emergency & critical carescoring tool

APACHE II

APACHE II is the most widely used ICU severity-of-illness score. It combines the Acute Physiology Score (12 variables using worst values in first 24h), age points, and chronic health evaluation to predict hospital mortality and enable ICU benchmarking.

inputs

°C
As decimal (e.g., 0.4 for 40%)
Used if FiO₂ <0.5
Used if FiO₂ ≥0.5
years

✓ when to use

Calculate within the first 24 hours of ICU admission using the worst physiological values during that period. Used for mortality prediction, ICU benchmarking, severity comparison across populations, and clinical trial stratification. Widely used for audit and quality improvement.

✗ when not to use

APACHE II predicts group-level mortality, not individual outcomes — do not use it as a sole determinant for individual patient treatment decisions. The score was developed in 1985 and may overestimate mortality for conditions where ICU care has substantially improved. Burns and cardiac surgery patients were excluded from the original derivation. APACHE III and IV exist but are less widely adopted.

clinical pearls

  • Use the WORST values in the first 24 hours — not the admission values or the most recent values. This is the most common scoring error.
  • The GCS component is scored as (15 − GCS), so a GCS of 3 contributes 12 points — it is the single highest-scoring acute physiology variable.
  • Chronic health points are 5 for non-operative or emergency surgical patients with severe organ insufficiency (liver cirrhosis, NYHA IV heart failure, chronic renal failure, immunocompromise), and 2 for elective surgical patients with the same. All other patients score 0.
  • APACHE II-derived mortality predictions should be interpreted cautiously in 2026 — ICU outcomes have improved substantially since 1985. The score is most useful for comparing severity between groups, not for predicting individual outcomes.
  • Oxygenation scoring depends on FiO₂: if ≥0.5, use A-a gradient; if <0.5, use PaO₂. This two-tier approach is unique to APACHE II and requires knowing both values.

related tools

sofaOrgan dysfunction scoring (daily)news2Ward-based early warninggcsCNS component of APACHE II