What is the APACHE II score?

APACHE II (Acute Physiology and Chronic Health Evaluation II) is a severity-of-disease scoring system used in ICUs. It uses 12 physiologic variables, age, and chronic health status to generate a score from 0 to 71. Higher scores correlate with higher predicted hospital mortality.

When should APACHE II be calculated?

APACHE II should be calculated within the first 24 hours of ICU admission using the worst values recorded. It is used for ICU benchmarking, risk stratification, and research. It should not be used for individual patient prognosis or to make care limitation decisions.

What is a high APACHE II score?

APACHE II scores above 25 are associated with predicted mortality rates exceeding 50%. Scores of 0–9 correlate with approximately 4% mortality, 10–14 with 15%, 15–19 with 25%, 20–24 with 40%, 25–29 with 55%, and 30–34 with 75%. These rates vary by diagnosis.

What are the limitations of APACHE II?

APACHE II was developed in 1985 and may not reflect modern ICU outcomes. It does not account for lead-time bias, treatment effects before ICU admission, or ICU readmissions. It performs less well for specific populations such as cardiac surgery and burns. Newer versions (APACHE III, IV) and SOFA score may be more appropriate in certain contexts.

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APACHE II Score

Acute Physiology and Chronic Health Evaluation II — ICU mortality risk prediction using worst values in first 24 hours.

Physiologic Variables (worst values in first 24h of ICU)

Temperature (°C)

Mean Arterial Pressure (mmHg)

Heart Rate (bpm)

Respiratory Rate (breaths/min)

FiO₂ (%) If FiO₂ ≥ 50%, enter A-a gradient. If < 50%, enter PaO₂.

A-a gradient (if FiO₂≥50%) or PaO₂ (if FiO₂<50%) (mmHg)

Arterial pH

Serum Sodium (mEq/L)

Serum Potassium (mEq/L)

Serum Creatinine (mg/dL)

Acute renal failure?

Hematocrit (%)

White Blood Cell Count (×10³/µL)

Glasgow Coma Scale (GCS) score

Age

Age (years)

Chronic Health (if severe organ insufficiency or immunocompromised)

Chronic Health Points

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Enter values and click Calculate.

0–4~4% mortality

5–9~8% mortality

10–14~15% mortality

15–19~25% mortality

20–24~40% mortality

25–29~55% mortality

30–34~75% mortality

≥35~85% mortality

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

About This Tool

What Is APACHE II?

The Acute Physiology and Chronic Health Evaluation II (APACHE II) is a severity-of-disease scoring system developed by Knaus et al. in 1985. It remains one of the most widely used ICU prognostic tools worldwide. The score comprises three components: the Acute Physiology Score (APS) based on 12 physiologic variables, an age adjustment, and chronic health points for patients with severe organ insufficiency or immunocompromised states. The total score ranges from 0 to 71, with higher scores indicating greater severity of illness and higher predicted mortality.

How It Is Calculated

Each of the 12 physiologic variables (temperature, MAP, heart rate, respiratory rate, oxygenation, arterial pH, sodium, potassium, creatinine, hematocrit, WBC, and GCS) is assigned 0 to 4 points based on deviation from normal, using the worst values recorded during the first 24 hours of ICU admission. Age contributes 0 to 6 points, and chronic health status adds 2 points (elective postoperative) or 5 points (emergency postoperative or nonoperative) for patients with severe chronic organ insufficiency or immunosuppression.

Clinical Utility and Limitations

APACHE II is primarily used for ICU quality benchmarking, clinical research stratification, and family prognostic discussions. It should not be used in isolation for individual patient treatment decisions, as mortality predictions are population-based estimates. The original validation cohort was from the early 1980s, and case-mix and ICU practices have evolved considerably since then. Newer iterations (APACHE III, IV) and competing scores (SAPS II, SOFA) address some limitations but are less widely adopted in routine clinical practice.

🔑 Clinical Pearls

Always use the most abnormal value in the first 24 hours — not the admission value or the most recent value.
If a lab value was not measured, assume it is normal (0 points) per the original methodology.
APACHE II was validated for hospital mortality prediction, not long-term outcomes.
The score performs best at population level — avoid using it to make withdrawal-of-care decisions for individual patients.

Key References

Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med. 1985;13(10):818-829.
Bouch DC, Thompson JP. Severity scoring systems in the critically ill. Continuing Education in Anaesthesia Critical Care & Pain. 2008;8(5):181-185.

Formula last verified: February 2026