APACHE II Score Calculator
Acute Physiology and Chronic Health Evaluation II — the most widely used ICU severity-of-illness scoring system worldwide. Combines 12 physiological variables, age, and chronic health status to estimate in-hospital mortality risk for critically ill adults within the first 24 hours of ICU admission.
Calculate APACHE II Score
Select the worst (most abnormal) value for each variable during the first 24 hours of ICU admission. All 12 physiological variables, age, and chronic health status are required. For a simpler organ-dysfunction tool, see the SOFA Score calculator.
Severe organ insufficiency or immunocompromised is defined as any of the following documented prior to this hospital admission:
- Liver: Biopsy-proven cirrhosis, portal hypertension, prior hepatic failure/encephalopathy/coma
- Cardiovascular: NYHA Class IV heart failure
- Respiratory: Chronic restrictive, obstructive, or vascular disease causing severe exercise limitation; documented chronic hypoxia, hypercapnia, polycythaemia, severe pulmonary hypertension, or ventilator dependence
- Renal: Chronic dialysis
- Immunocompromised: Receiving immunosuppressive therapy, chemotherapy, radiation, long-term steroids, or disease that suppresses immune function (e.g., leukaemia, lymphoma, AIDS)
APACHE II provides a population-level mortality estimate, not an individual prognosis. The original equation also includes a diagnostic category weight that adjusts mortality prediction based on the admission diagnosis (e.g., post-operative cardiac surgery vs. sepsis). This calculator provides the raw APACHE II score and an approximate mortality estimate without the diagnostic adjustment.
Understanding the APACHE II Score
The APACHE II (Acute Physiology and Chronic Health Evaluation II) was published in 1985 by William Knaus and colleagues at George Washington University. It was one of the first validated ICU severity-of-illness scoring systems and has remained the most widely cited and used globally, despite the subsequent development of APACHE III (1991) and APACHE IV (2006).
The score was derived from a cohort of 5,815 ICU admissions across 13 hospitals and was designed to answer a specific clinical question: given a patient’s physiology at ICU admission, what is their estimated risk of in-hospital death? It quantifies severity by combining the degree of acute physiological derangement, the patient’s age, and the burden of pre-existing chronic disease.
Score Composition
Three components:
A. Acute Physiology Score (APS) — 12 variables, each scored 0–4 based on deviation from normal (range: 0–60)
B. Age Points — 0 to 6 points
C. Chronic Health Points — 0, 2, or 5 points
Total = APS + Age + Chronic Health
Range: 0–71 (theoretical maximum)
Worked Example
A 68-year-old with community-acquired pneumonia, Day 1 ICU:
APS: Temp 38.8 (+1), MAP 62 (+2), HR 118 (+2), RR 30 (+1), PaO₂ 58 on FiO₂ 0.4 (+3), pH 7.30 (+2), Na 141 (0), K 4.1 (0), Cr 1.8 (+2), Hct 36 (0), WBC 18 (+1), GCS 13 (+2) = 16
Age: 68 → +5
Chronic: None → 0
APACHE II = 21 → Estimated mortality ~40%.
Why APACHE II endures: Despite being over 40 years old, APACHE II remains the most commonly used ICU scoring system in clinical research and quality benchmarking. Its persistence is due to simplicity (12 readily available variables), extensive validation across diverse populations, and widespread familiarity. While APACHE IV offers better calibration for modern ICU populations, it requires proprietary software, which limits adoption.
