APACHE II Score Calculator | ICU Mortality Prediction

Core Components (Worst value in 1st 24 hours of ICU admission)

TemperatureBody’s core temperature.

Mean Arterial PressureAverage arterial pressure.

Heart RateReflects cardiovascular stress.

Respiratory RateIndicates respiratory distress.

OxygenationUse A-a gradient if FiO₂ ≥ 50%, otherwise use PaO₂.

Arterial pHMeasures blood acidity or alkalinity.

Serum SodiumKey electrolyte for fluid balance.

Serum PotassiumCritical for cardiac and neuromuscular function.

Serum CreatinineIndicator of kidney function.

Acute Renal Failure?

HematocritProportion of red blood cells.

WBC CountIndicates inflammation or infection.

Glasgow Coma ScalePoints = 15 – GCS. (Open GCS Calculator)

Age & Chronic Health

Age Points

Severe Organ Insufficiency or Immunocompromisede.g., Cirrhosis, NYHA Class IV HF, Severe COPD, Chronic Dialysis, Immunosuppression.

APACHE II Score

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Predicted Mortality

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Show score breakdown

Recent Updates

APACHE II still discriminates well (AUC about 0.78–0.81), but because it was developed in 1985, its calibration has drifted: external validations repeatedly show it overestimates mortality in contemporary cohorts.
Use it for population-level comparison and ICU benchmarking rather than individual prognosis, and consider local/era-specific recalibration.
More complex successors (APACHE III, APACHE IV) add variables and can improve accuracy, but APACHE II remains widely used for its simplicity and extensive validation.

Source: Knaus WA, et al. APACHE II: a severity of disease classification system, 1985. Crit Care Med 1985;13(10):818-829. PMID: 3928249.

Key Knowledge Points

The APACHE II Score Calculator is a severity-of-disease classification system widely used in ICUs.
It uses the most abnormal value for each physiological variable within the first 24 hours of ICU admission.
The score consists of three parts: the Acute Physiology Score (APS), an Age Adjustment, and a Chronic Health Evaluation.
Like other scoring systems, APACHE II is a tool for population-level prediction and quality benchmarking, and should be used cautiously for individual patient prognosis.
The score is particularly valuable for research, resource allocation, and comparing outcomes between different ICUs.

About This APACHE II Score Calculator

This APACHE II Score Calculator provides a robust method for classifying the severity of illness in critically ill patients. Developed by Dr. William Knaus, the Acute Physiology and Chronic Health Evaluation II (APACHE II) system is one of the most widely used scoring systems in intensive care units (ICUs) worldwide. It calculates a score based on 12 physiological variables, age, and chronic health status, all recorded within the first 24 hours of ICU admission. The resulting score correlates with the risk of hospital mortality, making it an invaluable tool for prognostication, clinical research, and quality assessment.

Scoring System Explained

The total APACHE II score is the sum of three components:

Acute Physiology Score (APS): Points are assigned for the most deranged value of 12 physiological measurements during the initial 24 hours.
Age Points: Points are added based on the patient’s age.
Chronic Health Points: Points are added if the patient has a history of severe organ system insufficiency or is immunocompromised.

Clinical Interpretation

The total score, ranging from 0 to 71, is used to estimate the risk of hospital mortality. A higher score signifies more severe physiological derangement and a correspondingly higher risk of death. The figures below are approximate, derived from the original nonoperative cohort:

Score 0-4: ~4% mortality
Score 15-19: ~25% mortality
Score 30-34: ~73% mortality
Score ≥ 35: ~85% mortality

Mortality rates are also influenced by the specific reason for ICU admission, and more precise predictions can be made using disease-specific logistic regression equations.

Limitations

While the APACHE II score is a cornerstone of critical care assessment, it has limitations. It was developed in 1985, and medical care has advanced substantially since then. External validation studies repeatedly show that APACHE II overestimates mortality in contemporary cohorts (calibration drift), even though its ability to discriminate between survivors and non-survivors remains good (AUC ~0.78–0.81). It is therefore best used for population-level benchmarking rather than as the sole determinant of an individual patient’s prognosis, with local recalibration where needed. The score also does not account for changes in a patient’s condition after the first 24 hours.

Frequently Asked Questions (FAQ)

1. What does “worst” value in the first 24 hours mean?

For each of the 12 physiological variables, you should identify the value that deviates most from the normal range during the first 24 hours of ICU admission. This single most abnormal value is what gets assigned points.

2. How is the GCS score calculated within APACHE II?

The APACHE II GCS point value is calculated as (15 – Patient’s GCS). So, a patient with a GCS of 3 would get 12 points, while a patient with a GCS of 15 would get 0 points. If the patient is sedated, the pre-sedation GCS should be used.

3. What qualifies as “severe organ insufficiency” for the chronic health points?

This refers to a pre-existing condition that severely restricts function. Examples include NYHA Class IV heart failure, severe COPD with chronic hypoxia or hypercapnia, documented cirrhosis with portal hypertension, or chronic renal dialysis.

4. How does APACHE II compare to SAPS-3?

APACHE II uses data from the first 24 hours, while SAPS-3 uses data from only the first hour. Both are powerful prognostic tools, but APACHE II captures a broader window of initial physiological instability. SAPS-3 provides an earlier “snapshot” of severity.

5. Is there a newer version of APACHE?

Yes, APACHE III and APACHE IV have been developed. They are more complex and use more variables, which can improve predictive accuracy. However, APACHE II remains extremely popular and widely used due to its simplicity, extensive validation, and long history in clinical practice and research.

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⚠️ Disclaimer:

This tool is for informational and educational purposes only and is not a substitute for professional clinical judgment. All treatment decisions must be made by a qualified healthcare professional considering the individual patient’s full clinical context. This APACHE II Score Calculator is an aid, not a replacement for clinical expertise.