Clinical Approach to Rash

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 rash

Rash is one of the most common presenting complaints in primary care, accounting for approximately 5-8% of all outpatient visits. Skin conditions represent the fourth most common cause of human disease globally, affecting nearly one-third of the world’s population at any given time. In family medicine practice, dermatological complaints rank among the top ten reasons for consultation, with an estimated 36% of the population experiencing a skin problem each year. Despite their frequency, rashes present a significant diagnostic challenge due to the vast differential diagnosis and the importance of pattern recognition in reaching an accurate diagnosis.

Definition

A rash (or exanthem) is any change in the skin’s appearance, including alterations in color, texture, or sensation. Rashes represent cutaneous manifestations of inflammation, infection, immune dysregulation, or systemic disease. They may be localized or generalized, and their morphology provides critical diagnostic clues to the underlying etiology.

Classification by Duration

Category	Duration	Common Causes	Clinical Significance
Acute	Less than 2 weeks	Viral exanthems, acute urticaria, contact dermatitis, drug eruptions, cellulitis	Often self-limiting; requires exclusion of serious infections and drug reactions
Subacute	2 to 6 weeks	Pityriasis rosea, secondary syphilis, guttate psoriasis, persistent drug reactions	May indicate evolving chronic condition or resolving acute process
Chronic	Greater than 6 weeks	Eczema, psoriasis, chronic urticaria, lichen planus, cutaneous lupus	Requires systematic evaluation for underlying systemic disease; often needs specialist referral

Classification by Primary Lesion Morphology

Accurate identification of the primary lesion is the cornerstone of dermatological diagnosis. Primary lesions are the initial manifestation of disease before any modification by scratching, infection, or treatment.

Lesion Type	Description	Size Criteria	Common Conditions
Macule	Flat, circumscribed area of color change; non-palpable	Less than 1 cm	Vitiligo, café-au-lait spots, freckles, viral exanthems
Patch	Flat area of color change; non-palpable	Greater than 1 cm	Tinea versicolor, morphea, port-wine stain
Papule	Elevated, solid lesion; palpable	Less than 1 cm	Acne, warts, molluscum contagiosum, lichen planus
Plaque	Elevated, flat-topped lesion; palpable	Greater than 1 cm	Psoriasis, eczema, mycosis fungoides
Nodule	Solid, elevated lesion extending into dermis or subcutis	Greater than 1 cm depth	Erythema nodosum, lipoma, cyst, melanoma
Vesicle	Fluid-filled, elevated lesion; clear fluid	Less than 1 cm	Herpes simplex, varicella, dyshidrotic eczema
Bulla	Large fluid-filled lesion	Greater than 1 cm	Bullous pemphigoid, burns, Stevens-Johnson syndrome
Pustule	Elevated lesion containing purulent fluid	Variable	Folliculitis, acne, pustular psoriasis, impetigo
Wheal	Transient, edematous, pink papule or plaque	Variable	Urticaria, insect bites, dermatographism
Petechiae/Purpura	Non-blanching red-purple lesions from extravasated blood	Petechiae less than 3 mm; Purpura greater than 3 mm	Vasculitis, thrombocytopenia, meningococcemia

Secondary Lesions

Secondary lesions evolve from primary lesions due to natural progression, trauma, scratching, or secondary infection. Recognizing these helps determine chronicity and patient behavior.

Surface Changes

Scale: Accumulation of stratum corneum (psoriasis, tinea)

Crust: Dried serum, blood, or pus (impetigo, eczema)

Erosion: Superficial loss of epidermis; heals without scarring

Ulcer: Full-thickness loss extending into dermis; heals with scarring

Texture Changes

Lichenification: Thickened skin with accentuated markings (chronic eczema)

Excoriation: Linear erosions from scratching

Atrophy: Thinning of skin (steroid use, aging)

Scar: Fibrous tissue replacing normal skin after injury

Classification by Distribution Pattern

Pattern	Description	Diagnostic Significance
Localized	Confined to one body region	Contact dermatitis, herpes zoster, fungal infection
Generalized	Widespread involvement of multiple body areas	Viral exanthem, drug eruption, systemic disease
Symmetric	Bilateral distribution in similar locations	Endogenous process (eczema, psoriasis, drug reaction)
Asymmetric	Unilateral or irregular distribution	Exogenous cause (contact, infection, trauma)
Dermatomal	Following nerve distribution	Herpes zoster (shingles)
Photodistributed	Sun-exposed areas (face, neck, dorsal hands, V of chest)	Photosensitivity, lupus, polymorphous light eruption
Flexural	Body folds (axillae, groin, antecubital fossae)	Atopic dermatitis, inverse psoriasis, intertrigo
Extensor	Extensor surfaces (elbows, knees)	Psoriasis, dermatitis herpetiformis
Acral	Hands and feet	Hand-foot-and-mouth disease, dyshidrotic eczema, secondary syphilis

The Diagnostic Triad: Successful rash diagnosis relies on three key elements:

Morphology — What does the primary lesion look like?
Distribution — Where on the body is it located and what pattern does it follow?
Clinical Context — What is the patient’s history, associated symptoms, and timeline?

Mastering the dermatological vocabulary and systematically applying this triad will allow you to generate an accurate differential diagnosis for the majority of rashes encountered in clinical practice.

Impact on Quality of Life

Psychosocial Burden

Skin conditions significantly impact quality of life, often underestimated by clinicians. Studies show that patients with chronic skin diseases have depression and anxiety rates comparable to those with cancer, heart disease, and diabetes. Visible rashes can lead to social stigmatization, impaired relationships, and reduced work productivity. Pruritic conditions cause sleep disturbance in up to 60% of affected patients. Always assess the psychosocial impact when evaluating patients with rash.

2. Pathophysiology and Mechanisms

Understanding the underlying mechanisms of cutaneous eruptions

The skin is a complex organ serving as the primary barrier between the body and the external environment. Understanding the pathophysiological mechanisms that produce different types of rashes is essential for rational diagnosis and treatment. Rashes result from disturbances in normal skin function through inflammatory, immunological, infectious, or vascular processes.

Skin Anatomy and the Origin of Rash

Skin Layer	Key Structures	Pathological Processes	Resulting Lesion Types
Epidermis	Keratinocytes, melanocytes, Langerhans cells, stratum corneum	Hyperkeratosis, spongiosis, acantholysis, dyskeratosis	Scale, vesicles, erosions, color change
Dermal-Epidermal Junction	Basement membrane zone, hemidesmosomes	Autoimmune attack, separation	Bullae (bullous pemphigoid, epidermolysis bullosa)
Dermis	Collagen, blood vessels, nerves, mast cells, fibroblasts	Inflammation, vasodilation, edema, cellular infiltration	Papules, plaques, wheals, nodules
Subcutis	Adipose tissue, deeper vessels	Panniculitis, deep infection	Deep nodules, indurated plaques
Blood Vessels	Arterioles, capillaries, venules	Vasodilation, increased permeability, vasculitis, thrombosis	Erythema, wheals, purpura, livedo

Major Pathophysiological Mechanisms

Type I Hypersensitivity (Immediate)

Mechanism: IgE-mediated mast cell degranulation

Mediators: Histamine, leukotrienes, prostaglandins

Time course: Minutes to hours

Clinical examples: Urticaria, angioedema, anaphylaxis

Type IV Hypersensitivity (Delayed)

Mechanism: T-cell mediated inflammation

Mediators: Cytokines (interferon-gamma, tumor necrosis factor)

