Acute Physiology and Chronic Health Evaluation II (Knaus 1985). ICU severity / mortality.

What it is and when to use it

APACHE II (Acute Physiology and Chronic Health Evaluation II) is a severity-of-disease classification system that estimates the risk of in-hospital mortality in adult patients admitted to intensive care units. It is calculated from 12 acute physiological variables recorded during the first 24 hours of ICU admission (including temperature, mean arterial pressure, heart and respiratory rate, oxygenation, arterial pH, sodium, potassium, creatinine, haematocrit, white blood cell count and the Glasgow Coma Scale), together with age and chronic health status. The total score ranges from 0 to 71 points. It is one of the reference tools described in the critical-care literature and is widely used for severity stratification and for comparing case mix across ICUs.

How to interpret it

The total score combines the acute physiology subtotal, age points (0 to 6 according to age band) and points for severe chronic illness or immunocompromise (2 points if admission is elective postoperative, 5 points if non-operative or emergency postoperative). The higher the score, the greater the risk of in-hospital death: as a guide, low scores (roughly 0-9) are associated with an estimated mortality below 10%, whereas high scores (above 25-30) are associated with mortality exceeding 50-75%. Predicted mortality is derived from a logistic equation that incorporates the APACHE II score and a diagnosis-specific coefficient; the mortality percentage therefore depends on both the number of points and the admission diagnostic category.

Limitations and when not to use it

APACHE II was validated in adult ICU patients and does not apply to paediatric patients, pregnant women, burns patients or post-cardiac-surgery patients, for whom specific models are used. The score should be calculated from the worst values of the first 24 hours of ICU admission and is not designed for repeated calculation or daily monitoring of progress. It is a population-level prediction tool for severity stratification and cohort comparison, not an instrument for determining prognosis or limiting treatment in an individual patient. It does not assess response to treatment over the course of admission, nor does it track organ dysfunction dynamically as other scores do (e.g. SOFA).

Frequently asked questions

What is the difference between APACHE II and SOFA?

APACHE II is a mortality-prediction model calculated once from the first 24 hours of data, whereas SOFA quantifies and tracks daily dysfunction across six organ systems during admission. They are complementary: APACHE II for initial stratification and SOFA for following the clinical course.

When should APACHE II be calculated?

It should be calculated using the most deranged physiological values recorded during the first 24 hours of ICU admission; it is not recalculated daily.

Does a high APACHE II score mean the patient will die?

No. APACHE II estimates a probability of mortality at the population level, not an individual outcome; patients with high scores can survive, and it should not be used in isolation to limit treatment.