood cultures: Positive in 5-14% of community-acquired pneumonia
Pulse oximetry / arterial blood gas: Assess oxygenation and acid-base status

Second-Line Tests

Legionella urinary antigen: Sensitivity 70-80% for serogroup 1
Pneumococcal urinary antigen: Sensitivity 50-80%; remains positive despite antibiotics
Respiratory viral panel (PCR): Influenza, RSV, COVID-19, other respiratory viruses
CT chest: If chest radiograph equivocal, or for complications (abscess, empyema)
Bronchoscopy with BAL: For immunocompromised patients or non-responding pneumonia

If Suspecting Infective Endocarditis

First-Line Tests

Blood cultures: At least 3 sets from separate sites before antibiotics; bacteremia is typically continuous
Transthoracic echocardiography: Sensitivity 40-60% for vegetations; start here in native valve
ECG: New conduction abnormality suggests perivalvular extension

Second-Line Tests

Transesophageal echocardiography: Sensitivity greater than 90%; required if transthoracic negative and clinical suspicion high, prosthetic valve, or to assess complications
CT scan: Assess for embolic phenomena (splenic infarcts, renal infarcts)
PET-CT: Increasingly used for prosthetic valve endocarditis (Modified Duke criteria)

If Suspecting Intra-abdominal Infection / Abscess

First-Line Tests

CT abdomen and pelvis with contrast: Gold standard for abscess detection; sensitivity greater than 95%
Blood cultures: Often positive in biliary and hepatic infections
Liver function tests: Cholestatic pattern in biliary disease

Second-Line Tests

MRCP or ERCP: For biliary obstruction, cholangitis
Image-guided aspiration: Diagnostic and therapeutic; send for culture
Ultrasound: Good for gallbladder, some liver abscesses; operator-dependent

If Suspecting Meningitis / Encephalitis

First-Line Tests

Lumbar puncture: Opening pressure, cell count and differential, protein, glucose, Gram stain, culture
CT head (before LP if indicated): Required before LP if focal deficits, papilledema, altered consciousness, immunocompromise, or seizures
Blood cultures: Obtain before antibiotics; do not delay antibiotics for LP

Second-Line Tests

CSF PCR panel: Herpes simplex virus, enterovirus, varicella-zoster virus, bacterial PCR
CSF cryptococcal antigen: In HIV/immunocompromised patients
MRI brain: For encephalitis, abscess, or if diagnosis unclear
EEG: If encephalitis suspected or seizure activity

If Suspecting Tuberculosis

First-Line Tests

Chest radiograph: Upper lobe infiltrates, cavitation, miliary pattern, hilar lymphadenopathy
Sputum for acid-fast bacilli smear (3 samples): Early morning specimens; smear-positive indicates high infectivity
Sputum GeneXpert MTB/RIF: Rapid PCR test; also detects rifampicin resistance; sensitivity greater than 95% for smear-positive, 70% for smear-negative

Second-Line Tests

Sputum culture: Gold standard but takes 2-8 weeks; required for drug susceptibility testing
Interferon-gamma release assay (QuantiFERON, T-SPOT): Tests for TB infection (not active disease); useful in contacts and latent TB screening
CT chest: More sensitive than chest radiograph for subtle disease
Bronchoscopy with BAL: For smear-negative suspected pulmonary TB
Tissue biopsy: For extrapulmonary TB (lymph node, bone, etc.)

Systematic Investigation of Prolonged Fever / Fever of Unknown Origin

Staged Approach to FUO Investigation

A systematic, stepwise approach prevents unnecessary testing while ensuring serious diagnoses are not missed:

Stage 1 (Baseline): Complete history and physical examination, basic laboratory panel, blood cultures, urinalysis, chest radiograph
Stage 2 (Directed testing): Based on clinical clues from Stage 1—echocardiography if murmur, CT imaging if abdominal symptoms, serologies if epidemiological risk
Stage 3 (Advanced imaging): CT chest/abdomen/pelvis, PET-CT (increasingly used as it can identify occult infections, malignancies, and inflammatory conditions)
Stage 4 (Invasive testing): Bone marrow biopsy, liver biopsy, temporal artery biopsy, lymph node biopsy based on specific suspicion