Time course: 24-72 hours after exposure

Clinical examples: Contact dermatitis, drug reactions, tuberculin reaction

Autoimmune Mechanisms

Mechanism: Autoantibodies or autoreactive T-cells targeting skin components

Mediators: Autoantibodies, complement, cytotoxic T-cells

Time course: Chronic, relapsing

Clinical examples: Pemphigus, lupus, dermatomyositis

How Common Conditions Cause Rash

Condition	Pathophysiological Mechanism	Treatment Implication
Urticaria (hives)	Mast cell degranulation releases histamine, causing vasodilation and increased vascular permeability; fluid leaks into dermis creating wheals	Antihistamines block H1 receptors; severe cases may require epinephrine or corticosteroids
Atopic dermatitis (eczema)	Filaggrin gene mutations cause epidermal barrier dysfunction; Th2-dominant immune response leads to IgE elevation and eosinophilic inflammation	Emollients restore barrier; topical corticosteroids reduce inflammation; targeted biologics (dupilumab) block IL-4/IL-13
Psoriasis	Th17/Th1-mediated inflammation causes keratinocyte hyperproliferation (turnover reduced from 28 days to 3-4 days); IL-17 and IL-23 are key cytokines	Topical vitamin D analogues normalize keratinocyte differentiation; biologics target TNF-alpha, IL-17, or IL-23
Contact dermatitis (allergic)	Hapten binds to skin proteins; Langerhans cells present antigen to T-cells; sensitized T-cells release cytokines on re-exposure causing inflammation	Allergen avoidance is primary; topical corticosteroids suppress T-cell response
Viral exanthem	Direct viral cytopathic effect on keratinocytes and/or immune complex deposition; interferon response contributes to inflammation	Usually self-limiting; supportive care; specific antivirals for herpes viruses
Drug eruption (morbilliform)	Type IV hypersensitivity; drug or metabolite acts as hapten; T-cell mediated inflammation in dermis	Drug withdrawal; corticosteroids for severe reactions; identify and document culprit drug
Stevens-Johnson syndrome/Toxic epidermal necrolysis	Drug-induced cytotoxic T-cell and natural killer cell activation; keratinocyte apoptosis mediated by Fas-Fas ligand interaction and granulysin	Immediate drug cessation critical; supportive care in burn unit; IVIG or cyclosporine may be considered
Cellulitis	Bacterial invasion of dermis and subcutis (usually Streptococcus or Staphylococcus); inflammatory response causes erythema, edema, warmth, pain	Antibiotics targeting gram-positive organisms; elevation; address portal of entry
Tinea (dermatophyte infection)	Fungal invasion of stratum corneum; host inflammatory response to fungal antigens creates annular, scaling plaques with active advancing border	Topical antifungals for limited disease; oral antifungals for extensive or hair/nail involvement
Vasculitis (cutaneous)	Immune complex deposition in vessel walls; complement activation; neutrophil recruitment causes vessel wall damage and red blood cell extravasation	Identify and treat underlying cause; immunosuppression for systemic vasculitis; non-blanching purpura is the hallmark

Understanding Vascular Changes in Rash

The Blanching Test: A simple but critical clinical maneuver

Blanching erythema: Pressing the skin causes temporary whitening as blood is displaced from dilated vessels — indicates vasodilation (inflammation, infection)
Non-blanching erythema (purpura): Pressing does not change color — indicates extravasated blood (vasculitis, thrombocytopenia, trauma)

Non-blanching rashes require urgent evaluation as they may indicate serious conditions such as meningococcemia, vasculitis, or coagulopathy.

Vascular Change	Mechanism	Clinical Appearance	Differential Diagnosis
Vasodilation	Histamine, prostaglandins, nitric oxide	Blanching erythema	Urticaria, viral exanthem, drug reaction
Increased permeability	Histamine, bradykinin, VEGF	Edema, wheals	Urticaria, angioedema
Red blood cell extravasation	Vessel wall damage, platelet dysfunction	Non-blanching petechiae and purpura	Vasculitis, thrombocytopenia, trauma
Vessel occlusion	Thrombosis, emboli, cryoglobulins	Livedo reticularis, necrosis, ulceration	Antiphospholipid syndrome, calciphylaxis, cholesterol emboli

The Itch-Scratch Cycle

Pruritus (itching) is a cardinal symptom accompanying many rashes and significantly impacts quality of life. Understanding its mechanism helps explain both pathology and treatment approaches.

Pruritogenic Pathway

Peripheral mediators: Histamine, proteases, cytokines (IL-31), substance P

Sensory neurons: Unmyelinated C-fibers transmit itch signals via spinothalamic tract

Central processing: Thalamus and somatosensory cortex

Motor response: Scratching provides temporary relief but causes tissue damage

The Vicious Cycle

Inflammation → Itch → Scratching → More inflammation

Scratching causes mechanical damage to keratinocytes, releasing more inflammatory mediators

This leads to lichenification in chronic cases

Treatment implication: Breaking the itch-scratch cycle is as important as treating the underlying cause

Epidermal Barrier Dysfunction

The “Brick and Mortar” Model

The stratum corneum functions like a brick wall: corneocytes (keratinocytes that have lost their nuclei) are the “bricks,” and lipid lamellae (ceramides, cholesterol, fatty acids) are the “mortar.” Disruption of either component compromises barrier function.

Filaggrin deficiency (genetic or acquired): Bricks are defective — seen in atopic dermatitis
Lipid depletion (overwashing, low humidity): Mortar is missing — seen in xerosis and irritant dermatitis
Consequence: Increased transepidermal water loss (TEWL), allergen penetration, and susceptibility to infection

This is why emollients are first-line therapy for eczema — they restore the “mortar” and support barrier function.

Key Immune Cells in Cutaneous Inflammation

Cell Type	Location	Function	Associated Conditions
Langerhans cells	Epidermis	Antigen presentation to T-cells; initiate adaptive immune response	Contact dermatitis, Langerhans cell histiocytosis
Mast cells	Dermis (perivascular)	Release histamine and other mediators; immediate hypersensitivity	Urticaria, mastocytosis, anaphylaxis
T-helper cells (Th1)	Recruited to dermis	Interferon-gamma production; cell-mediated immunity	Psoriasis, contact dermatitis
T-helper cells (Th2)	Recruited to dermis	IL-4, IL-5, IL-13 production; promote IgE and eosinophils	Atopic dermatitis, drug reactions
T-helper cells (Th17)	Recruited to dermis	IL-17, IL-22 production; neutrophil recruitment	Psoriasis, pustular diseases
Neutrophils	Recruited to dermis/epidermis	Phagocytosis; release of proteases and reactive oxygen species	Pustular psoriasis, Sweet syndrome, bacterial infections
Eosinophils	Recruited to dermis	Release of cytotoxic granules; role in allergic inflammation	Drug reactions, bullous pemphigoid, eosinophilic dermatoses

Often Overlooked: The Koebner Phenomenon

The Koebner phenomenon (isomorphic response) is the development of skin lesions at sites of trauma in patients with certain dermatoses. It occurs because the pathological process is primed to occur wherever the skin is injured.

Classic examples: Psoriasis, vitiligo, lichen planus
Clinical significance: New lesions appearing in scratch marks, surgical scars, or areas of friction suggest these diagnoses
Patient education: Advise patients to avoid scratching or trauma to minimize new lesion development

3. History Taking

A comprehensive approach to eliciting the rash history

Red Flags — Require Urgent Evaluation

Rapidly spreading purpura or petechiae — Meningococcemia, vasculitis, disseminated intravascular coagulation
Mucosal involvement — Stevens-Johnson syndrome, toxic epidermal necrolysis, pemphigus
Skin pain out of proportion to appearance — Necrotizing fasciitis, Stevens-Johnson syndrome
Fever with widespread rash and systemic symptoms — Sepsis, drug reaction with eosinophilia and systemic symptoms (DRESS)

Blistering involving greater than 10% body surface area — Toxic epidermal necrolysis, staphylococcal scalded skin syndrome
Facial or airway swelling — Angioedema, anaphylaxis
Nikolsky sign positive (skin sloughs with gentle pressure) — Pemphigus, toxic epidermal necrolysis
Immunocompromised patient with new rash — Opportunistic infections, disseminated herpes or varicella zoster

Systematic History: The “RASHES” Approach

Use the mnemonic “RASHES” to ensure comprehensive history taking for any cutaneous eruption:

R — Rash characteristics: What does it look like? Is it itchy, painful, or asymptomatic? What was the first lesion like?
A — Anatomical distribution and spread: Where did it start? Where has it spread? Is it localized or generalized?
S — Systemic symptoms: Any fever, malaise, joint pain, sore throat, or other associated symptoms?
H — History of exposures: New medications, contacts, travel, occupational exposures, sick contacts, sexual history?
E — Evolution and timeline: When did it start? How has it changed? Is it getting better, worse, or staying the same?
S — Similar episodes and skin history: Has this happened before? Any history of eczema, psoriasis, allergies, or atopy?

