APACHE II

Why use it

Gold standard for ICU severity benchmarking. Predicts in-hospital mortality risk. Required for many clinical trials and quality metrics. Enables case-mix comparison across ICUs.

About

APACHE II (Acute Physiology and Chronic Health Evaluation II) is the most widely used ICU severity scoring system worldwide. It uses 12 acute physiological variables (worst values in first 24h of ICU), age points, and chronic health points to estimate in-hospital mortality risk. The score ranges from 0-71, with higher scores indicating greater severity.

When to use

• Within first 24 hours of ICU admission
• ICU quality benchmarking and research
• Prognostication and goals of care discussions
• Clinical trial enrollment criteria

Formula

Pearls & pitfalls

• Use WORST values from the first 24 hours of ICU admission.
• GCS is inverted: APACHE GCS points = 15 - actual GCS.
• Chronic health points: +5 for non-elective surgical/medical patients with severe organ insufficiency; +2 for elective surgical.
• APACHE II was calibrated in 1985 — modern ICU mortality may be lower at equivalent scores.
• Does NOT account for diagnosis — same score may have different mortality depending on underlying condition.
• APACHE III and IV exist but APACHE II remains the most commonly referenced version.

Next steps

Score 0-9: ~4% mortality. Score 10-14: ~8%. Score 15-19: ~12%. Score 20-24: ~30%. Score 25-29: ~50%. Score 30-34: ~73%. Score ≥ 35: ~85%. Use for prognostic discussions but NEVER as sole basis for treatment decisions.

Evidence

Developed by Knaus et al. (Crit Care Med, 1985) from 5,815 ICU admissions at 13 hospitals. Extensively validated worldwide. Standard ICU severity metric for > 35 years despite known limitations.

References

1. Knaus WA et al. APACHE II: A severity of disease classification system. Crit Care Med. 1985;13(10):818-829. — link