Investigation	When to Order	What It May Reveal
Erythrocyte Sedimentation Rate (ESR)	All patients with prolonged fever	Non-specific marker; ESR greater than 100 suggests endocarditis, osteomyelitis, malignancy, giant cell arteritis
Ferritin	Suspected hemophagocytic lymphohistiocytosis or Adult-onset Still disease	Markedly elevated (often greater than 10,000 ng/mL in Still disease); elevated in hemophagocytic lymphohistiocytosis
Lactate Dehydrogenase (LDH)	Suspected lymphoma, hemolysis, Pneumocystis pneumonia	Elevated in lymphoma, hemophagocytic lymphohistiocytosis, Pneumocystis (in HIV patients)
Antinuclear Antibody (ANA)	Suspected autoimmune disease	Positive in systemic lupus erythematosus, mixed connective tissue disease; low specificity if low titer
ANCA (anti-neutrophil cytoplasmic antibodies)	Suspected vasculitis	c-ANCA in granulomatosis with polyangiitis; p-ANCA in microscopic polyangiitis
CT Chest / Abdomen / Pelvis	Unexplained fever after basic workup	Occult abscess, lymphadenopathy, hepatosplenomegaly, masses, pulmonary infiltrates
PET-CT (18F-FDG)	FUO with negative conventional workup	Identifies areas of increased metabolic activity; useful for occult infections, vasculitis, malignancy; sensitivity 80-90% in FUO
Bone Marrow Biopsy	Suspected hematological malignancy, granulomatous disease, hemophagocytic lymphohistiocytosis	Leukemia, lymphoma, granulomas (TB, histoplasmosis), hemophagocytosis
Temporal Artery Biopsy	Age greater than 50 with elevated ESR, headache, visual symptoms	Giant cell arteritis (gold standard); obtain adequate length (greater than 2 cm)
Liver Biopsy	Unexplained hepatomegaly, granulomatous hepatitis suspected	Granulomas (TB, sarcoidosis, drug-induced), malignancy, hepatitis

Empiric Treatment Trials as Diagnostic Tools

When to Consider Empiric Therapy

In certain clinical scenarios, empiric treatment trials can serve as diagnostic tools when other investigations are inconclusive. Response to therapy supports the diagnosis.

Naproxen Test: NSAIDs (naproxen 250 mg twice daily) may reduce fever in malignancy but not infection—though evidence is limited and this test has fallen out of favor
Empiric Anti-tuberculosis therapy: Consider in high-risk patients (endemic area, HIV, suggestive imaging) with negative initial workup; response within 2-3 weeks supports diagnosis
Corticosteroid trial: Only after exhaustive exclusion of infection; may be diagnostic for Adult-onset Still disease, temporal arteritis, or other inflammatory conditions
Drug withdrawal: Discontinue all non-essential medications and observe for 72-96 hours; resolution supports drug fever

Caution: Empiric corticosteroids can mask infection and should only be considered when infection has been reasonably excluded. Never give empiric corticosteroids without first excluding tuberculosis and other infections.

7. Pattern Recognition and Clinical Decision-Making

Practical algorithms and decision pathways

Step 1: Is This Urgent?

