The APACHE II score ranges from 0 to 71, with higher scores indicating greater severity of illness and higher predicted hospital mortality. A score of 0–9 generally corresponds to a predicted mortality of less than 10%. Scores of 10–19 correspond to approximately 10–25% mortality. Scores of 20–29 suggest mortality in the range of 40–50%, and scores above 30 carry predicted mortality rates exceeding 50%, often reaching 70–80% or higher.

The score is most meaningful when interpreted in the context of the patient's specific diagnosis, as mortality predictions vary significantly by disease category. For example, the same APACHE II score may carry different prognostic implications in a post-surgical patient compared to a patient with septic shock. The score should be used alongside clinical judgment, not as a sole determinant of care decisions.

The APACHE II score should be calculated within the first 24 hours of ICU admission using the worst (most abnormal) physiological values recorded during that period. It is primarily used for ICU quality benchmarking, comparing case-mix severity across units, and stratifying patients in clinical research trials.

Clinicians also use the APACHE II score to support goals-of-care discussions with patients and families, providing an objective framework for communicating illness severity. It can help guide resource allocation decisions and is sometimes used as an inclusion criterion in clinical trials to ensure comparable disease severity between treatment groups.

APACHE II was developed and validated in 1985 using data from 5,815 ICU admissions. Advances in critical care since then — including early goal-directed therapy, lung-protective ventilation, and modern sepsis management — mean that actual mortality rates for a given APACHE II score are often lower than the original predictions. More recent scoring systems (APACHE III, IV, [SOFA](/tools/sofa-score), SAPS) may provide better calibration for current ICU populations.

The score uses the worst values in the first 24 hours, which means it cannot be calculated at the time of ICU admission and does not capture clinical trajectory or response to treatment. Patients who deteriorate after the first 24 hours will not have this reflected in their APACHE II score. Additionally, the chronic health component relies on subjective assessment of pre-existing organ insufficiency, introducing some variability between assessors.

The score was not designed to guide individual treatment decisions (such as whether to withdraw care) and should never be used as the sole basis for such decisions. It performs best as a tool for population-level risk stratification and quality assessment.