APACHE II Calculator (ICU severity score, Knaus 1985)

Understanding the APACHE II calculator — concept and principle

This calculator works out the APACHE II score, a 0–71 measure of illness severity used in intensive care, from the worst values in the first 24 hours: twelve physiological measurements, age and chronic health. It shows the acute physiology points, age points, chronic health points and which oxygenation rule was used. ICU clinicians, researchers and students use it.

Physiology + age + chronic health — with the default inputs: APACHE II 18.

How it is calculated

APACHE II = acute physiology points + age points + chronic health points
Twelve measurements score 0–4 each by distance from normal: temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, pH, sodium, potassium, creatinine (doubled in acute kidney failure), haematocrit, white cell count, plus 15 − Glasgow Coma Scale
Oxygenation: A–a gradient if FiO₂ ≥ 50% (A–a = FiO₂ × 713 − PaCO₂ ÷ 0.8 − PaO₂), otherwise PaO₂
Age: under 45 → 0; 45–54 → 2; 55–64 → 3; 65–74 → 5; 75+ → 6
Chronic health: 5 for non-surgical or emergency surgical patients, 2 after elective surgery, if severe organ failure or immunosuppression existed before admission

The principle

Knaus and colleagues (1985) built APACHE II to compare the severity of illness of ICU patients. The more a vital sign or lab value deviates from normal, the more points it adds, because larger physiological disturbances are linked to higher risk. Age and pre-existing severe disease add further points. The total correlated with hospital mortality in the original study; a mortality estimate also requires a diagnostic category weight, which is not shown here.

Worked example

Defaults: age 67, temperature 38.7 °C, MAP 65, heart rate 118, respiratory rate 28, FiO₂ 40%, PaO₂ 68, pH 7.31, Na 134, K 4.1, creatinine 1.8 mg/dL, haematocrit 36, WBC 16.5, GCS 14, no chronic health points.

Points: temperature 1, MAP 2, heart rate 2, respiratory rate 1, PaO₂ 1, pH 2, sodium 0, potassium 0, creatinine 2, haematocrit 0, WBC 1, GCS 15 − 14 = 1.
Acute physiology = 13; oxygenation rule PaO₂ (FiO₂ below 50%); A–a gradient —.
Age 67 → 5 points; chronic health 0.
APACHE II = 13 + 5 + 0 = 18.

Each point is the calculator re-run with only “Age (years)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Age (years)	APACHE II score (0–71)
16.8	13
33.5	13
50.2	15
67	18
83.8	19
100	19
134	19

How to read the result

Higher scores indicate more severe illness. The score was designed for groups of patients and research comparisons, not to decide the care of one person. Use the worst values over the first 24 hours, not a single snapshot.

Common mistakes

Using admission values instead of the worst values in 24 hours.
Forgetting to double creatinine points in acute kidney failure.
Using PaO₂ when FiO₂ is 50% or more.
Scoring GCS in sedated patients without a consistent rule.

Keep in mind: Not a diagnosis or prognosis for an individual. ICU decisions are made by the clinical team.

Frequently asked questions

What is a high APACHE II score?

Higher totals mean more severe illness; the original study showed rising mortality with higher scores.

Why is creatinine doubled in acute kidney failure?

The original method gives extra weight to acute kidney failure.

Can APACHE II predict one patient's outcome?

Not reliably; it was designed for groups.

Sources: Knaus WA, Draper EA, Wagner DP, Zimmerman JE, "APACHE II: a severity of disease classification system", Crit Care Med 1985;13(10):818–829 · MSD Manual Professional Edition, "APACHE II Scoring System" table

Checked 9 Nov 2026. Knaus WA, Draper EA, Wagner DP, Zimmerman JE, “APACHE II: a severity of disease classification system”, Crit Care Med 1985;13(10):818–829; point table as reproduced in MSD Manual Professional Edition, table “Acute Physiologic Assessment and Chronic Health Evaluation (APACHE) II Scoring System”.

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