APACHE II Score

How critically ill is this patient, and what is the risk of dying during this ICU admission? The APACHE II score has been answering that question in intensive care units worldwide since Knaus et al. published it in 1985. The calculator for APACHE II takes the three component scores and computes the total, which maps to a predicted in-hospital mortality probability via a validated logistic regression equation.

APACHE II Score Structure: Three Components

The APACHE II score (maximum 71 points) consists of three components:

• Acute Physiology Score (APS, 0–60 points): the sum of points assigned to 12 physiological variables measured during the first 24 hours in the ICU — temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, arterial pH, serum sodium, serum potassium, serum creatinine, hematocrit, white blood cell count, and Glasgow Coma Scale. Each variable receives 0–4 points based on deviation from normal; the worst value in the first 24 hours is used.
• Age points (0–6): ≤44 years = 0; 45–54 = 2; 55–64 = 3; 65–74 = 5; ≥75 = 6
• Chronic Health Points (2 or 5): 2 points for elective post-operative patients with severe organ insufficiency or immunocompromise; 5 points for non-operative or emergency post-operative patients with the same conditions

Use this online calculator once the APS, age, and chronic health components are calculated. The SOFA score calculator provides the alternative severity assessment preferred in current sepsis guidelines.

Predicted Mortality: The APACHE II Equation

In Knaus et al. (1985), the predicted in-hospital mortality R is derived from the logistic regression:

ln(R/(1−R)) = −3.517 + (APACHE II score × 0.146) + diagnostic category weight

Mortality by approximate score range (without diagnostic category adjustment):

• Score 0–9: predicted mortality approximately 4%
• Score 10–14: approximately 8%
• Score 15–19: approximately 15%
• Score 20–24: approximately 25%
• Score 25–29: approximately 40%
• Score 30–34: approximately 55%
• Score ≥35: approximately 73%+

Limitations and Appropriate Use

APACHE II was validated on an ICU population from the mid-1980s — before widespread use of lung-protective ventilation, modern vasopressors, and evidence-based sepsis protocols. Predicted mortality from the original equation systematically overestimates mortality in modern ICUs. Contemporary use should compare APACHE II-predicted vs. observed mortality as a performance benchmark rather than as an individual patient prognosis tool. For individual patients, APACHE II should inform clinical decision-making alongside bedside assessment, not replace it. The qSOFA calculator provides a simpler 3-variable score for rapid sepsis risk stratification outside the ICU. The sepsis and critical care calculators cover the full suite of ICU severity scoring tools.

Differences from APACHE III and IV

APACHE III (1991) and APACHE IV (2006) added variables, expanded the diagnostic category list, and recalibrated mortality predictions against larger modern datasets. APACHE III adds arterial blood urea nitrogen, serum albumin, bilirubin, blood glucose, and urine output, achieving better discrimination (AUROC ≈ 0.88) than APACHE II (AUROC ≈ 0.84). APACHE IV further improved calibration using contemporary ICU populations. Despite these advances, APACHE II remains the most widely cited version in published research due to its 40-year literature base, enabling cross-study comparison of patient severity across decades of critical care research.