Rash Characteristics (The “R”)

Question	Why It Matters	Diagnostic Clue
“What did the rash look like when it first appeared?”	Primary lesion morphology is key to diagnosis	Vesicles suggest viral or autoimmune blistering disease; wheals suggest urticaria
“Is it itchy, painful, burning, or none of these?”	Symptom quality narrows differential	Itch: eczema, urticaria, scabies. Pain: herpes zoster, cellulitis. Burning: contact dermatitis
“Does it come and go, or is it always there?”	Transient versus persistent lesions	Transient (less than 24 hours): urticaria. Persistent: most other dermatoses
“Have individual spots changed or have new ones appeared?”	Evolution pattern	Target lesions evolving: erythema multiforme. Spreading border: tinea

Targeted Questions by Suspected Cause

Suspected Cause	Key Features	Ask This Question
Drug eruption	Symmetric, morbilliform, onset 7-14 days after new drug	“Have you started any new medications in the past 2-6 weeks, including over-the-counter drugs, supplements, or herbal remedies?”
Contact dermatitis	Localized to contact area, geometric borders, vesicles	“Have you used any new soaps, detergents, lotions, cosmetics, or jewelry? Does the rash match an area that touched something specific?”
Viral exanthem	Prodromal symptoms, maculopapular, truncal predominance	“Did you have a sore throat, runny nose, fever, or feel unwell before the rash appeared? Has anyone around you been sick?”
Herpes zoster (shingles)	Dermatomal, unilateral, painful vesicles	“Did you have burning or tingling pain in that area before the rash appeared? Have you ever had chickenpox?”
Scabies	Intense nocturnal pruritus, web spaces, family members affected	“Is the itching worse at night? Does anyone else in your household have similar itching?”
Psoriasis	Well-demarcated plaques with silvery scale, extensor surfaces	“Have you ever had similar patches before? Do you have any joint pain or stiffness? Is there a family history of psoriasis?”
Atopic dermatitis (eczema)	Flexural distribution, chronic relapsing, intense itch	“Do you have a history of asthma, hay fever, or food allergies? Did you have eczema as a child?”
Urticaria (hives)	Transient wheals, individual lesions last less than 24 hours	“Do the spots disappear within 24 hours and new ones appear elsewhere? Can you identify any triggers like foods, stress, or temperature changes?”
Cellulitis	Unilateral, warm, tender, spreading erythema	“Did you have any cut, scratch, insect bite, or break in the skin before this started? Do you have diabetes or circulation problems?”
Tinea (fungal infection)	Annular with central clearing, scaling border	“Have you been in contact with animals, used shared gym equipment, or walked barefoot in public areas? Does anyone in your household have a similar rash?”
Secondary syphilis	Palms and soles involvement, generalized, non-pruritic	“Have you had any genital sores in the past few months? Have you had new sexual partners recently?”
Photosensitive eruption	Sun-exposed distribution, spares shaded areas	“Does the rash appear or worsen after sun exposure? Are you taking any new medications? Does it spare areas covered by clothing, watch straps, or under the chin?”

Medication and Social History

Medications That Commonly Cause Rash

Antibiotics — Penicillins, sulfonamides, cephalosporins (morbilliform eruptions, urticaria, fixed drug eruption)
Anticonvulsants — Phenytoin, carbamazepine, lamotrigine (DRESS syndrome, Stevens-Johnson syndrome)
Allopurinol — High risk for Stevens-Johnson syndrome and toxic epidermal necrolysis, especially with renal impairment
Nonsteroidal anti-inflammatory drugs — Urticaria, angioedema, fixed drug eruption
Angiotensin-converting enzyme inhibitors — Angioedema (can occur years after starting)
Chemotherapy agents — Various eruptions including acneiform rash (epidermal growth factor receptor inhibitors)
Immune checkpoint inhibitors — Morbilliform eruptions, vitiligo, lichenoid reactions

Social and Occupational History

Occupation: Healthcare workers (latex allergy, hand dermatitis), hairdressers (contact dermatitis), construction workers (cement dermatitis), food handlers (hand eczema)
Hobbies: Gardening (phytophotodermatitis, plant dermatitis), swimming (chlorine dermatitis, swimmer’s itch)
Travel: Endemic fungal infections, leishmaniasis, arthropod bites, viral exanthems
Pets and animals: Tinea from cats and dogs, scabies from close contacts
Sexual history: Secondary syphilis, HIV seroconversion illness, genital herpes
Living situation: Scabies and bedbugs in close living quarters, homeless shelters, nursing homes
Recent activities: Hot tub (Pseudomonas folliculitis), hiking (tick bites, poison ivy)

Timeline and Evolution

Onset Pattern	Typical Duration	Suggests
Minutes to hours	Resolves within 24-48 hours	Urticaria, angioedema, anaphylaxis
Hours to days	1-2 weeks	Viral exanthem, drug eruption, cellulitis
Days	2-6 weeks	Pityriasis rosea (herald patch first), contact dermatitis
Weeks to months	Chronic or relapsing	Psoriasis, eczema, lichen planus, chronic urticaria
Recurrent episodes	Episodic	Herpes simplex, fixed drug eruption, erythema multiforme

The Atopic Triad

Always ask about personal and family history of the “atopic triad”:

Atopic dermatitis (eczema)
Allergic rhinitis (hay fever)
Asthma

Patients with one atopic condition have increased risk of others and are more prone to contact allergies, drug reactions, and food allergies. A positive atopic history significantly influences your differential diagnosis and management approach.

4. Physical Examination

A systematic approach to examining the patient with rash

Systematic Framework: Use the “ABCDE” approach for complete dermatological examination:

A — Asymmetry and Arrangement: Is the rash symmetric or asymmetric? Linear, grouped, annular, or scattered?
B — Border and Body site: What are the margins like? Where on the body is it located?
C — Color: What is the predominant color? Does it blanch?
D — Diameter and Distribution: How big are individual lesions? What is the overall distribution pattern?
E — Evolution and Extras: How has it changed? Check nails, hair, mucous membranes

General Inspection

Overall appearance: Well or unwell? Febrile? Signs of systemic illness?
Skin extent: Estimate percentage of body surface area involved (palm of patient’s hand ≈ 1% body surface area)
Distribution pattern: Generalized, localized, dermatomal, photodistributed, flexural, extensor?
Symmetry: Symmetric suggests endogenous cause; asymmetric suggests exogenous cause
Configuration: Linear (contact, Koebner), annular (tinea, granuloma annulare), grouped (herpes), reticular (livedo)

Vital Signs

Vital Sign	What to Look For	Clinical Significance
Temperature	Fever greater than 38°C (100.4°F)	Suggests infection, drug reaction with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome, or systemic vasculitis
Heart Rate	Tachycardia	May indicate sepsis, anaphylaxis, pain, or systemic inflammatory response
Blood Pressure	Hypotension	Concerning for sepsis or anaphylaxis; urgent intervention required
Respiratory Rate	Tachypnea	May suggest anaphylaxis with respiratory involvement or sepsis
Oxygen Saturation	Hypoxia	Concerning for anaphylaxis, severe sepsis, or pulmonary involvement in systemic disease

Primary Lesion Assessment

Careful identification of the primary lesion is the most important step in dermatological diagnosis. Examine areas with fresh, unmodified lesions.

Assessment	Technique	What It Tells You
Palpation	Gently run fingers over lesion	Flat (macule/patch) vs raised (papule/plaque/nodule); texture (smooth, rough, scaly)
Diascopy	Press glass slide firmly against lesion	Blanching = vascular (erythema); Non-blanching = extravasated blood (purpura) or pigment
Dermoscopy	Handheld magnifying device with light source	Detailed structures: burrows (scabies), scale pattern, vascular patterns, pigment distribution
Wood’s lamp	Ultraviolet light examination in dark room	Coral red = erythrasma; Blue-green = Pseudomonas; Enhanced depigmentation = vitiligo
Nikolsky sign	Gentle lateral pressure on skin	Positive (skin shears off) = pemphigus, toxic epidermal necrolysis, staphylococcal scalded skin
Auspitz sign	Remove scale from plaque	Pinpoint bleeding = psoriasis (dilated capillaries in dermal papillae)
Darier sign	Stroke lesion firmly	Urtication (wheal formation) = mastocytosis

Mucosal Membranes and Skin Appendages

Oral Mucosa

Examine: Buccal mucosa, tongue, palate, gingiva, lips

Look for: Erosions, ulcers, white patches, vesicles

Significance: Mucosal involvement suggests Stevens-Johnson syndrome, pemphigus, lichen planus, or erythema multiforme major

Nails

Examine: All fingernails and toenails

Look for: Pitting, onycholysis, oil spots, subungual hyperkeratosis, dystrophy

Significance: Nail changes support psoriasis, lichen planus, fungal infection, or connective tissue disease

Hair and Scalp

Examine: Scalp, eyebrows, body hair

Look for: Scale, erythema, hair loss, broken hairs, excoriation

Significance: Scalp involvement in psoriasis, seborrheic dermatitis, tinea capitis, discoid lupus

Don’t Forget Hidden Areas

A complete skin examination should include areas patients may not show voluntarily:

Scalp — Often affected in psoriasis, seborrheic dermatitis, tinea
Ear canals and behind ears — Psoriasis, seborrheic dermatitis
Umbilicus — Psoriasis predilection site
Natal cleft — Psoriasis, intertrigo
Genitalia — Lichen sclerosus, psoriasis, sexually transmitted infections, fixed drug eruption
Palms and soles — Secondary syphilis, hand-foot-and-mouth disease, pustular psoriasis, dyshidrotic eczema
Web spaces of fingers and toes — Scabies, tinea pedis