Clinical Scenario	Urgency Level	Immediate Action
Fever with hypotension (systolic blood pressure less than 90 mmHg) or signs of shock	EMERGENT	Activate sepsis protocol: IV access, blood cultures, lactate, broad-spectrum antibiotics within 1 hour, fluid resuscitation (30 mL/kg crystalloid), consider ICU admission
Fever with petechial/purpuric rash	EMERGENT	Assume meningococcemia until proven otherwise: immediate blood cultures, lumbar puncture (if no contraindication), empiric ceftriaxone or cefotaxime, contact precautions
Fever with altered mental status or seizures	EMERGENT	Suspect meningitis/encephalitis: CT head if indicated, lumbar puncture, empiric antibiotics plus acyclovir (for HSV encephalitis), supportive care
Neutropenic fever (absolute neutrophil count less than 500/μL)	EMERGENT	Blood cultures from all lumens, empiric anti-pseudomonal beta-lactam within 1 hour (do not wait for culture results), daily assessment for complications
Fever with severe localized pain out of proportion to examination	EMERGENT	Suspect necrotizing fasciitis: surgical consultation immediately, broad-spectrum antibiotics, urgent imaging (CT or MRI), prepare for operative debridement
Fever greater than 41°C (hyperpyrexia)	EMERGENT	Differentiate fever from hyperthermia; if hyperthermia (heat stroke, NMS, serotonin syndrome): active cooling, specific treatment; if fever: aggressive workup and empiric treatment
Fever with rigors and localizing symptoms	URGENT	Suggests bacteremia: blood cultures, targeted imaging (renal ultrasound for pyelonephritis, abdominal CT for abscess), empiric antibiotics after cultures
Fever in returning traveler from malaria-endemic area	URGENT	Thick and thin blood smears immediately (repeat every 12 hours × 3 if negative), rapid malaria antigen test, empiric treatment if high suspicion and delay in diagnosis
Fever with new heart murmur	URGENT	Suspect endocarditis: 3 sets of blood cultures before antibiotics, echocardiography (transthoracic then transesophageal if needed), cardiology consultation
Low-grade fever in stable patient without red flags	ROUTINE	Complete history and examination, baseline investigations, symptomatic treatment, close follow-up, most will be self-limiting viral infections

Step 2: Classify by Duration

Acute Fever (less than 7 days)

Most likely: Viral infection (70-80%)

Approach: Focus on excluding serious bacterial infection; supportive care if no red flags; targeted workup based on localizing symptoms

Proceed to Algorithm A

Subacute Fever (7-21 days)

Most likely: Prolonged viral, localized bacterial, early systemic disease

Approach: More extensive workup indicated; consider occult infections (abscess, endocarditis, TB); review medications

Proceed to Algorithm B

Prolonged Fever / FUO (greater than 3 weeks)

Most likely: Infection, malignancy, or inflammatory condition (Big Three)

Approach: Systematic staged investigation; consider referral to infectious disease or rheumatology; PET-CT increasingly useful

Proceed to Algorithm C

Step 3: Follow the Appropriate Algorithm

Algorithm A: Acute Fever (less than 7 days)

Clinical Scenario	Most Likely Diagnosis	Action
Fever + coryza + sore throat + myalgias, no localizing signs	Viral upper respiratory infection	Symptomatic treatment, antipyretics, hydration; advise return if worsening or not improving in 5-7 days
Fever + productive cough + dyspnea + focal chest findings	Community-acquired pneumonia	Chest radiograph, sputum culture, blood cultures if moderate-severe; empiric antibiotics based on severity (CURB-65 or PSI score)
Fever + dysuria + frequency + flank pain	Pyelonephritis	Urinalysis, urine culture, blood cultures if systemic symptoms; empiric antibiotics; renal imaging if not responding or recurrent
Fever + right upper quadrant pain + positive Murphy sign	Acute cholecystitis	Ultrasound abdomen, liver function tests; surgical consultation; antibiotics covering gram-negative and anaerobes
Fever + severe headache + neck stiffness + photophobia	Bacterial meningitis	Do not delay antibiotics; blood cultures, CT head (if indicated), lumbar puncture; empiric ceftriaxone + vancomycin ± ampicillin (if Listeria risk)
Fever + localized erythema/warmth/swelling of skin	Cellulitis or erysipelas	Mark borders to monitor spread; blood cultures if systemic symptoms; oral antibiotics if mild, IV if severe or not responding
Fever + diarrhea (watery or bloody) ± vomiting	Acute gastroenteritis	Stool studies if bloody, severe, or prolonged; rehydration; antibiotics only if specific indication (severe, invasive pathogen)

