APACHE II Score Calculator — ICU Severity and Mortality
Calculate the APACHE II score from the worst values in the first 24 hours in ICU — 12 physiology variables, age and chronic health — with the estimated hospital mortality for non-operative and post-operative patients.
How it works
APACHE II = Acute Physiology Score (12 variables, 0–4 points each, worst value in the first 24 h; GCS scored as 15 − GCS; creatinine doubled in acute renal failure) + age points (≤ 44: 0, 45–54: 2, 55–64: 3, 65–74: 5, ≥ 75: 6) + chronic health points (5 for non-operative or emergency post-operative, 2 for elective post-operative). Oxygenation: A–a gradient when FiO₂ ≥ 0.5 (A–a = FiO₂ × 713 − PaCO₂/0.8 − PaO₂), otherwise PaO₂. Range 0–71.
APACHE II (Acute Physiology and Chronic Health Evaluation II; Knaus et al., Critical Care Medicine 1985) is the most widely used ICU severity-of-illness score. Each physiology variable gets 0–4 points for how far its worst value in the first 24 hours lies from normal; age and severe chronic disease add points. The mortality figures shown are the approximate hospital death rates by score band reported for non-operative and post-operative patients in the original cohort; the full APACHE II equation also uses a diagnostic-category weight, so treat the percentage as a population estimate, not an individual prediction.
Worked example
A 67-year-old medical ICU patient with temperature 38.6 °C (1), MAP 65 mmHg (2), heart rate 115 (2), respiratory rate 26 (1), PaO₂ 68 mmHg on 40% oxygen (1), pH 7.30 (2), sodium 134 (0), potassium 4.1 (0), creatinine 1.6 mg/dL (2), hematocrit 34% (0), WBC 16 (1) and GCS 14 (1) has an Acute Physiology Score of 13. Adding 5 age points and no chronic health points gives APACHE II = 18 / 71, with an approximate non-operative hospital mortality of 25%.
Sources: Knaus WA et al. APACHE II: a severity of disease classification system. Crit Care Med 1985
Frequently asked questions
Which values do I enter in APACHE II?
Use the worst value of each variable in the first 24 hours after ICU admission — the one that earns the most points, whether high or low. Score GCS as observed (or estimated before sedation).
What is a high APACHE II score?
Scores above about 20 are associated with hospital mortality of 30–40% or more, and scores of 35 and above with mortality above 80% in the original cohort. Scores under 10 carry a mortality below about 10%.
When is the A–a gradient used instead of PaO₂?
When FiO₂ is 0.5 (50%) or higher, oxygenation is scored on the alveolar–arterial gradient. Below 0.5 it is scored on the PaO₂ itself.
What counts as chronic health points?
Severe organ insufficiency diagnosed before admission — cirrhosis with portal hypertension, NYHA class IV heart failure, severe chronic lung disease, chronic dialysis — or an immunocompromised state. They add 5 points for medical or emergency surgical patients and 2 points after elective surgery.
Is APACHE II still used?
Yes — newer versions (APACHE III and IV) are more accurate but proprietary, so APACHE II remains common in research, audits and many ICUs worldwide. SOFA is used alongside it to track organ failure over time.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.