Lymph Node Examination

Region	Drains	Associated Conditions
Cervical	Head, neck, oropharynx	Viral exanthems, cellulitis of face/scalp, lymphoma, metastatic disease
Axillary	Upper limb, chest wall, breast	Cellulitis of arm, cat-scratch disease, melanoma staging
Inguinal	Lower limb, genitalia, perineum	Cellulitis of leg, sexually transmitted infections, genital herpes
Generalized lymphadenopathy	Multiple regions	Viral infections, drug reactions (DRESS), lymphoma, HIV, secondary syphilis

Special Signs and Maneuvers

Sign	How to Perform	Positive Finding	Indicates
Nikolsky sign	Apply lateral shearing pressure to normal-appearing skin adjacent to a blister	Epidermis separates from dermis	Pemphigus vulgaris, toxic epidermal necrolysis, staphylococcal scalded skin syndrome
Asboe-Hansen sign	Apply pressure to the top of an intact blister	Blister spreads laterally	Pemphigus vulgaris, bullous pemphigoid
Auspitz sign	Gently scrape silvery scale from a plaque	Pinpoint bleeding spots appear	Psoriasis
Koebner phenomenon	Observe for lesions in areas of trauma (scratch marks, surgical scars)	New lesions develop at sites of skin injury	Psoriasis, vitiligo, lichen planus
Dermatographism	Firmly stroke normal skin with a tongue depressor	Linear wheal develops within minutes	Physical urticaria, atopic diathesis
Burrow ink test	Apply ink to suspected area, wipe off with alcohol	Ink retained in serpiginous track	Scabies

Expected Findings by Etiology

Condition	Primary Lesion	Distribution	Key Examination Finding
Atopic dermatitis	Poorly defined erythematous patches and plaques with scale	Flexural (antecubital, popliteal fossae), face in children	Lichenification, excoriations, xerosis, infraorbital folds (Dennie-Morgan lines)
Psoriasis	Well-demarcated erythematous plaques with silvery scale	Extensor surfaces (elbows, knees), scalp, sacrum	Auspitz sign positive, nail pitting, onycholysis
Contact dermatitis	Vesicles, papules, or plaques with erythema	Geometric or linear pattern matching contact	Sharp borders, confined to contact area, may see allergic ID reaction elsewhere
Urticaria	Wheals (raised, erythematous, edematous plaques)	Anywhere; may be generalized	Individual lesions resolve in less than 24 hours, dermatographism may be present
Cellulitis	Ill-defined erythema with warmth and swelling	Unilateral; lower leg most common	Tender, warm, portal of entry may be visible, lymphangitic streaking, regional adenopathy
Herpes zoster	Grouped vesicles on erythematous base	Dermatomal, unilateral, does not cross midline	Pain often precedes rash, may see crusting in older lesions
Tinea corporis	Annular scaly patch with raised border	Trunk, extremities; asymmetric	Central clearing, active scaly edge, may see satellite lesions
Scabies	Papules, vesicles, burrows	Web spaces, wrists, waistband, genitalia	Burrows (pathognomonic), excoriations, nodules on genitalia, spares head in adults
Drug eruption (morbilliform)	Symmetric maculopapular eruption	Starts on trunk, spreads to extremities; usually spares face	May have mild pruritus, mucosal sparing differentiates from Stevens-Johnson syndrome
Pityriasis rosea	Oval salmon-colored patches with collarette scale	“Christmas tree” pattern on back following skin lines	Herald patch precedes generalized eruption by 1-2 weeks

Examination Tips for Rash Assessment

Good lighting is essential: Natural daylight is ideal; fluorescent lighting can alter color perception
Expose the skin fully: A partial examination leads to missed diagnoses; examine the entire skin surface when possible
Find fresh lesions: Primary morphology is best assessed on new, unmodified lesions
Always do diascopy: The blanching test takes seconds and has critical diagnostic importance
Photograph the rash: Lesions evolve; documentation allows comparison and specialist consultation
Consider systemic examination: Joint examination for psoriasis, respiratory exam for sarcoidosis, abdominal exam for livedo

When to Perform Systemic Examination

System	Examine If	Looking For
Musculoskeletal	Psoriasis, lupus, dermatomyositis, reactive arthritis	Joint swelling, tenderness, limited range of motion, dactylitis
Respiratory	Sarcoidosis, connective tissue disease, anaphylaxis	Wheeze, stridor, crackles, reduced breath sounds
Cardiovascular	Vasculitis, endocarditis, Kawasaki disease	Murmurs, peripheral pulses, blood pressure in all limbs
Abdominal	Henoch-Schönlein purpura, liver disease, inflammatory bowel disease	Hepatosplenomegaly, tenderness, signs of portal hypertension
Neurological	Herpes zoster, neurofibromatosis, tuberous sclerosis	Sensory changes in dermatomal distribution, café-au-lait spots, ash leaf macules

5. Differential Diagnosis

Systematic approach organized by probability, morphology, and clinical features

The differential diagnosis for rash is vast, encompassing hundreds of conditions. A systematic approach using morphology, distribution, and clinical context allows efficient narrowing of possibilities. This section organizes differentials by presentation acuity and probability to guide clinical reasoning.

Acute Rash (Duration: Less than 2 weeks)

Probability	Condition	Key Features	Red Flags
COMMON (approximately 70%)	Viral exanthem	Maculopapular, truncal predominance, prodromal symptoms, self-limiting	High fever, petechiae, mucosal involvement
COMMON	Urticaria (acute)	Transient wheals lasting less than 24 hours, intensely pruritic, dermographism	Angioedema, respiratory symptoms, hypotension
COMMON	Contact dermatitis (irritant or allergic)	Localized to contact area, geometric borders, vesicles or erythema	Widespread involvement, systemic symptoms
COMMON	Cellulitis	Unilateral, warm, tender, spreading erythema with ill-defined borders	Rapid spread, crepitus, bullae, severe pain, systemic toxicity
COMMON	Insect bites and stings	Papules or wheals at bite sites, grouped, exposed areas	Anaphylaxis, extensive local reaction, signs of infection
LESS COMMON (approximately 20%)	Drug eruption (morbilliform)	Symmetric maculopapular rash, onset 7-14 days after drug initiation	Mucosal involvement, facial edema, fever, eosinophilia
LESS COMMON	Herpes zoster (shingles)	Dermatomal, unilateral, grouped vesicles on erythematous base, pain	Ophthalmic involvement (V1), disseminated in immunocompromised
LESS COMMON	Herpes simplex	Grouped vesicles on erythematous base, recurrent at same site	Eczema herpeticum, encephalitis symptoms, neonatal exposure
LESS COMMON	Erythema multiforme	Target lesions with three zones, acral distribution, often follows herpes simplex	Mucosal involvement (erythema multiforme major), widespread blistering
UNCOMMON BUT SERIOUS (approximately 10%)	Stevens-Johnson syndrome / Toxic epidermal necrolysis	Mucosal erosions, skin pain, dusky erythema progressing to blistering, Nikolsky positive	Greater than 10% body surface area detachment, respiratory involvement
UNCOMMON BUT SERIOUS	Meningococcemia	Petechiae and purpura, fever, rapidly progressive, ill-appearing	Hypotension, altered mental status, rapidly spreading purpura
UNCOMMON BUT SERIOUS	Necrotizing fasciitis	Pain out of proportion to examination, rapidly spreading, dusky discoloration	Crepitus, bullae, systemic toxicity, wooden-hard induration
UNCOMMON BUT SERIOUS	Drug reaction with eosinophilia and systemic symptoms (DRESS)	Facial edema, fever, lymphadenopathy, internal organ involvement, eosinophilia	Hepatitis, nephritis, pneumonitis, myocarditis

Chronic Rash (Duration: Greater than 6 weeks)

Step-by-Step Approach to Chronic Rash:

Step 1: Identify the primary lesion morphology — Is it papulosquamous, eczematous, vesiculobullous, or other?
Step 2: Assess distribution pattern — Is it flexural, extensor, photodistributed, or generalized?
Step 3: Consider the “Big Five” chronic rashes — Eczema, psoriasis, tinea, contact dermatitis, and drug reaction
Step 4: Look for systemic clues — Nail changes, mucosal involvement, joint symptoms, organ involvement