Score Interpretation & Mortality Estimates
The APACHE II score correlates with in-hospital mortality in a graded, non-linear fashion. The mortality estimates below are approximate and derived from the original 1985 validation cohort and subsequent large observational studies. Actual mortality varies significantly by admission diagnosis, institution, era of treatment, and case mix.
| APACHE II Score | Severity | Approx. Mortality | Clinical Implication |
|---|---|---|---|
| 0–4 | Minimal | ~4% | Low acuity; may not require ICU-level care |
| 5–9 | Mild | ~8% | Standard ICU monitoring and support |
| 10–14 | Moderate | ~15% | Active organ support likely; close monitoring |
| 15–19 | Moderately Severe | ~25% | Significant physiological derangement; escalating support |
| 20–24 | Severe | ~40% | Multi-organ compromise; family discussions warranted |
| 25–29 | Very Severe | ~55% | High mortality risk; goals-of-care review |
| 30–34 | Critical | ~75% | Profound derangement; proactive palliative care input |
| ≥ 35 | Extreme | ~85% | Very high mortality; urgent goals-of-care discussion imperative |
APACHE II mortality estimates were calibrated using data from the early 1980s. Modern ICU care has significantly improved survival at every score level. A patient scoring 25 in 2025 may have a true mortality closer to 35–40% rather than 55%, depending on the diagnostic category and institution. Always interpret the score as a relative severity indicator rather than an absolute mortality prediction.
Scoring Components & Clinical Considerations
The APACHE II score comprises three distinct components. Understanding how each contributes to the total — and where each is most susceptible to confounders — is essential for accurate scoring and meaningful clinical interpretation.
The APS accounts for the largest portion of the APACHE II score (up to 60 points) and captures the degree of acute physiological derangement. Each of the 12 variables is scored on a symmetric scale: 0 points for values within the normal range, with increasing points (1–4) for deviations above or below normal. The worst value during the first 24 hours of ICU admission is used.
The 12 variables span cardiovascular (MAP, HR), respiratory (RR, oxygenation), metabolic (temperature, pH/HCO₃, Na, K), renal (creatinine), haematological (haematocrit, WBC), and neurological (GCS) domains. A key design principle is that both extremes are penalised — severe bradycardia scores the same as severe tachycardia, reflecting the clinical reality that physiological derangement in either direction carries risk.
The oxygenation component has a unique conditional structure: if FiO₂ ≥ 0.5, the A-a gradient is used; if FiO₂ < 0.5, PaO₂ alone is used. This design accommodates the non-linear relationship between FiO₂ and PaO₂ at different levels of oxygen supplementation.
Age contributes 0–6 points and serves as a surrogate for diminishing physiological reserve. The age thresholds were empirically derived from the original cohort: patients under 45 receive no additional points, while those 75 and older receive the maximum of 6. This reflects the well-documented association between advancing age and reduced ability to recover from acute critical illness.
The age component has been criticised for being a crude proxy — a fit 78-year-old and a frail 78-year-old receive the same 6 points. Frailty indices and functional status assessments may better capture physiological age, but these are not incorporated into the APACHE II framework. Some clinicians advocate for combining APACHE II with a frailty assessment (e.g., Clinical Frailty Scale) for a more comprehensive risk estimation in elderly patients.
The chronic health component adds 0, 2, or 5 points based on the presence of severe organ insufficiency or immunocompromised status prior to the current admission. The scoring distinguishes between elective post-operative patients (+2) and emergency post-operative or non-operative medical patients (+5) with the same chronic conditions.
This distinction reflects the observation that patients with chronic organ failure who are admitted emergently have worse outcomes than those admitted for planned procedures. The definitions of “severe organ insufficiency” are specific and well-defined (cirrhosis, NYHA IV heart failure, chronic dialysis, chronic ventilator dependence, or immunosuppression) and should not be applied loosely to patients with milder chronic disease.
A significant limitation is the binary nature of this component — a patient with one qualifying condition receives the same points as a patient with three. The Charlson Comorbidity Index provides a more granular assessment of comorbidity burden but is not part of the APACHE system.
The full APACHE II mortality prediction equation includes a diagnostic category weight — a coefficient that adjusts the mortality estimate based on the patient’s primary reason for ICU admission. There are 50+ diagnostic categories, each with a specific weight reflecting the baseline mortality associated with that condition. For example, post-operative cardiac surgery has a lower diagnostic weight (better prognosis for a given score) than sepsis or cardiac arrest.
