What is the APACHE II Score?

APACHE II (Acute Physiology and Chronic Health Evaluation II) is a disease severity scoring system for ICU patients published by Knaus et al. in 1985. It sums 12 physiologic variables (worst values during the first 24 hours of ICU care), age points, and chronic health points, for a total of 0–71. The higher the score, the more severe the patient's condition at the population level.

How is the APACHE II Score calculated?

Three components are summed: (A) Acute Physiology Score — 12 variables (temperature, MAP, heart rate, respiratory rate, oxygenation, pH/HCO3, sodium, potassium, creatinine, hematocrit, WBC, and 15 minus GCS), each 0–4 points (creatinine doubled with acute renal failure); (B) age points: under 44 years = 0, 45–54 = 2, 55–64 = 3, 65–74 = 5, 75 and above = 6; (C) chronic health points: 5 for non-operative or post-op emergency patients with severe organ failure/immunocompromise, 2 for post-op elective, 0 if none.

What do the worst values in the first 24 hours mean?

For each physiologic variable, APACHE II uses the value farthest from normal during the first 24 hours after ICU admission — not the admission value or the average. If a variable was not measured, it is assumed normal (0 points). This rule matters because the score was designed to compare severity across ICU patients.

When to use AaDO2 vs PaO2?

If FiO2 is 0.5 (50%) or higher, use the alveolar-arterial oxygen gradient (AaDO2): 500 or more = 4 points, 350–499 = 3, 200–349 = 2, below 200 = 0. If FiO2 is below 0.5, use PaO2 alone: above 70 = 0, 61–70 = 1, 55–60 = 3, below 55 = 4. This calculator selects automatically based on the FiO2 you enter.

What does the APACHE II mortality estimate table mean?

The Knaus table maps score ranges to observed hospital death rates in the study population (e.g. score 0–4 about 4%, 20–24 about 40%, 35 and above about 85%). These are population statistics from 1985 data, not predictions for individual patients — patients with the same score can have very different outcomes depending on diagnosis, therapy, and facilities.

Can this APACHE II calculator be used for clinical decisions?

No. This calculator is an educational tool for understanding how APACHE II scoring works. Prognostic scores are population-based and must not be used to determine individual prognosis, triage decisions, or withdrawal of therapy. All clinical decisions must be made by the ICU team caring for the patient, considering the complete clinical picture.

MEDICAL CALCULATOR · EDUCATIONAL

APACHE II Score Calculator

Compute ICU disease severity score (Acute Physiology and Chronic Health Evaluation II, Knaus 1985): 12 physiologic variables + age points + chronic health points. Total 0–71, complete with per-variable breakdown and mortality estimate table as population context.

⚠ Important limits — read first

This calculator is an educational tool. APACHE II is a population prognostic score from a 1985 study — it is not a prediction of outcome for individual patients and must not be used for triage decisions, individual prognosis, or withdrawal of therapy.

All clinical decisions must be made by the ICU team caring for the patient. Use the worst values during the first 24 hours of ICU care for each variable, per the original definition.

1 · Physiologic variables (worst values, first 24 hours)

2 · Age & chronic health

Age

years · <44: 0 · 45–54: 2 · 55–64: 3 · 65–74: 5 · ≥75: 6

Patient age

0 points

Chronic health

history of severe organ failure / immunocompromise

Patient has history of severe organ failure or immunocompromise Patient category

0 points

Non-operative / post-op emergency: 5 · post-op elective: 2 · no history: 0

Example case

Auto-fill one severe ICU patient scenario to see how the calculator works.

3 · Results

Total APACHE II Score

range 0–71

Acute Physiology Score (12 variables): 0
Age points: 0
Chronic health points: 0
Mortality estimate (population context): ≈4%

Mortality figures come from the Knaus table (1985) and describe observed death rates in the study population — not the probability of death for individual patients.

Per-variable breakdown

Point details per component (worst values, first 24 hours)
Component	Input value	Points
Total APACHE II		0

Mortality estimate table (Knaus — population context)

Observed hospital death rates per score range in the original study. Not individual predictions.
APACHE II Score	Mortality estimate

Methodology note: each variable uses the worst value (farthest from normal) during the first 24 ICU hours. Unmeasured variables are assumed normal (0 points). Creatinine points are doubled with acute renal failure. GCS points = 15 − actual GCS.

Scope boundary: this tool is an APACHE II scoring calculator for adult ICU patients. It does not cover APACHE III/IV, pediatric scores (PRISM/PIM), or other sepsis scores like SOFA/qSOFA — use the appropriate score calculator for those needs.

Frequently asked questions (FAQ)

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APACHE II Score Calculator

1 · Physiologic variables (worst values, first 24 hours)

2 · Age & chronic health

Age

Chronic health

Example case

3 · Results

Per-variable breakdown

Mortality estimate table (Knaus — population context)

Frequently asked questions (FAQ)

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