Algorithm B: Subacute Fever (7-21 days)

Clinical Scenario	Consider	Action
Persistent fever despite initial antibiotics	Wrong diagnosis, resistant organism, abscess, drug fever	Reassess source; consider imaging (CT) to look for abscess or undrained collection; review antibiotics; consider stopping and observing for drug fever
Fever + weight loss + night sweats	Tuberculosis, lymphoma, endocarditis	Chest radiograph, CT imaging, blood cultures, consider TB workup (sputum, Quantiferon), LDH, peripheral smear
Fever + new or changing heart murmur	Infective endocarditis	Multiple blood cultures, echocardiography (TTE then TEE if needed), apply Duke criteria
Fever + vague abdominal pain + recent surgery	Intra-abdominal abscess	CT abdomen/pelvis with contrast; image-guided drainage if collection found
Fever + back pain (especially localized point tenderness)	Vertebral osteomyelitis, epidural abscess	MRI spine (gold standard), blood cultures, ESR/CRP; neurosurgical consultation if epidural abscess
Fever + joint pain/swelling (single joint)	Septic arthritis	Urgent joint aspiration for cell count, Gram stain, culture; blood cultures; empiric antibiotics after aspiration

Algorithm C: Prolonged Fever / Fever of Unknown Origin

Stage	Investigations	Expected Yield
Stage 1: Basic Workup	Complete history review (especially medications, travel, exposures), thorough physical examination, CBC with differential, CMP, LFTs, ESR, CRP, urinalysis, blood cultures × 3, chest radiograph, HIV test	Identifies clues that direct further workup in approximately 50% of cases
Stage 2: Directed Testing	Based on Stage 1 clues: echocardiography (murmur), CT abdomen (abdominal symptoms), serologies (epidemiological risk), autoimmune panel (inflammatory symptoms), tuberculin skin test or IGRA	Identifies diagnosis in additional 20-30% of cases
Stage 3: Advanced Imaging	CT chest/abdomen/pelvis (if not done), PET-CT (valuable for localizing occult infection, malignancy, or vasculitis), MRI (specific indications)	PET-CT has 80-90% sensitivity in FUO; helps direct biopsy
Stage 4: Invasive Testing	Bone marrow biopsy (cytopenias, suspected hematological malignancy, granulomatous disease), temporal artery biopsy (age greater than 50, elevated ESR, headache), liver biopsy (hepatomegaly, granulomatous hepatitis), lymph node biopsy (lymphadenopathy)	Provides definitive diagnosis in select cases; should be guided by prior findings

“What Do I Do If…” Decision Reference

Clinical Situation	Immediate Action	Next Step
Patient on antibiotics develops new fever	Consider: Drug fever, Clostridioides difficile infection, resistant organism, undrained source	Stool test for C. difficile, repeat cultures, imaging for occult source, trial of drug withdrawal if appropriate
Fever persists beyond expected course	Question the diagnosis; consider complications or alternative etiology	Repeat imaging, broaden differential, consider specialist consultation
All workup is negative but fever continues	Consider: Drug fever, factitious fever, undiagnosed autoimmune condition, rare infections	Drug withdrawal trial, supervised temperature measurement, PET-CT, autoimmune workup, consider empiric therapeutic trials
Patient has indwelling device and fever	Device-related infection is likely until proven otherwise	Blood cultures (including through device), consider device removal if not responding; echocardiography if cardiac device or central line
Immunocompromised patient with any fever	Treat as potentially serious infection; broader differential needed	Blood cultures, chest imaging, empiric broad-spectrum antibiotics; consider opportunistic infections based on immune defect
Fever with eosinophilia	Consider: Drug reaction, parasitic infection, eosinophilic disorders, lymphoma	Medication review, travel history, stool ova and parasites, peripheral smear, consider bone marrow if persistent
Recurrent fevers with symptom-free intervals	Consider: Relapsing infection (malaria, brucellosis), periodic fever syndrome, lymphoma, autoimmune	Detailed pattern documentation, genetic testing if periodic fever syndrome suspected, malaria smears, CT imaging

