APACHE II Calculator

About the APACHE II Score

The Acute Physiology and Chronic Health Evaluation II (APACHE II) was published by Knaus et al. in 1985 and remains one of the most widely validated and used ICU scoring systems in the world. It estimates the probability of in-hospital mortality based on 12 acute physiological variables, patient age, and chronic health status, assessed within the first 24 hours of ICU admission. The original derivation study is indexed on PubMed.

APACHE II Predicted Mortality Reference

APACHE II Score	Non-Surgical ICU Mortality	Post-Emergency Surgery
0–4	~4%	~1%
5–9	~8%	~3%
10–14	~15%	~7%
15–19	~25%	~12%
20–24	~40%	~30%
25–29	~55%	~35%
30–34	~73%	~73%
≥35	~85%	~88%

A newer, more accurate successor, APACHE IV, was derived from a larger and more contemporary cohort of over 110,000 ICU admissions, but it requires a diagnostic-category lookup that most simplified online calculators, including this one, do not implement.

Limitations

Two of the twelve physiological variables, serum creatinine and urine output patterns, overlap directly with renal function assessment; a patient's Creatinine Clearance is often already being tracked in parallel and can help contextualise whether an elevated creatinine score reflects acute injury or chronic baseline disease.

• APACHE II was derived from 1979–1982 ICU data, case mix, treatments, and outcomes have changed significantly since then.
• It is designed for group mortality prediction (populations), not individual patient prognosis.
• Diagnosis-specific mortality may differ substantially from the overall predicted mortality.
• Other systems (APACHE III, APACHE IV, SOFA, SAPS II) may be more accurate for specific populations or contemporary practice.
• APACHE II should never be used as a basis for withdrawal of care decisions for individual patients.

Managing high-severity patients

Patients scoring in the higher severity bands frequently require vasopressor infusions and closely titrated IV fluids; our IV Drip Rate Calculator covers the drop-rate and mcg/kg/min conversions used at the bedside for these infusions.

Frequently Asked Questions

When should APACHE II be calculated?+

APACHE II uses the worst value for each physiological variable recorded during the first 24 hours of ICU admission. Using values from later in the stay, or averaging values across the day, does not match the original derivation methodology and will produce an inaccurate score.

Can APACHE II be used to decide whether to withdraw care?+

No. APACHE II was derived to predict mortality across groups of similar patients, not to forecast the outcome of a specific individual. It should never be used alone as the basis for withholding or withdrawing treatment. Those decisions require full clinical assessment, discussion with the patient or family, and multidisciplinary input.

What is the maximum possible APACHE II score?+

The maximum is 71 points: up to 60 from the Acute Physiology Score (12 variables scored 0-4 each, plus the Glasgow Coma Scale component), up to 6 from age, and up to 5 from chronic health status. In practice, scores above 40 are unusual, and higher scores correspond to progressively higher predicted mortality.

Related Tools on MediCalc Pro

For neurological assessment in ICU patients, see Glasgow Coma Scale. For pneumonia severity and ICU admission criteria, see CURB-65 Score.

References

• Knaus WA, et al. "APACHE II: A severity of disease classification system." Crit Care Med. 1985;13(10):818-829.
• Zimmerman JE, et al. "Acute Physiology and Chronic Health Evaluation (APACHE) IV: Hospital mortality assessment for today's critically ill patients." Crit Care Med. 2006;34(5):1297-1310.