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APACHE II Score

APACHE II grades ICU illness severity (0–71) from the worst values in the first 24 hours of 12 physiologic variables, plus age and chronic health points. Higher scores mean higher hospital mortality; scores ≥25 indicate very severe illness.

  • Age (years)
  • Temperature (rectal/core) (°C or °F)
  • Mean arterial pressure (mmHg or kPa)
  • Heart rate (/min)
  • Respiratory rate (/min)
  • FiO₂ (% or fraction) (%)
  • PaO₂ (arterial) (mmHg or kPa)
  • PaCO₂ (needed if FiO₂ ≥50%) (mmHg or kPa)
  • Arterial pH
  • Sodium (mmol/L or mEq/L)
  • Potassium (mmol/L or mEq/L)
  • Serum creatinine (mg/dL or µmol/L)
  • Acute renal failure (yes/no)
  • Hematocrit (%)
  • WBC (×10⁹/L or ×10³/µL)
  • Glasgow Coma Scale
  • Chronic health (severe organ insufficiency or immunocompromise): None (0); Yes: non-operative or emergency post-op (5); Yes: elective post-op (2)
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Interpretation

ResultMeaning
0–9Lower severity
10–19Moderate severity
20–29High severity
≥30Very high severity

Formula

APACHE II = Acute Physiology Score (12 variables, worst value in first 24 h: temperature, MAP, HR, RR, oxygenation [A–a gradient if FiO₂ ≥0.5, else PaO₂], arterial pH, Na, K, creatinine [double points in acute renal failure], hematocrit, WBC, 15 − GCS) + age points (≤44 0, 45–54 2, 55–64 3, 65–74 5, ≥75 6) + chronic health points (5 non-operative/emergency post-op, 2 elective post-op). A–a = FiO₂ × 713 − PaCO₂/0.8 − PaO₂.

Clinical pearls

  • Use the WORST value in the first 24 hours of ICU admission for each variable.
  • Chronic health points apply only if severe organ insufficiency (biopsy-proven cirrhosis, NYHA IV, severe COPD, chronic dialysis) or immunocompromise was present BEFORE admission.
  • Validated for ICU populations at admission; not for serial tracking or individual triage decisions.

Examples

apache ii 67 yo temp 39.2 map 65 hr 115 rr 28 fio2 40% pao2 65 ph 7.30 na 132 k 3.2 cr 1.6 hct 35 wbc 18 gcs 13apache 2 for 80 year old elective post-op, AKI with creatinine 2.5, otherwise normal vitals and labs, FiO2 21%, PaO2 90, GCS 15

FAQ

What is a high APACHE II score?

Scores above about 25 indicate very severe illness with high predicted hospital mortality; the maximum is 71.

Which oxygenation value does APACHE II use?

If FiO₂ ≥0.5, the A–a oxygen gradient is scored; if FiO₂ <0.5, the PaO₂ is scored.

Evidence

  1. Knaus WA et al. APACHE II: a severity of disease classification system. Crit Care Med 1985;13:818–29.

Pulmonary & Critical Care · Reviewed 2026-10-14 · Methodology