What does the APACHE II score measure?

It measures severity of illness in critically ill adults during the first 24 hours of ICU admission. The total ranges from 0 to 71 and correlates with risk of in-hospital death, making it a benchmark for comparing case mix and outcomes between units.

Which variables go into the score?

Twelve acute physiologic variables: temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, arterial pH, sodium, potassium, creatinine, haematocrit, white cell count, and the Glasgow Coma Scale. Age points and a chronic health adjustment are added on top.

How is oxygenation scored?

When the inspired oxygen fraction is 0.5 or higher, the alveolar-arterial gradient is scored. When it is below 0.5, the arterial PaO2 is scored instead. The tool switches automatically based on the FiO2 you enter.

Why does acute renal failure double the creatinine points?

The original APACHE II scoring weights renal dysfunction more heavily when it is acute. Ticking the acute renal failure box doubles the points assigned to creatinine, reflecting its stronger association with mortality in that setting.

How accurate is the mortality estimate?

The mortality bands are approximate and were derived from a 1985 cohort. Actual risk also depends on the admitting diagnosis, which APACHE II adjusts for with a diagnostic weight not included in this simplified calculator. Treat the output as a severity benchmark, not a precise prediction.

APACHE II Score Calculator

APACHE II (Acute Physiology and Chronic Health Evaluation II) is the most widely used severity-of-illness score in intensive care. By converting twelve physiologic measurements, the patient’s age, and their chronic health into a single number, it lets clinicians benchmark how sick a patient is and how that compares across units and studies.

How it works

The total has three parts:

APACHE II = Acute Physiology Score
          + Age points
          + Chronic Health points

Each of the 12 acute variables is scored 0 to 4 by how far it deviates from normal, using the worst value in the first 24 hours. The Glasgow Coma Scale contributes 15 - GCS points. The oxygenation term uses the A-a gradient when FiO2 is at least 0.5 and the PaO2 otherwise, and creatinine points are doubled in acute renal failure.

Age adds 0 to 6 points, and chronic severe organ insufficiency or immunocompromise adds 2 points for elective post-operative patients or 5 points otherwise.

Example and notes

A 60-year-old (3 age points) with a moderately deranged physiology score of 12 and no chronic illness totals 15, placing them in a roughly 25 percent predicted mortality band. Enter the single worst value of each variable across the first ICU day, and remember the mortality figures are population estimates that also depend on the admitting diagnosis.