Probability	Condition	Approximate Frequency	Key Distinguishing Features
COMMON	Atopic dermatitis (eczema)	15-20% of children; 2-10% of adults	Flexural distribution, intense pruritus, personal or family history of atopy, chronic relapsing course
COMMON	Psoriasis	2-3% of population	Well-demarcated plaques with silvery scale, extensor surfaces, nail changes, Auspitz sign
COMMON	Seborrheic dermatitis	3-5% of population	Greasy yellowish scale, nasolabial folds, scalp, eyebrows, central face
COMMON	Tinea (dermatophyte infections)	10-20% of population	Annular with central clearing, active scaly border, asymmetric, potassium hydroxide positive
COMMON	Chronic urticaria	0.5-1% of population	Wheals lasting less than 24 hours, recurring for greater than 6 weeks, often no identifiable cause
LESS COMMON	Lichen planus	0.5-1% of population	Purple, polygonal, planar papules with Wickham striae; oral involvement common
LESS COMMON	Pityriasis rosea	Common in young adults	Herald patch followed by “Christmas tree” distribution, collarette scale, self-limiting
LESS COMMON	Rosacea	2-10% of fair-skinned populations	Central facial erythema, telangiectasia, papulopustules, flushing triggers
LESS COMMON	Acne vulgaris	85% of adolescents	Comedones, papules, pustules, nodules on face, chest, back; sebaceous distribution
UNCOMMON	Cutaneous lupus erythematosus	Variable	Malar rash, photosensitivity, discoid lesions with scarring, systemic features
UNCOMMON	Dermatomyositis	Rare	Heliotrope rash (eyelids), Gottron papules (knuckles), proximal muscle weakness
UNCOMMON	Cutaneous T-cell lymphoma (mycosis fungoides)	Rare	Persistent patches and plaques in non-sun-exposed areas, poikiloderma, pruritus

Morphology-Based Differential

Papulosquamous (Scaly Papules/Plaques)

Psoriasis

Lichen planus

Pityriasis rosea

Secondary syphilis

Tinea corporis

Seborrheic dermatitis

Cutaneous T-cell lymphoma

Eczematous (Erythema, Scale, Vesicles)

Atopic dermatitis

Contact dermatitis (allergic/irritant)

Nummular eczema

Stasis dermatitis

Dyshidrotic eczema

Asteatotic eczema

Seborrheic dermatitis

Vesiculobullous (Blisters)

Herpes simplex and zoster

Bullous pemphigoid

Pemphigus vulgaris

Dermatitis herpetiformis

Stevens-Johnson syndrome / Toxic epidermal necrolysis

Bullous impetigo

Contact dermatitis (severe)

Purpuric (Non-Blanching)

Vasculitis (leukocytoclastic, IgA)

Thrombocytopenia

Meningococcemia

Disseminated intravascular coagulation

Senile purpura

Trauma

Pigmented purpuric dermatosis

Distribution-Based Differential

Distribution Pattern	Think Of	Key Discriminators
Flexural (antecubital, popliteal, neck)	Atopic dermatitis, inverse psoriasis, intertrigo, candidiasis	Atopy history, satellite pustules (candida), lack of scale (inverse psoriasis)
Extensor (elbows, knees)	Psoriasis, dermatitis herpetiformis, granuloma annulare	Silvery scale (psoriasis), grouped vesicles (dermatitis herpetiformis)
Photodistributed (face, V-neck, dorsal hands)	Lupus, polymorphous light eruption, drug photosensitivity, dermatomyositis	Sparing of shaded areas (under chin, behind ears), medication history
Dermatomal (unilateral, band-like)	Herpes zoster, zosteriform metastases (rare)	Pain preceding rash, grouped vesicles, does not cross midline
Palms and soles	Secondary syphilis, hand-foot-and-mouth disease, pustular psoriasis, dyshidrotic eczema, erythema multiforme	Sexual history (syphilis), systemic symptoms, nail involvement
Scalp	Psoriasis, seborrheic dermatitis, tinea capitis, discoid lupus	Scale character, hair loss, scarring (discoid lupus)
Intertriginous (skin folds)	Candidiasis, intertrigo, inverse psoriasis, erythrasma	Satellite pustules (candida), coral-red fluorescence (erythrasma)

Drug-Induced Rash

Eruption Type	Common Culprit Drugs	Characteristics	Time to Onset
Morbilliform (exanthematous)	Penicillins, sulfonamides, anticonvulsants, allopurinol	Symmetric maculopapular eruption starting on trunk, may become confluent	7-14 days (first exposure); 1-3 days (re-exposure)
Urticaria / Angioedema	Penicillins, nonsteroidal anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, contrast media	Wheals, may have angioedema; can be IgE-mediated or direct mast cell activation	Minutes to hours (immediate); days to years for angiotensin-converting enzyme inhibitor angioedema
Fixed drug eruption	Sulfonamides, nonsteroidal anti-inflammatory drugs, tetracyclines, paracetamol	Round, dusky red-violaceous patch; recurs at same site with re-exposure	30 minutes to 8 hours on re-exposure
Stevens-Johnson syndrome / Toxic epidermal necrolysis	Sulfonamides, anticonvulsants (carbamazepine, phenytoin, lamotrigine), allopurinol, nonsteroidal anti-inflammatory drugs	Mucosal erosions, targetoid lesions, skin pain, epidermal detachment	1-3 weeks (first exposure)
Drug reaction with eosinophilia and systemic symptoms (DRESS)	Anticonvulsants, allopurinol, sulfonamides, vancomycin, minocycline	Facial edema, fever, lymphadenopathy, eosinophilia, hepatitis or other organ involvement	2-8 weeks
Photosensitivity	Tetracyclines (doxycycline), fluoroquinolones, thiazides, amiodarone, nonsteroidal anti-inflammatory drugs	Exaggerated sunburn or eczematous eruption in sun-exposed areas	Hours to days after sun exposure while on drug
Lichenoid eruption	Angiotensin-converting enzyme inhibitors, thiazides, antimalarials, beta-blockers, gold	Lichen planus-like: purple, polygonal papules; may have oral involvement	Months to years
Acute generalized exanthematous pustulosis	Antibiotics (aminopenicillins, macrolides), calcium channel blockers	Fever, widespread sterile pustules on erythematous background, neutrophilia	1-5 days
Acneiform eruption	Corticosteroids, epidermal growth factor receptor inhibitors, lithium, isoniazid	Monomorphic papulopustules without comedones; sudden onset	Days to weeks

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

Clinical Clue	Think This First	Next Step
Herald patch followed by “Christmas tree” pattern	Pityriasis rosea	Reassurance; self-limiting in 6-8 weeks; consider rapid plasma reagin if sexually active
Well-demarcated plaques with silvery scale on elbows and knees	Psoriasis	Check nails, scalp, natal cleft; ask about joint symptoms
Intensely pruritic papules in web spaces, wrists, genitalia	Scabies	Look for burrows; examine close contacts; skin scraping
Annular lesion with central clearing and active scaly border	Tinea corporis	Potassium hydroxide preparation; ask about animal contact
Grouped vesicles on erythematous base in dermatomal distribution	Herpes zoster	Start antivirals within 72 hours; check for ophthalmic involvement if facial
Targetoid lesions with three zones, acral distribution	Erythema multiforme	Check for herpes simplex trigger; examine mucosae
Non-blanching petechiae and purpura with fever	Meningococcemia or vasculitis	Urgent sepsis workup; blood cultures; consider lumbar puncture
Mucosal erosions with skin pain and epidermal detachment	Stevens-Johnson syndrome / Toxic epidermal necrolysis	Stop all suspect drugs immediately; urgent dermatology and possible burn unit transfer
Facial edema with fever, lymphadenopathy, and rash 2-6 weeks after new drug	Drug reaction with eosinophilia and systemic symptoms (DRESS)	Stop drug; check liver function tests, renal function, complete blood count with differential
Rash on palms and soles in sexually active patient	Secondary syphilis	Rapid plasma reagin or venereal disease research laboratory test; full sexually transmitted infection screen
Unilateral warm, tender, spreading erythema on lower leg	Cellulitis	Mark borders; look for portal of entry; consider risk factors for unusual organisms
Purple polygonal papules with lacy white pattern on surface	Lichen planus	Examine oral mucosa, nails, genitalia; consider hepatitis C testing

6. Diagnostic Investigations

A stepwise, cost-effective approach guided by clinical suspicion

Many rashes can be diagnosed clinically without investigations. However, when the diagnosis is uncertain, when systemic disease is suspected, or when treatment decisions require confirmation, targeted investigations become essential. This section outlines a rational approach to diagnostic testing.

Key Principle: The majority of rashes encountered in primary care can be diagnosed by history and physical examination alone. Investigations should be guided by clinical suspicion, not performed routinely.

