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APACHE II

ICU severity of illness from the worst values in the first 24 hours

Sources last checked: 2026-10-13

APACHE II

ICU severity of illness from the worst values in the first 24 hours

Temperature (rectal/core)
Mean arterial pressure
Heart rate
Respiratory rate (ventilated or not)
Oxygenation
Arterial pH (or venous HCO₃⁻ if no blood gas)
Serum sodium
Serum potassium
Serum creatinine
Haematocrit
White cell count
Age
Severe organ insufficiency or immunocompromise (chronic health)
Fill in every field to see the result.

Use the worst value of each variable in the first 24 h of ICU admission. Chronic health points require organ insufficiency or immunocompromise documented BEFORE admission (biopsy-proven cirrhosis with portal hypertension, NYHA IV, severe chronic respiratory disease, chronic dialysis, or immunosuppression). Not validated for burns, cardiac surgery or children; calibration drifts in modern ICU cohorts.

Source: Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med 1985;13(10):818-829.Open

Interpretation

0–9Lower severity. Predicted death rate rises steeply with each band; individual risk needs the diagnostic-category equation from the original paper.
10–19Moderate severity of illness.
20–29High severity of illness.
≥ 30Very high severity of illness.

What is the APACHE II?

APACHE II (Acute Physiology and Chronic Health Evaluation II) scores the severity of illness of adult ICU patients from 12 physiological variables measured as the worst value in the first 24 hours, plus age and chronic health points. Knaus and colleagues developed it in 1985 from 5815 ICU admissions in 13 hospitals, and it remains one of the most widely reported severity scores in trials and ICU audit.

When to use it

  • Describing case mix and severity in ICU audit, research and trial reports
  • Benchmarking observed versus expected mortality across units (with the diagnostic-category equation)
  • Teaching the components of acute physiological derangement

When not to use it

  • To make triage, withdrawal or individual treatment decisions — it was built for groups, not individuals
  • Children, burns and patients after cardiac surgery (excluded or poorly calibrated)
  • Values outside the first 24 hours of ICU admission — the score is defined on that window

How it is calculated

APACHE II = Acute Physiology Score (12 variables, worst value in first 24 h; creatinine points doubled in acute renal failure; GCS scores 15 − GCS) + age points + chronic health points. Range 0–71.

Clinical pearls and limitations

  • Use the worst value of each variable in the first 24 h of ICU admission. Chronic health points require organ insufficiency or immunocompromise documented BEFORE admission (biopsy-proven cirrhosis with portal hypertension, NYHA IV, severe chronic respiratory disease, chronic dialysis, or immunosuppression). Not validated for burns, cardiac surgery or children; calibration drifts in modern ICU cohorts.
  • Creatinine points are doubled in acute renal failure — the most commonly missed rule.
  • For FiO₂ ≥ 0.5 score the A–a gradient; below 0.5 score the PaO₂.
  • Use the venous bicarbonate row only when no arterial blood gas is available.
  • In sedated patients, score the GCS as assessed before sedation or as best assessable — practice varies, so document your approach.
  • Calibration has drifted in modern European ICUs; APACHE II tends to overestimate mortality today.

Frequently asked questions

What is the maximum APACHE II score?

71 points, although scores above about 55 are rarely seen.

Does APACHE II give a mortality percentage?

Only with the original logistic equation, which adds a weight for the main diagnostic category and emergency surgery. The raw score alone does not give an individual probability.

Which values do I use?

The worst (most deranged) value of each physiological variable during the first 24 hours after ICU admission.

What counts as chronic health for APACHE II?

Organ insufficiency or immunocompromise present before admission: biopsy-proven cirrhosis with portal hypertension, NYHA class IV heart failure, severe chronic respiratory disease, chronic dialysis, or immunosuppression.

Evidence and guidelines

  1. Primary source: Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med 1985;13(10):818-829.
  2. Point table reproduced in: Mean SOFA score in comparison with APACHE II score in predicting mortality in surgical patients with sepsis. Cureus 2023;15(3):e36653 (Appendix, Table 7).
  3. Point table reproduced in: Assessment of postoperative complications using E-PASS and APACHE II. Patient Saf Surg 2018;12:3.

Have a follow-up question?

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