This calculator provides the raw APACHE II score and an approximate mortality estimate based on score alone, without the diagnostic adjustment. For formal mortality prediction using the complete APACHE II equation, the diagnostic category must be specified. In clinical practice, the raw score is most commonly used for severity description and unit benchmarking, while the full equation is used in research and quality improvement programmes.
Special Populations & Considerations
APACHE II was developed in a mixed medical–surgical adult ICU population in the United States in the early 1980s. Its calibration and performance vary across different patient populations, healthcare systems, and eras. These limitations should inform how the score is interpreted in specific clinical contexts.
APACHE II is not validated in children. Normal physiological ranges for heart rate, respiratory rate, blood pressure, and creatinine are age-dependent in paediatric patients. Paediatric-specific severity scores (PIM-3, PELOD-2) should be used instead. Do not apply APACHE II to any patient under 16 years of age.
Post-cardiac surgery patients may have transient physiological extremes (hypothermia from bypass, haemodynamic instability, coagulopathy) that inflate APS points without reflecting sustained organ dysfunction. APACHE II typically overestimates mortality in this population. The EuroSCORE II or STS Score are better calibrated for cardiac surgical risk prediction.
Major burns and polytrauma produce acute physiological derangement that may resolve rapidly with resuscitation. Day 1 APACHE II scores in these populations may overestimate mortality. Burn-specific scores (ABSI, revised Baux) and trauma-specific scores (ISS, TRISS) provide better calibration. APACHE II at 48–72 hours may be more informative than the admission score.
APACHE II requires 12 laboratory and clinical parameters, some of which (ABG, A-a gradient) may not be routinely available in resource-limited ICUs. Modified versions of APACHE have been proposed for low-resource settings, but the standard score should not be calculated with missing or assumed values, as this significantly impairs accuracy. The MEWS or qSOFA may be more practical alternatives.
Temporal calibration drift: APACHE II mortality estimates were derived from 1979–1982 data. Advances in ICU care — lung-protective ventilation, early goal-directed therapy, modern vasopressor strategies, improved infection control — mean that actual mortality at any given score is now substantially lower than the original estimates suggest. Studies from the 2010s consistently show observed mortality 20–40% lower than APACHE II predictions across most diagnostic categories.
Clinical Workflow & Using APACHE II Effectively
APACHE II is most valuable when used within a structured clinical workflow — not as an isolated number but as part of a comprehensive severity assessment that informs resource allocation, quality benchmarking, and prognostic discussions.
APACHE II is designed to be calculated once using the worst values from the first 24 hours of ICU admission. Collect all 12 APS variables, document the patient’s age, and determine the chronic health status based on the specific criteria (not clinical impression). Use the worst value — the most physiologically abnormal — for each parameter during this period.
Timing matters: calculating APACHE II before a full 24-hour set of observations is available may underestimate severity (if the patient deteriorates after initial assessment) or overestimate it (if early resuscitation rapidly corrects initial derangements).
The raw APACHE II score should always be interpreted in the context of the admission diagnosis. An APACHE II of 18 in a patient with diabetic ketoacidosis (often rapidly reversible) carries a very different prognosis from an APACHE II of 18 in a patient with metastatic carcinoma and septic shock. The diagnostic category weight in the full APACHE II equation partially captures this, but clinical judgement remains essential.
While APACHE II is not designed for serial reassessment (unlike SOFA), many clinicians recalculate at 48–72 hours to assess trajectory. A rising APACHE II suggests worsening organ function despite treatment, while a declining score supports clinical improvement. This serial use is not part of the original validation but is common in practice.