Troubleshooting Refractory Fever

Ask These Questions When Fever Does Not Resolve

Is the diagnosis correct? Revisit the history and examination; consider alternative diagnoses
Is there an undrained source? Abscesses require drainage; antibiotics alone are insufficient
Is the antimicrobial appropriate? Review culture results, sensitivities, dosing, and penetration to site
Is there a device or foreign body that needs removal? Infected devices often require removal for cure
Is this drug fever? One of the most commonly missed diagnoses; consider stopping non-essential medications
Is there a second diagnosis? Patients can have multiple simultaneous problems
Is the patient immunocompromised? Broader differential and longer treatment courses may be needed
Has enough time passed? Some infections (endocarditis, osteomyelitis) take days to defervesce even with appropriate treatment

Sepsis Recognition and Response

Sepsis Screening Criteria (qSOFA at Bedside)

If a patient with suspected infection has ≥2 of the following, consider sepsis and escalate care:

Respiratory rate ≥22 breaths per minute
Altered mental status (Glasgow Coma Scale less than 15)
Systolic blood pressure ≤100 mmHg

Hour-1 Bundle for Sepsis:

Measure lactate level (repeat within 2-4 hours if initial lactate greater than 2 mmol/L)
Obtain blood cultures before antibiotics
Administer broad-spectrum antibiotics
Begin rapid administration of 30 mL/kg crystalloid for hypotension or lactate ≥4 mmol/L
Apply vasopressors if hypotensive during or after fluid resuscitation to maintain MAP ≥65 mmHg

8. Clinical Pearls and Pitfalls

Practical wisdom — learn from successes and avoid common mistakes

Must-Know Clinical Pearls

Fever versus hyperthermia: Fever is a regulated rise in the hypothalamic set point and responds to antipyretics; hyperthermia is a failure of thermoregulation and does not respond to antipyretics. This distinction is critical because treatment differs completely.

Relative bradycardia is a valuable clue: Heart rate should increase approximately 10-15 beats per minute for each 1°C rise in temperature. When it does not (pulse-temperature dissociation), consider typhoid fever, drug fever, Legionella, brucellosis, or factitious fever.

The “Big Three” dominate FUO: Infections, malignancies, and non-infectious inflammatory diseases account for 75-90% of fever of unknown origin diagnoses. A systematic approach targeting these categories is essential.

Drug fever is frequently missed: It can occur with almost any medication, typically appears 7-10 days after starting a drug, and characteristically presents with a patient who looks well despite high temperatures. Always review the medication list.

Rigors strongly suggest bacteremia: True shaking chills with teeth chattering indicate a high likelihood of bloodstream infection (bacteremia), abscess, pyelonephritis, or malaria. This finding should prompt blood cultures and urgent evaluation.

Malaria must always be considered in travelers: Any febrile illness in a patient who has traveled to a malaria-endemic area within the past year must be evaluated for malaria. It can be rapidly fatal if missed, and the diagnosis is easily made with blood smears.

PET-CT has transformed FUO workup: 18F-FDG PET-CT has 80-90% sensitivity in identifying the source of FUO and is increasingly used early in the workup, particularly when conventional imaging is negative.

Undiagnosed FUO usually has good prognosis: In approximately 10-25% of classic FUO cases, no diagnosis is established despite thorough investigation. These patients generally do well, suggesting benign, self-limited conditions.

Critical Pitfalls to Avoid

Assuming absence of fever excludes serious infection: Elderly patients, immunocompromised patients, and those on corticosteroids or antipyretics may not mount a fever despite severe infection. A normal temperature does not rule out sepsis in these populations.

Delaying antibiotics in sepsis: Every hour of delay in antibiotic administration in septic shock is associated with increased mortality. Obtain cultures but do not delay antibiotics—the goal is administration within 1 hour of recognition.

