APACHE II is a severity-of-disease classification system applied within 24 hours of ICU admission. It uses 12 physiological measurements, age, and chronic health status to predict hospital mortality. Scores range from 0 to 71, with higher scores indicating greater severity.

It is primarily used for: ICU benchmarking and quality assessment, stratifying patients in clinical trials, guiding goals-of-care discussions, and comparing case-mix severity across ICUs. It is not intended as the sole determinant of treatment decisions for individual patients.

Use the worst (most abnormal) values during the first 24 hours of ICU admission. If a lab test was not performed, use normal values. The GCS should be assessed before sedation when possible.

Chronic health points (2 or 5) are given for severe organ insufficiency or immunocompromised state documented prior to admission: liver (cirrhosis, portal HTN), cardiovascular (NYHA IV), respiratory (chronic restrictive/obstructive/vascular disease), renal (chronic dialysis), or immunocompromised.