When to Investigate

Clinical Scenario	Rationale for Investigation	Initial Tests to Consider
Uncertain diagnosis after clinical assessment	Confirm diagnosis before initiating treatment	Skin biopsy, potassium hydroxide preparation, culture
Suspected systemic disease manifesting in skin	Identify underlying condition requiring specific treatment	Complete blood count, metabolic panel, autoantibodies, imaging
Rash unresponsive to appropriate treatment	Reconsider diagnosis; rule out secondary infection or malignancy	Skin biopsy, culture, patch testing
Red flags present	Exclude serious or life-threatening conditions	Urgent blood work, blood cultures, skin biopsy
Chronic urticaria (greater than 6 weeks)	Identify underlying trigger (found in less than 10% of cases)	Complete blood count, thyroid function, consider limited panel

Baseline Investigations When Indicated

Investigation	Purpose	What to Look For	Practical Points
Complete blood count with differential	Assess for infection, eosinophilia, hematologic abnormalities	Eosinophilia (drug reaction, parasites); leukocytosis (infection); thrombocytopenia (purpura)	Essential if systemic symptoms present or drug reaction suspected
Comprehensive metabolic panel	Renal and hepatic function, electrolytes	Elevated liver enzymes (drug reaction with eosinophilia and systemic symptoms, hepatitis); renal impairment	Important for drug reactions and systemic vasculitis
Erythrocyte sedimentation rate and C-reactive protein	Markers of inflammation	Elevated in vasculitis, connective tissue disease, infection	Non-specific; useful for monitoring disease activity
Urinalysis	Screen for renal involvement	Proteinuria, hematuria (vasculitis, lupus, Henoch-Schönlein purpura)	Simple screening test for systemic involvement

Bedside and Office-Based Tests

Test	Technique	Indication	Interpretation
Potassium hydroxide (KOH) preparation	Scrape scale onto slide, add 10-20% potassium hydroxide, apply heat, examine under microscope	Suspected fungal infection (tinea, candidiasis)	Positive: branching hyphae (dermatophytes) or pseudohyphae and budding yeast (Candida)
Tzanck smear	Unroof vesicle, scrape base, stain with Giemsa or Wright stain	Suspected herpes simplex or varicella zoster	Positive: multinucleated giant cells (does not distinguish herpes simplex from varicella zoster)
Wood’s lamp examination	Examine skin in dark room under ultraviolet A light (365 nm)	Pigmentary disorders, fungal infections, bacterial infections	Coral-red: erythrasma; blue-green: Pseudomonas; bright white: vitiligo; yellow-green: some tinea capitis
Dermoscopy	Handheld magnification with polarized or non-polarized light	Pigmented lesions, scabies, vascular lesions	Enhances visualization of structures not visible to naked eye; scabies shows triangular “delta wing” mite
Diascopy	Press glass slide against lesion	Distinguish vascular dilation from extravasated blood	Blanching: erythema (vascular); Non-blanching: purpura or pigment
Skin scraping for scabies	Apply mineral oil to burrow, scrape with blade, examine under microscope	Suspected scabies	Positive: mites, eggs, or fecal pellets (scybala) visible

Targeted Investigations by Suspected Etiology

If Suspecting Autoimmune or Connective Tissue Disease

First-Line Tests

Antinuclear antibody (ANA): Screening for lupus, dermatomyositis; positive in greater than 95% of systemic lupus erythematosus
Anti-double-stranded DNA: Specific for systemic lupus erythematosus; correlates with disease activity
Complete blood count: Cytopenias in lupus; eosinophilia in eosinophilic conditions
Complement levels (C3, C4): Low in active lupus

Second-Line Tests

Extractable nuclear antigens (anti-Ro, anti-La, anti-Smith, anti-ribonucleoprotein): Define specific connective tissue disease subtype
Creatine kinase and aldolase: Elevated in dermatomyositis with muscle involvement
Myositis-specific antibodies: Anti-Mi-2, anti-Jo-1 for dermatomyositis subtyping
Skin biopsy with direct immunofluorescence: Gold standard for bullous diseases and lupus

If Suspecting Vasculitis

First-Line Tests

Complete blood count, metabolic panel, urinalysis: Assess systemic involvement
Erythrocyte sedimentation rate and C-reactive protein: Inflammatory markers
Skin biopsy: Histology shows leukocytoclastic vasculitis; direct immunofluorescence for IgA (Henoch-Schönlein purpura)

Second-Line Tests

Antineutrophil cytoplasmic antibodies (ANCA): c-ANCA for granulomatosis with polyangiitis; p-ANCA for microscopic polyangiitis
Hepatitis B and C serology: Associated with polyarteritis nodosa and cryoglobulinemia
Cryoglobulins: If livedo, purpura, or ulceration present
Imaging: Chest x-ray, CT angiography as indicated for systemic involvement

If Suspecting Drug Eruption

For Morbilliform Eruption

Timeline review: Most important diagnostic tool
Complete blood count with differential: Eosinophilia supports drug etiology
Usually clinical diagnosis: Biopsy rarely needed unless diagnosis uncertain

For Severe Drug Reactions (DRESS, Stevens-Johnson syndrome, Toxic epidermal necrolysis)

Complete blood count: Eosinophilia, atypical lymphocytes (DRESS)
Liver function tests: Hepatitis in DRESS
Renal function: Nephritis in DRESS
Skin biopsy: Confirms diagnosis; shows necrotic keratinocytes

If Suspecting Infection

Suspected Infection	Diagnostic Tests	Expected Findings
Bacterial (cellulitis, impetigo)	Usually clinical diagnosis; wound culture if purulent; blood cultures if systemic	Growth of Staphylococcus aureus or Streptococcus pyogenes
Fungal (tinea, candidiasis)	Potassium hydroxide preparation; fungal culture (takes 2-4 weeks)	Hyphae or yeast on microscopy; species identification on culture
Viral (herpes simplex, varicella zoster)	Polymerase chain reaction of vesicle fluid (gold standard); viral culture; Tzanck smear (rapid but less sensitive)	Polymerase chain reaction positive; giant cells on Tzanck
Syphilis	Rapid plasma reagin or venereal disease research laboratory (screening); Treponema pallidum particle agglutination or fluorescent treponemal antibody absorption (confirmatory)	Positive serology; dark-field microscopy of chancre if primary
Scabies	Skin scraping with mineral oil; dermoscopy	Mites, eggs, or fecal pellets on microscopy; “delta wing” sign on dermoscopy

Skin Biopsy

When to Biopsy

Diagnosis uncertain after clinical assessment
Suspected malignancy
Suspected autoimmune blistering disease
Rash unresponsive to appropriate treatment
Need to exclude specific diagnoses (vasculitis, cutaneous T-cell lymphoma)
Documentation required before systemic therapy

Biopsy Type	Technique	Best For
Punch biopsy (3-4 mm)	Cylindrical sample through epidermis and dermis	Most inflammatory dermatoses; standard technique for rash diagnosis
Shave biopsy	Tangential removal of superficial skin	Epidermal lesions, suspected superficial skin cancers (not for melanoma)
Excisional biopsy	Complete removal of lesion with margin	Suspected melanoma, small nodules
Incisional biopsy	Partial removal from large lesion	Large tumors, panniculitis, deep nodules

Biopsy Tips

Biopsy an established lesion: Very early or very late lesions may be non-diagnostic
Avoid modified lesions: Do not biopsy excoriated, infected, or treated areas
Include lesional and perilesional skin: Essential for bullous diseases
Request direct immunofluorescence: If bullous disease or lupus suspected (requires special transport medium)
Provide clinical information: Pathologists need clinical context for accurate interpretation

Patch Testing for Contact Dermatitis

Indications

Suspected allergic contact dermatitis
Chronic eczema unresponsive to treatment
Occupational dermatitis
Eyelid or hand dermatitis

Practical Points

Usually performed by dermatologist or allergist
Standard series tests common allergens (nickel, fragrance, preservatives, rubber chemicals)
Patient must avoid topical steroids on back for 1 week before test
Readings at 48 and 96 hours after application

Empiric Treatment Trials as Diagnostic Tools

Therapeutic Trials in Dermatology

When diagnosis is uncertain and conditions are low-risk, response to empiric therapy can support the diagnosis:

Suspected tinea: Trial of topical antifungal for 2-4 weeks — improvement supports fungal diagnosis
Suspected scabies: Treat patient and close contacts with permethrin — resolution in 2-4 weeks supports diagnosis
Suspected eczema: Trial of emollients and topical corticosteroid — improvement supports eczema diagnosis
Suspected drug eruption: Withdraw suspected drug — improvement within days to weeks supports drug causation

Caution: Do not use empiric trials when diagnosis of serious condition is possible or when delay could cause harm.

