ICU severity of illness — acute physiology + age + chronic health, 0 to 71
Vital signs (worst in first 24 h)
Core temperature
Mean arterial pressure (mmHg)
Heart rate (beats/min)
Respiratory rate (breaths/min)
Laboratory values
Arterial pH (arterial blood gas required — no serum HCO₃⁻ substitute in this tool)
Serum sodium (mmol/L)
Serum potassium (mmol/L)
Serum creatinine
Haematocrit (%)
White cell count (×10³/mm³)
Oxygenation
FiO₂ ≥ 0.5 (use A–a gradient instead of PaO₂)
PaO₂
Neurological and demographic
Glasgow Coma Scale (3–15)
Age (years)
Acute renal failure (doubles the creatinine points)
Chronic health status
No severe organ insufficiency or immunocompromise
Severe organ insufficiency / immunocompromised — non-operative or emergency post-op
Severe organ insufficiency / immunocompromised — elective post-op
Enter every physiological value, GCS and age
Evidence
Original publication
Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Derived from 5 815 ICU admissions; the score (acute physiology + age + chronic health, 0–71) was closely correlated with subsequent in-hospital death, with risk further modified by the principal diagnostic category.
Critical Care Medicine, 1985
Open source
Guideline
APACHE II is calculated from the worst values in the first 24 h of ICU admission and is widely used for cohort severity adjustment and research. It is not designed to guide individual treatment decisions; newer models (APACHE IV, SAPS 3, APACHE II with diagnosis-specific coefficients) give better-calibrated individual estimates.
Last reviewed: 2026-09
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MedScore is an educational and clinical decision-support tool. It does not provide a diagnosis, a treatment, or a drug-dosing recommendation, and it does not replace clinical judgement or local guidelines. Confirm every result against the original source before acting on it. The treating clinician remains responsible for all decisions.