When should APACHE II be calculated?

APACHE II should be calculated using the worst values from the first 24 hours of ICU admission. It was designed as an admission severity score and is less reliable when calculated at other time points.

What is a high APACHE II score?

Scores above 25 are considered very high and correspond to mortality rates exceeding 50%. Scores above 35 are associated with mortality rates above 80%. However, individual outcomes vary significantly.

Is APACHE II still used today?

Yes, APACHE II remains widely used despite newer versions (APACHE III, IV). Its simplicity, extensive validation across diverse populations, and free availability (unlike proprietary newer versions) contribute to its continued relevance.

How does APACHE II compare to SOFA?

APACHE II predicts overall mortality at ICU admission, while SOFA (Sequential Organ Failure Assessment) tracks organ dysfunction over time. SOFA is better for monitoring disease trajectory, while APACHE II provides a baseline severity assessment.

Can APACHE II be used for individual patient prognosis?

APACHE II provides population-level mortality estimates and should not be used for individual patient prognostication or decisions about treatment withdrawal. Individual patients may do much better or worse than predicted.

Does APACHE II account for the admitting diagnosis?

The original APACHE II system includes diagnostic category-specific mortality coefficients, but many implementations (including this calculator) use the score alone. Adding diagnostic weighting improves accuracy for specific conditions.

APACHE II Score Calculator

Calculate the APACHE II score to estimate ICU mortality risk. Uses acute physiology, age, and chronic health status to predict in-hospital death probability.

⚠️ Medical Disclaimer: This is a simplified APACHE II calculator for educational purposes. ICU scoring should use institutional tools with full lab data.

Age

years

Temperature

°C

Mean Arterial Pressure

mmHg

Heart Rate

bpm

Respiratory Rate

/min

FiO₂

PaO₂

mmHg

GCS

Post-Operative Status

Severe Chronic Health Problem

APACHE II Score3

Predicted In-Hospital Mortality

~4%

APACHE II Score: 3

Total APACHE II Score

Acute physiology + age + chronic health

Predicted Mortality

~4%

Approximate in-hospital mortality rate

APS Points

Acute Physiology Score component

Age Points

Age 55 years

GCS Points

GCS 15 → 0 points

Chronic Health Points

Chronic health + surgical status

APACHE II Score	Approx. Mortality	Severity
0-4	~4%	Low
5-9	~8%	Low-Moderate
10-14	~15%	Moderate
15-19	~25%	Moderate-High
20-24	~40%	High
25-29	~55%	Very High
30-34	~73%	Critical
35+	~85%	Extreme

Planning notes, formulas, and examples

About the APACHE II Score Calculator

The APACHE II (Acute Physiology and Chronic Health Evaluation II) Score Calculator estimates in-hospital mortality for critically ill patients admitted to the intensive care unit. Developed by Knaus et al. in 1985, it remains one of the most widely validated and used ICU severity scoring systems worldwide.

APACHE II combines three components: the Acute Physiology Score (APS) based on 12 physiologic variables measured within the first 24 hours of ICU admission, an age adjustment, and a chronic health evaluation. The total score ranges from 0 to 71, with higher scores indicating greater severity of illness and higher predicted mortality.

This calculator uses a simplified version focused on key physiologic parameters, age points, and chronic health status to produce an approximate mortality estimate for benchmarking, research, and severity discussions. It is most reliable when the first-24-hour ICU values are collected consistently and interpreted alongside the full clinical picture.

When This Page Helps

APACHE II provides an objective, standardized measure of illness severity that supports clinical decision-making, prognostic communication with families, quality benchmarking across ICUs, and research stratification. By quantifying disease severity, it helps clinicians set appropriate expectations and allocate resources effectively.

The score also facilitates comparison of patient populations across different studies and institutions, making it invaluable for critical care research and quality improvement programs.

How to Use the Inputs

Collect the worst physiologic values from the first 24 hours of ICU admission.
Enter temperature, mean arterial pressure, heart rate, and respiratory rate.
Enter the GCS score (lowest in 24 hours).
Enter FiO₂ and PaO₂ values for oxygenation assessment.
Select the post-operative status if applicable.
Indicate whether the patient has severe chronic health conditions.
Review the total score and corresponding mortality estimate.

Formula used

APACHE II = Acute Physiology Score + Age Points + Chronic Health Points

Age Points: <45=0, 45-54=2, 55-64=3, 65-74=5, ≥75=6
Chronic Health Points: 0-5 depending on organ insufficiency and surgical status
APS: Sum of individual physiologic variable points (0-4 each)

Total range: 0-71

Example Calculation

Result: APACHE II = 14, Predicted Mortality ~15%

A 65-year-old patient with the given vital signs has moderate physiologic derangements. Age contributes 5 points, and the total score of 14 corresponds to approximately 15% in-hospital mortality.

Tips & Best Practices

Use the worst (most abnormal) values from the first 24 hours, not the most recent values.
For GCS in sedated patients, use the pre-sedation score if available.
APACHE II was validated for adults only — do not use for pediatric patients.
Consider APACHE II alongside clinical judgment, not as a replacement for it.
Serial APACHE II scores are less validated than the initial admission score.
Remember that APACHE II may underperform in specific diagnostic groups like trauma or burns.

Evolution of APACHE Scoring

The original APACHE system (1981) used 34 physiologic variables and proved impractical for routine use. APACHE II simplified this to 12 variables and gained widespread adoption. APACHE III (1991) and APACHE IV (2006) improved calibration but are proprietary, limiting their accessibility. APACHE II remains the most commonly cited version in medical literature.

Limitations

APACHE II has known limitations: it was developed in 1985 and may not reflect modern ICU outcomes after improvements in critical care; it performs variably across different diagnostic groups; and it does not account for interventions received before ICU admission or treatment limitations in place.

Quality Improvement Applications

ICUs commonly use APACHE II for benchmarking by comparing observed mortality to APACHE II-predicted mortality. A standardized mortality ratio (SMR) less than 1.0 suggests the ICU is performing better than expected, while an SMR greater than 1.0 may indicate opportunities for improvement.

Sources & Methodology

Last updated: March 26, 2026

Methodology

This page estimates APACHE II from the variables collected in the calculator, combining an acute physiology subtotal with age points and chronic-health points into an admission-severity score. Because this implementation uses a reduced variable set rather than the full 12-variable APACHE II data collection sheet, the result should be treated as an approximate severity estimate rather than a complete ICU audit-grade APACHE II submission.

The score is intended for severity framing and discussion, not for individual end-of-life decisions or withdrawal-of-care thresholds. APACHE II is most defensible when based on the worst values from the first 24 hours of ICU admission and interpreted in the context of the underlying diagnosis and treatment course.

Sources

APACHE II: a severity of disease classification system (PubMed / Critical Care Medicine)
The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults (PubMed / Chest) — Later APACHE development paper useful for context on calibration and limits of APACHE II-style mortality prediction.

Frequently Asked Questions

APACHE II should be calculated using the worst values from the first 24 hours of ICU admission. It was designed as an admission severity score and is less reliable when calculated at other time points.