7. Pattern Recognition and Clinical Decision-Making

Practical algorithms and decision pathways for rash evaluation

Efficient rash diagnosis requires systematic thinking combined with pattern recognition. This section provides practical algorithms to guide clinical decision-making from initial triage through definitive management.

Step 1: Is This Urgent?

Clinical Scenario	Urgency Level	Immediate Action
Rapidly spreading petechiae or purpura with fever	EMERGENT	IV access, blood cultures, empiric antibiotics for meningococcemia; do not delay for lumbar puncture
Urticaria with angioedema, stridor, or hypotension	EMERGENT	Epinephrine intramuscular, airway management, IV fluids, call for help
Mucosal erosions with skin pain and blistering (suspected Stevens-Johnson syndrome or toxic epidermal necrolysis)	EMERGENT	Stop all suspect drugs immediately; urgent dermatology consultation; consider burn unit transfer
Severe pain out of proportion to skin findings with systemic toxicity	EMERGENT	Consider necrotizing fasciitis; surgical consultation; broad-spectrum antibiotics; imaging if stable
Widespread erythema with fever, facial edema, and lymphadenopathy	URGENT	Consider drug reaction with eosinophilia and systemic symptoms (DRESS); stop suspect drugs; check liver function tests, renal function, complete blood count
Herpes zoster involving ophthalmic division (forehead, nose tip)	URGENT	Ophthalmology referral same day; start antivirals within 72 hours of rash onset
Rapidly spreading unilateral erythema with fever (cellulitis)	URGENT	Mark borders; assess for abscess; determine if oral or intravenous antibiotics needed; consider admission criteria
Chronic stable rash without systemic symptoms	ROUTINE	Systematic evaluation; can schedule follow-up; consider dermatology referral if uncertain
Localized pruritic rash without red flags	ROUTINE	Clinical diagnosis; empiric treatment; follow-up if no improvement

Step 2: Identify the Primary Lesion

The Single Most Important Step: Correctly identifying the primary lesion morphology immediately narrows your differential from hundreds of conditions to a manageable list.

Macules and Patches

Flat, color change only

→ Proceed to Algorithm A

Think: viral exanthem, drug eruption, vitiligo, tinea versicolor

Papules and Plaques

Raised, solid lesions

→ Proceed to Algorithm B

Think: psoriasis, eczema, lichen planus, contact dermatitis

Vesicles and Bullae

Fluid-filled blisters

→ Proceed to Algorithm C

Think: herpes, bullous pemphigoid, contact dermatitis, Stevens-Johnson syndrome

Wheals

Transient, edematous plaques

→ Proceed to Algorithm D

Think: urticaria, angioedema, urticarial vasculitis

Pustules

Pus-filled lesions

→ Proceed to Algorithm E

Think: folliculitis, acne, pustular psoriasis, rosacea

Purpura

Non-blanching red-purple

→ Proceed to Algorithm F

Think: vasculitis, thrombocytopenia, meningococcemia

Step 3: Follow the Appropriate Algorithm

Algorithm A: Maculopapular Rash

Clinical Scenario	Most Likely Diagnosis	Action
Acute onset, prodromal symptoms, truncal predominance, self-limiting	Viral exanthem	Supportive care; reassurance; isolation if contagious
New medication in past 2-6 weeks, symmetric, pruritic	Drug eruption (morbilliform)	Stop suspect drug; antihistamines; monitor for progression
Herald patch followed by “Christmas tree” pattern, collarette scale	Pityriasis rosea	Reassurance; symptomatic treatment; resolves in 6-8 weeks
Palms and soles involvement, sexually active, generalized	Secondary syphilis	Rapid plasma reagin and confirmatory test; treat with penicillin; partner notification

Algorithm B: Papulosquamous Rash

Clinical Scenario	Most Likely Diagnosis	Action
Well-demarcated plaques, silvery scale, extensor surfaces, nail pitting	Psoriasis	Topical therapy; assess for psoriatic arthritis; consider systemic therapy if extensive
Poorly demarcated, flexural, intense itch, atopic history	Atopic dermatitis	Emollients, topical corticosteroids, trigger avoidance
Annular with central clearing, scaly advancing border	Tinea corporis	Potassium hydroxide to confirm; topical antifungal; oral if extensive
Purple, polygonal papules with Wickham striae, oral involvement	Lichen planus	Topical corticosteroids; consider hepatitis C testing; biopsy if uncertain
Localized to contact area, geometric borders, vesicles may be present	Contact dermatitis	Identify and avoid allergen; topical corticosteroids; patch testing if recurrent

Algorithm C: Vesiculobullous Rash

Clinical Scenario	Most Likely Diagnosis	Action
Grouped vesicles on erythematous base, dermatomal, unilateral, painful	Herpes zoster	Antivirals within 72 hours; pain management; check for ophthalmic involvement
Grouped vesicles, recurrent at same site, prodromal tingling	Herpes simplex	Antivirals; episodic or suppressive therapy; counseling
Tense bullae on normal or erythematous skin, elderly patient, pruritic	Bullous pemphigoid	Biopsy with direct immunofluorescence; topical or systemic corticosteroids
Flaccid bullae, mucosal erosions, Nikolsky sign positive	Pemphigus vulgaris	Urgent dermatology referral; biopsy with direct immunofluorescence; systemic immunosuppression
Mucosal erosions, skin pain, dusky erythema, recent new medication	Stevens-Johnson syndrome / Toxic epidermal necrolysis	Stop all suspect drugs immediately; supportive care; consider burn unit

Algorithm D: Urticarial Rash

Clinical Scenario	Most Likely Diagnosis	Action
Individual wheals lasting less than 24 hours, duration less than 6 weeks	Acute urticaria	Antihistamines; identify and avoid trigger if possible; epinephrine if anaphylaxis
Individual wheals lasting less than 24 hours, duration greater than 6 weeks	Chronic spontaneous urticaria	Second-generation antihistamines (up to 4x dose); consider omalizumab if refractory
Individual wheals lasting greater than 24 hours, painful more than pruritic, leaves bruising	Urticarial vasculitis	Skin biopsy; investigate for underlying cause; may need systemic therapy
Wheals with systemic symptoms (fever, arthralgias, elevated inflammatory markers)	Urticarial vasculitis or systemic disease	Full workup including complement levels, autoantibodies; biopsy

Algorithm E: Purpuric Rash

Clinical Scenario	Most Likely Diagnosis	Action
Petechiae and purpura with fever, rapidly progressive, ill-appearing	Meningococcemia	Immediate IV antibiotics; do not delay for investigations; intensive care
Palpable purpura on lower extremities, may have arthralgias, abdominal pain	IgA vasculitis (Henoch-Schönlein purpura) or leukocytoclastic vasculitis	Skin biopsy; urinalysis; assess for systemic involvement; supportive care
Non-palpable petechiae with low platelet count	Thrombocytopenia (immune thrombocytopenic purpura, drug-induced, bone marrow failure)	Complete blood count; peripheral smear; hematology referral
Easy bruising on sun-damaged skin, forearms of elderly	Senile (actinic) purpura	Reassurance; skin protection; no treatment needed

“What Do I Do If…” Decision Reference

Clinical Situation	Immediate Action	Next Step
Patient develops rash while on antibiotics for infection	Assess severity; if mild morbilliform without mucosal involvement, can often continue if antibiotic essential	Monitor closely; stop if worsening; document allergy if drug stopped
Rash appeared after starting a medication weeks ago but now stable	If mild and not progressing, drug may not need to be stopped immediately	Discuss risk-benefit with patient; consider alternative if available; dermatology input
Multiple family members have itchy rash	High suspicion for scabies	Treat all household contacts simultaneously; wash bedding and clothing; repeat treatment in 1 week
Rash not responding to topical steroids	Reconsider diagnosis; check compliance and technique	Consider fungal infection (potassium hydroxide), secondary infection, contact allergy to steroid, or wrong diagnosis
Pregnant patient with new rash	Consider pregnancy-specific dermatoses; assess for systemic symptoms	Pemphigoid gestationis, polymorphic eruption of pregnancy, intrahepatic cholestasis; obstetric and dermatology input
Immunocompromised patient with vesicular rash	Low threshold for herpes simplex virus or varicella zoster virus; may disseminate	Start antivirals empirically; send polymerase chain reaction; consider admission
Localized rash at site of new jewelry, watch, or belt buckle	Allergic contact dermatitis to nickel highly likely	Remove allergen; topical corticosteroid; counsel on nickel avoidance
Patient insists on specific diagnosis but examination is non-diagnostic	Be honest about uncertainty; document findings carefully	Photograph rash; arrange follow-up; consider biopsy or dermatology referral