One of the most important applications of APACHE II is in calculating the Standardised Mortality Ratio (SMR): the ratio of observed deaths to APACHE II–predicted deaths. An SMR < 1 suggests that the ICU is performing better than expected; an SMR > 1 suggests worse-than-expected outcomes. This metric is widely used in national ICU audit programmes (e.g., ICNARC in the UK, ANZICS in Australia/New Zealand).
For meaningful SMR comparisons, the APACHE II score must be calculated consistently across institutions — the same timing window, the same variable definitions, and the same handling of missing data. Inconsistent scoring practices are the most common source of inaccurate SMR calculations and can create misleading comparisons between units.
APACHE II can provide objective data to support prognostic discussions, but it should never be used as the sole basis for treatment limitation decisions. Communicate the score as a population-level estimate: “Among patients with similar physiological derangement, approximately X% survive to hospital discharge.” Emphasise that individual outcomes depend on diagnosis, treatment response, premorbid function, and patient values.
Avoid presenting APACHE II as deterministic (e.g., “You have a 40% chance of dying”). Instead, frame it as one of several data points — along with clinical trajectory, response to initial treatment, and specialist opinion — that together inform a nuanced discussion about prognosis and treatment goals.
Common Pitfalls & Limitations
APACHE II is a powerful severity tool but is frequently misapplied or misinterpreted. The following pitfalls represent the most common sources of error and misunderstanding in clinical practice and research.
The single most important limitation of APACHE II is that its mortality coefficients were derived from 1979–1982 data. ICU care has fundamentally changed since then: lung-protective ventilation, protocolised sepsis management, improved surgical techniques, advanced renal replacement, and reduced hospital-acquired infection rates have all significantly improved survival. Using the original mortality equation without acknowledging this temporal drift leads to systematic overestimation of death risk.
Modern validation studies consistently show that observed mortality is 20–40% lower than APACHE II predicts across most diagnostic categories. For individual prognostication, the APACHE IV equation (2006) or institution-specific recalibrated models are more accurate. When using APACHE II for quality benchmarking, ensure the expected mortality model is updated for your population and era.
The GCS component (scored as 15 − GCS, range 0–12) is the single highest-scoring individual variable. A sedated, intubated patient assessed at GCS 3 contributes 12 points — more than any other variable can score. This can dramatically inflate the APACHE II score in patients who have been sedated and intubated for airway protection but have no intrinsic neurological pathology.
The recommended approach is to use the pre-sedation GCS or the GCS assessed during a sedation hold. If neither is available, many clinicians assume a GCS of 15 (0 points) for patients without suspected neurological injury. Whatever approach is chosen, it must be applied consistently across all patients to maintain scoring validity.
APACHE II uses the worst values in the first 24 hours of ICU admission. However, patients who receive extensive resuscitation in the emergency department or ward before ICU transfer may have partially corrected physiology at the time of ICU admission, resulting in a lower APACHE II score than their true severity warrants. Conversely, patients transferred directly from the scene or from facilities without resuscitation capacity may present with more extreme derangements.
This lead-time bias means that APACHE II may underestimate severity in well-resourced systems with aggressive pre-ICU care and overestimate severity in systems with limited pre-ICU capacity. Awareness of this bias is important when comparing SMRs across institutions with different patterns of pre-ICU care.
The chronic health component is frequently misscored. Common errors include applying the +5 points to elective post-operative patients (who should receive only +2), assigning chronic health points for mild chronic disease that does not meet the strict APACHE II definitions (e.g., applying liver points for fatty liver disease rather than biopsy-proven cirrhosis), or failing to apply chronic health points in patients who clearly qualify.
The definitions are specific: only biopsy-proven cirrhosis with portal hypertension, NYHA IV heart failure, chronic dialysis dependence, chronic ventilator dependence, or documented immunosuppression qualify. “Chronic kidney disease stage 3” or “moderate COPD” do not meet the threshold. Training and standardisation of the chronic health assessment are essential for scoring accuracy.
