APACHE II (Acute Physiology and Chronic Health Evaluation II) is the most widely used severity-of-illness score in intensive care. Developed by Knaus and colleagues in 1985, it converts twelve physiologic measurements, the patient’s age, and their chronic health into a single number from 0 to 71 that lets clinicians benchmark disease severity and estimate in-hospital mortality risk.
How it works
The total is the sum of three independent components:
APACHE II = Acute Physiology Score (APS)
+ Age points
+ Chronic Health pointsAcute Physiology Score (0–60)
Each of the 12 acute variables is scored 0, 1, 2, 3, or 4 by how far it deviates from the normal range, using the single worst value recorded in the first 24 hours of ICU admission:
| Variable | Abnormal if… |
|---|---|
| Temperature (rectal, °C) | Below 30 or above 41 scores 4 |
| Mean arterial pressure (mmHg) | Below 50 or above 160 scores 4 |
| Heart rate (beats/min) | Below 40 or above 180 scores 4 |
| Respiratory rate (breaths/min) | Below 6 or above 50 scores 4 |
| Oxygenation (see below) | Depends on FiO2 |
| Arterial pH | Below 7.15 or above 7.70 scores 4 |
| Serum sodium (mmol/L) | Below 111 or above 180 scores 4 |
| Serum potassium (mmol/L) | Below 2.5 or above 7.0 scores 4 |
| Serum creatinine (mg/dL) | Score doubled in acute renal failure |
| Haematocrit (%) | Below 20 or above 60 scores 4 |
| White cell count (×10³/mm³) | Below 1 or above 40 scores 4 |
| Glasgow Coma Scale | Contributes 15 − GCS points |
Oxygenation rule: when FiO2 ≥ 0.5, score the alveolar-arterial oxygen gradient; when FiO2 < 0.5, score PaO2 directly. The tool switches automatically based on the FiO2 entered.
Acute renal failure: ticking this box doubles the creatinine score, reflecting the original Knaus weighting for renal dysfunction that is acute rather than chronic.
Age points (0–6)
| Age (years) | Points |
|---|---|
| Under 45 | 0 |
| 45–54 | 2 |
| 55–64 | 3 |
| 65–74 | 5 |
| 75 and above | 6 |
Chronic Health points (0–5)
A history of severe chronic organ insufficiency or immunocompromise adds 2 points for elective post-operative patients and 5 points for all others (non-operative or emergency post-operative).
Interpreting the total
Higher scores correspond to higher predicted mortality, but the actual risk also depends on the admitting diagnosis — APACHE II uses a diagnostic category multiplier in the original paper that is not included in this simplified calculator. Treat the score as a severity benchmark for comparing patients and populations, not a precise prediction for an individual.
Example
For example: a 60-year-old patient (3 age points) admitted as an emergency with an APS of 12 and no qualifying chronic condition totals 15. A patient of similar age admitted electively post-operatively with the same physiology would score 17 (the 5 chronic-health points apply for non-elective presentations). Enter the single worst value of each variable across the first ICU day, and note that the mortality figures are population estimates derived from the 1985 Knaus cohort — case mix, treatment intensity, and outcomes have changed considerably since then.