When to Refer to Dermatology

Urgent Referral (Within 24-48 Hours)

Suspected Stevens-Johnson syndrome or toxic epidermal necrolysis
Suspected pemphigus or bullous pemphigoid
Rapidly progressive or unusual presentations
Drug reaction with eosinophilia and systemic symptoms (DRESS)
Suspected cutaneous malignancy
Erythroderma (greater than 90% body surface area involved)

Routine Referral

Uncertain diagnosis after systematic evaluation
Rash unresponsive to appropriate first-line treatment
Chronic dermatoses requiring systemic therapy
Patch testing for suspected allergic contact dermatitis
Biopsy for diagnostic confirmation
Psoriasis requiring biologic therapy

Troubleshooting Refractory Rash

Ask These Questions When Treatment Fails

Is the diagnosis correct? Reconsider differential; consider biopsy
Is there secondary infection? Bacterial or viral superinfection of eczema or other dermatoses
Is the patient using treatment correctly? Adequate amount, frequency, duration, and technique
Is there contact allergy to the treatment? Allergy to topical corticosteroid or vehicle
Are triggers being avoided? Ongoing allergen exposure, irritant contact, or non-compliance with avoidance
Is treatment strength adequate? May need stronger topical corticosteroid or systemic therapy
Is there an underlying systemic cause? Consider thyroid disease, diabetes, HIV, malignancy
Are there multiple overlapping conditions? For example, eczema with superimposed tinea (“eczema-tinea” or “tinea incognito”)

8. Clinical Pearls and Pitfalls

Practical wisdom — learn from successes and avoid common mistakes

Must-Know Clinical Pearls

The primary lesion is everything: Invest time in identifying the primary lesion morphology before it is modified by scratching, infection, or treatment. This single observation narrows your differential more than any other finding.

Always do diascopy: The blanching test takes two seconds and differentiates vascular dilation (blanching erythema) from extravasated blood (non-blanching purpura). Non-blanching rashes require urgent evaluation.

Distribution tells the story: Symmetric rashes suggest endogenous causes (eczema, psoriasis, drug reactions); asymmetric rashes suggest exogenous causes (infection, contact, trauma).

Medication history is critical: Always ask about medications started in the past 2-8 weeks, including over-the-counter drugs, supplements, and herbal remedies. Drug eruptions are common and easily missed.

Examine the whole skin: A partial examination leads to missed diagnoses. Check scalp, nails, mucous membranes, and hidden areas (genitalia, natal cleft, web spaces) when the diagnosis is unclear.

If it’s scaly, scrape it: A potassium hydroxide preparation takes minutes and can diagnose or exclude fungal infection. Treating eczema with antifungals or tinea with steroids delays healing.

Photograph the rash: Rashes evolve. A photograph provides documentation for monitoring, allows comparison at follow-up, and facilitates remote consultation with dermatology.

Think of scabies when multiple family members itch: Scabies is highly contagious and often missed. The classic distribution (web spaces, wrists, waistband, genitalia) with nocturnal pruritus should prompt treatment of all close contacts.

Critical Pitfalls to Avoid

Missing Stevens-Johnson syndrome or toxic epidermal necrolysis: Any patient with mucosal erosions, skin pain, and a recent new medication should be evaluated for these life-threatening drug reactions. Early drug cessation improves survival.

Dismissing purpura as “bruising”: Non-blanching lesions always require explanation. Petechiae with fever can indicate meningococcemia or other sepsis; palpable purpura suggests vasculitis requiring systemic evaluation.

Treating tinea with topical corticosteroids: Steroids initially improve tinea by reducing inflammation, but the infection spreads and becomes harder to diagnose (tinea incognito). Always do potassium hydroxide before prescribing steroids for annular scaly rashes.

Ignoring herpes zoster in the V1 distribution: Zoster involving the forehead, nose, or periorbital area can cause sight-threatening keratitis and requires same-day ophthalmology referral, even if the eye appears uninvolved.

Attributing all rashes to “allergy”: Most rashes are not allergic. Overuse of the term “allergy” leads to unnecessary dietary restrictions and avoidance behaviors. Reserve the term for documented IgE-mediated or confirmed allergic contact reactions.

Forgetting secondary syphilis: The “great imitator” can present as almost any rash. A generalized eruption involving the palms and soles in a sexually active patient should prompt syphilis serology.

Stopping angiotensin-converting enzyme inhibitors too late for angioedema: Angiotensin-converting enzyme inhibitor-induced angioedema can occur years after starting the medication and can be life-threatening. Stop the drug at first suspicion and never rechallenge.

Underestimating the psychosocial impact: Skin diseases significantly affect quality of life. Depression and anxiety are common. Address the emotional burden, not just the physical symptoms.

Key Takeaways

Accurate diagnosis begins with morphology: Learn to identify primary lesions (macule, papule, vesicle, wheal, pustule, purpura) — this skill is more valuable than memorizing disease lists.
Distribution provides diagnostic clues: Flexural suggests eczema; extensor suggests psoriasis; dermatomal suggests zoster; photodistributed suggests lupus or drug photosensitivity; palms and soles suggests syphilis or erythema multiforme.
Red flags demand urgent action: Non-blanching purpura with fever, mucosal involvement with skin pain, rapidly spreading erythema, and Nikolsky sign positive are emergencies.
Drug reactions are common and treatable: A thorough medication history including timeline is essential. Most drug eruptions improve rapidly after the culprit is stopped.
Potassium hydroxide preparation prevents misdiagnosis: A simple bedside test distinguishes fungal infection from inflammatory dermatoses, preventing inappropriate treatment.
Many rashes are clinical diagnoses: Investigations should be targeted, not routine. History and examination alone diagnose the majority of rashes in primary care.
Emollients are underused: For eczema and many other inflammatory dermatoses, regular emollient use is as important as topical corticosteroids and reduces the need for active treatment.
When uncertain, biopsy and photograph: A skin biopsy provides diagnostic certainty; a photograph documents the presentation for future reference and consultation.
Consider systemic disease: Some rashes are windows into internal disease — lupus, dermatomyositis, vasculitis, and internal malignancy can all present with skin findings.
Follow up and reassess: If treatment fails, reconsider the diagnosis. Tinea incognito, secondary infection, and contact allergy to treatments are common causes of treatment failure.

Quick Reference Algorithm

Systematic Approach to Any Rash:

Assess urgency: Is the patient stable? Are there red flags requiring immediate intervention?
Identify the primary lesion: What is the morphology before modification by scratching or treatment?
Determine the distribution: Is it localized or generalized? Symmetric or asymmetric? What pattern does it follow?
Perform the blanching test: Does the rash blanch with pressure? Non-blanching requires explanation.
Take a focused history: Use the “RASHES” mnemonic — Rash characteristics, Anatomical distribution, Systemic symptoms, History of exposures, Evolution, Similar episodes.
Review medications: Any new drugs in the past 2-8 weeks? Include over-the-counter products and supplements.
Examine the whole skin: Check nails, scalp, mucous membranes, and hidden areas for additional clues.
Formulate a differential: Generate a probability-ranked list based on morphology, distribution, and clinical context.
Investigate if needed: Potassium hydroxide, biopsy, serology, or other tests guided by clinical suspicion.
Treat and follow up: Initiate appropriate therapy; arrange reassessment to confirm response or reconsider diagnosis.

Common Rash Mimics and Look-Alikes

Condition Often Confused	Key Distinguishing Features
Psoriasis vs. Eczema	Psoriasis: well-demarcated, silvery scale, extensor surfaces, nail pitting. Eczema: poorly demarcated, flexural, lichenification, atopic history.
Tinea vs. Eczema (nummular)	Tinea: annular with central clearing, active scaly border, potassium hydroxide positive. Nummular eczema: coin-shaped, no central clearing, potassium hydroxide negative.
Cellulitis vs. Stasis dermatitis	Cellulitis: acute, unilateral, tender, warm, fever. Stasis dermatitis: chronic, bilateral, associated with venous insufficiency, hemosiderin staining.
Urticaria vs. Urticarial vasculitis	Urticaria: wheals last less than 24 hours, pruritic. Urticarial vasculitis: wheals last greater than 24 hours, painful more than pruritic, leave bruising.
Pityriasis rosea vs. Secondary syphilis	Pityriasis rosea: herald patch, collarette scale, “Christmas tree” pattern, spares palms/soles. Secondary syphilis: no herald patch, palms and soles involved, lymphadenopathy, positive serology.
Contact dermatitis vs. Atopic dermatitis	Contact: localized to contact area, geometric borders, exposure history. Atopic: flexural distribution, personal or family atopic history, chronic relapsing.