Critical Care
APACHE II Score
ICU severity of illness and mortality risk.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The APACHE II (Acute Physiology and Chronic Health Evaluation II) score quantifies severity of illness in critically ill adults and is used to estimate hospital mortality and to benchmark intensive-care performance. It is calculated within the first 24 hours of ICU admission from the worst values of twelve acute physiological variables (such as temperature, mean arterial pressure, heart and respiratory rate, oxygenation, pH, electrolytes, creatinine, haematocrit, white cell count and the Glasgow Coma Scale), plus points for age and for chronic health problems. The summed score ranges up to 71 and, with a diagnostic weighting, generates a predicted mortality.
Interpreting the result
Higher scores indicate greater severity and higher predicted mortality. Broadly, lower scores (under about 10) carry a lower mortality, intermediate scores (around 10–19) a moderate mortality, and scores of 20 or more a substantially higher mortality risk. The score is a population-level prognostic and benchmarking tool; it estimates risk for groups and should not be used to dictate the prognosis or treatment of any individual patient.
Worked example
A 70-year-old (age 5 points) admitted after emergency surgery with a deranged physiology contributing 14 acute points and 5 chronic-health points scores 24. A score above 20 indicates high severity and a high predicted mortality, used for risk stratification rather than individual prognostication.
Critical actions
Use the WORST value in the first 24h. Creatinine points double in acute renal failure (select the higher band manually). Oxygenation uses A-a gradient if FiO₂ ≥0.5, otherwise PaO₂ (mmHg).
Pearls / pitfalls
- Use the worst value of each variable from the first 24 hours of ICU admission.
- Creatinine points are doubled in acute renal failure — select the higher band manually.
- Oxygenation uses the A–a gradient when FiO₂ is 0.5 or higher, and the PaO₂ (in mmHg) otherwise.
- It is a group-level prognostic and audit tool, not an individual death predictor or a treatment-limitation rule.
Evidence & validation
Developed by Knaus and colleagues from a large multicentre ICU database, APACHE II became the most widely used general ICU severity score and remains a standard reference for case-mix adjustment and research, despite the existence of later systems such as APACHE IV, SAPS and SOFA.
Frequently asked questions
When should APACHE II be calculated?
It is calculated within the first 24 hours of ICU admission, using the worst recorded value of each variable during that period. Values recorded later are not used for the standard score.
Can APACHE II predict an individual patient's outcome?
No. It estimates mortality risk for groups of similar patients and is used for benchmarking and research. It should not be used to predict the outcome of, or limit treatment for, any individual.
How is acute renal failure handled?
The points assigned for the serum creatinine are doubled when there is acute renal failure. This needs to be applied manually, as it can substantially increase the total score.
Is APACHE II still relevant given newer scores?
Yes. Although APACHE IV, SAPS and SOFA exist, APACHE II remains widely used for case-mix adjustment, audit and research because of its long track record and familiarity.
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.
- Vincent JL, Moreno R. Clinical review: scoring systems in the critically ill. Crit Care. 2010;14:207.
About the creator
Knaus WA et al.
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