APACHEscores

APACHE scores, or Acute Physiology and Chronic Health Evaluation scores, are a family of severity-of-disease classification systems used in intensive care units to estimate the risk of in-hospital death and to quantify a patient’s acute illness burden. They are designed to aid clinical decision-making, resource planning, and research benchmarking.

APACHE II is the best known and historically most widely used version. It was introduced in 1985 by Knaus and colleagues at the University of Alberta, followed by APACHE III in the early 1990s and APACHE IV in the 2000s. APACHE II remains widely cited, though many centers have migrated to APACHE IV for improved discrimination.

APACHE II uses 12 acute physiological variables collected within the first 24 hours of ICU admission. These include body temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation (PaO2 or the A-a gradient with FiO2), arterial pH, serum sodium, potassium, creatinine, hematocrit, white blood cell count, and the Glasgow Coma Scale score. In addition, points are assigned for age and for chronic health conditions. The total APACHE II score ranges from 0 to 71, with higher scores indicating more severe illness. The predicted mortality is estimated using a logistic regression equation tied to the score and other factors.

APACHE IV expands the model with additional variables, including more detailed demographics and comorbidity information, and generally provides improved mortality discrimination. Both systems require data within the first 24 hours in the ICU and are recommended to supplement, not replace, clinician judgment. They are not universally applicable to pediatric patients or all hospital settings, and calibration can vary by population.

In practice, APACHE scores are used for benchmarking, outcome research, and to guide discussions about prognosis and resource needs in critical care.