APACHE II was designed as a population-level prognostic tool, not an individual triage instrument. It cannot reliably determine whether a specific patient will survive or die. Even at the highest score ranges (≥ 35, ~85% mortality), 15% of patients survive — and these survivors cannot be prospectively identified by the score alone.
Using APACHE II as a threshold for ICU admission or treatment withdrawal is ethically problematic and clinically unsound. Professional guidelines uniformly recommend against using any single severity score as a sole decision-making tool for resource allocation. The score should inform — not replace — holistic clinical assessment and shared decision-making with patients and families.
APACHE II does not include several parameters now recognised as important in critical illness severity: serum lactate, procalcitonin, troponin, B-type natriuretic peptide, and functional status/frailty. It also does not capture the response to initial treatment — a patient who rapidly improves with resuscitation may carry the same Day 1 score as a patient who continues to deteriorate.
Quick Reference Summary
variables (APS)
(APS + Age + Chronic)
(worst values, Day 1)
at score ≥ 35
| Component | Variables | Max Points |
|---|---|---|
| APS (A) | Temp, MAP, HR, RR, O₂, pH, Na, K, Cr, Hct, WBC, GCS | 60 |
| Age (B) | Patient age in years | 6 |
| Chronic Health (C) | Severe organ insufficiency / immunocompromised | 5 |
| Total | A + B + C | 71 |
The Golden Rule: APACHE II describes how sick a patient was on ICU Day 1 — it does not determine what will happen to them. Use it for population-level benchmarking, quality improvement, and as one input to prognostic discussions. Never use a single severity score as the sole basis for individual treatment decisions.
Disclaimer & References
For Educational Purposes Only. This calculator and the accompanying clinical information are intended as educational tools for healthcare professionals. They do not replace clinical judgement. Results should be interpreted in the full clinical context. Lab reference ranges vary by institution — verify with your own laboratory. Drug dosages should be confirmed against current prescribing information.
References
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med. 1985;13(10):818–829. DOI: 10.1097/00003246-198510000-00009
- Knaus WA, Wagner DP, Draper EA, et al. The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults. Chest. 1991;100(6):1619–1636. DOI: 10.1378/chest.100.6.1619
- Zimmerman JE, Kramer AA, McNair DS, Malila FM. Acute Physiology and Chronic Health Evaluation (APACHE) IV: hospital mortality assessment for today’s critically ill patients. Crit Care Med. 2006;34(5):1297–1310. DOI: 10.1097/01.CCM.0000215112.84523.F0
- Vincent JL, Moreno R. Clinical review: scoring systems in the critically ill. Crit Care. 2010;14(2):207. DOI: 10.1186/cc8204
- Bouch DC, Thompson JP. Severity scoring systems in the critically ill. Contin Educ Anaesth Crit Care Pain. 2008;8(5):181–185. DOI: 10.1093/bjaceaccp/mkn033
- Sadaka F, EthmaneAbouElMaali C, Cytron MA, Fowler K, Javaux VM, O’Brien J. Predicting Mortality of Patients With Sepsis: A Comparison of APACHE II and APACHE III Scoring Systems. J Clin Med Res. 2017;9(11):907–910. DOI: 10.14740/jocmr3083w
- Capuzzo M, Valpondi V, Sgarbi A, et al. Validation of severity scoring systems — SAPS II and APACHE II — in a single-center population. Intensive Care Med. 2000;26(12):1779–1785. DOI: 10.1007/s001340000715
- Wong DT, Crofts SL, Gomez M, McGuire GP, Byrick RJ. Evaluation of predictive ability of APACHE II system and hospital outcome in Canadian intensive care unit patients. Crit Care Med. 1995;23(7):1177–1183. DOI: 10.1097/00003246-199507000-00005
- Minne L, Abu-Hanna A, de Jonge E. Evaluation of SOFA-based models for predicting mortality in the ICU: a systematic review. Crit Care. 2008;12(6):R161. DOI: 10.1186/cc7160