Failing to consider infective endocarditis: Endocarditis is a “great mimicker” that can present with fever alone. Any patient with prolonged fever and risk factors (valve disease, injection drug use, prosthetic valve, indwelling devices) should have blood cultures and echocardiography.

Stopping the workup after initial negative results: A negative chest radiograph does not exclude pneumonia (CT is more sensitive). Negative blood cultures do not exclude endocarditis (culture-negative endocarditis exists). Persistence and systematic investigation are key.

Forgetting to check the medication list: Drug fever is one of the most commonly missed diagnoses. Any new medication started in the preceding weeks should be suspected, especially beta-lactam antibiotics, anticonvulsants, and allopurinol.

Not asking about travel history: A travel history can completely change the differential diagnosis. Malaria, typhoid, dengue, and other tropical infections are easily missed when travel is not elicited.

Using antipyretics to treat hyperthermia: Antipyretics work by lowering the hypothalamic set point; they are ineffective in hyperthermia syndromes (heat stroke, neuroleptic malignant syndrome, serotonin syndrome) where the set point is normal but thermoregulation has failed.

Empiric corticosteroids without excluding infection: Never start empiric corticosteroids for possible inflammatory disease without first reasonably excluding infection, especially tuberculosis. Corticosteroids can cause catastrophic dissemination of undiagnosed infections.

Key Takeaways

Fever is a controlled elevation of body temperature mediated by prostaglandin E2 acting on the hypothalamus; it differs fundamentally from hyperthermia, which is a failure of thermoregulation.
Duration is the most important classification: acute (less than 7 days), subacute (7-21 days), and prolonged/FUO (greater than 3 weeks). Each has a distinct differential and approach.
The systematic “FEVER” mnemonic ensures comprehensive history taking: Features, Exposures, Vulnerability factors, Exact localizing symptoms, Recent events and Rx.
Red flags requiring urgent evaluation include hemodynamic instability, altered mental status, petechial rash, immunocompromise with fever, severe localized pain, and hyperpyrexia.
Physical examination should be systematic (head to extremities) but recognize that many serious infections (bacteremia, endocarditis, occult abscess) may have normal or minimal findings.
The “Big Three” categories (infections, malignancies, non-infectious inflammatory diseases) account for most cases of fever of unknown origin; a systematic staged approach is essential.
Drug fever should be considered in every patient with unexplained fever who is receiving medications; the classic presentation is a patient who appears well despite high temperatures with relative bradycardia.
Malaria must be excluded in any febrile patient with travel to an endemic area within the past year—it is a medical emergency that is easily diagnosed and treated if suspected.
Blood cultures should be obtained before antibiotics whenever possible, but antibiotic administration should not be delayed in sepsis—the one-hour goal is critical.
When fever does not resolve, systematically ask: Is the diagnosis correct? Is there an undrained source? Is there a device that needs removal? Is this drug fever?

Quick Reference Algorithm

Systematic Approach to Fever:

Assess urgency: Is the patient hemodynamically stable? Are there red flags requiring immediate intervention?
Take a thorough history: Use the “FEVER” mnemonic—Features, Exposures, Vulnerability factors, Exact localizing symptoms, Recent events and Rx
Perform systematic examination: Head to extremities approach, including vital signs with assessment for relative bradycardia
Classify by duration: Acute, subacute, or prolonged—this determines the differential diagnosis and investigative approach
Order baseline investigations: CBC, inflammatory markers (CRP, procalcitonin), blood cultures, urinalysis, basic metabolic panel, chest radiograph
Pursue targeted workup: Based on clinical clues from history and examination—let the patient tell you where to look
Review medications: Consider drug fever in any patient with unexplained fever, especially if appearing well
Escalate systematically if undiagnosed: Advanced imaging (CT, PET-CT), serologies, invasive testing (biopsies) based on clinical suspicion
Reassess regularly: The clinical picture may evolve; what was unclear initially may declare itself with time or repeated examination
Know when to treat empirically: In severe illness, empiric treatment may be necessary while investigations continue; document reasoning and plan for de-escalation