# APACHE II Score Calculator

Calculate the APACHE II (Acute Physiology and Chronic Health Evaluation II) ICU severity score. APACHE II = Acute Physiology Score (12 variables, each 0–4 using the worst value in the first 24 h) + Age points (0–6) + Chronic Health points (0/2/5). Range 0–71; higher scores indicate worse prognosis. Oxygenation uses the A-a gradient when FiO₂ ≥ 0.5, otherwise PaO₂. Creatinine points are doubled when acute renal failure is indicated. GCS contributes 15 − GCS. Also reports the base-model predicted mortality: R = 1/(1 + e^(−logit)), logit = −3.517 + 0.146 × score (diagnostic-category weight not included). Thresholds cross-verified against Knaus 1985 (Crit Care Med) and the SFAR scoring table. Not a substitute for clinical judgement. Not medical advice.

> Canonical page: https://elysiatools.com/en/tools/apache-ii-score

- **Category:** Health

- **Keywords:** APACHE II, APACHE2, ICU, critical care, severity of illness, mortality prediction, intensive care, Knaus, physiology score, intensive care scoring

## Overview

The APACHE II Score Calculator estimates an ICU severity score from 12 acute physiology variables, age, and chronic health status. It returns the APACHE II score from 0 to 71, component scores, and a base-model predicted mortality estimate; it does not replace clinical judgment or provide medical advice.

## Inputs

- **Temperature (°C)** (number): Core/rectal temperature °C.
- **Mean Arterial Pressure (mmHg)** (number): Mean arterial pressure mmHg.
- **Heart Rate (/min)** (number): Heart rate beats/min.
- **Respiratory Rate (/min)** (number): Respiratory rate breaths/min (vented or spontaneous).
- **FiO₂** (number): Fraction of inspired oxygen (0.21–1.0). If ≥0.5, A-a gradient is used.
- **PaO₂ (mmHg)** (number): Arterial PaO₂ mmHg. Used when FiO₂ < 0.5.
- **A-a Gradient (mmHg)** (number): Alveolar-arterial gradient mmHg. Used when FiO₂ ≥ 0.5.
- **Arterial pH** (number): Arterial pH.
- **Sodium (mmol/L)** (number): Serum sodium mmol/L.
- **Potassium (mmol/L)** (number): Serum potassium mmol/L.
- **Creatinine (mg/dL)** (number): Serum creatinine mg/dL.
- **Acute renal failure (×2 creatinine points)** (checkbox): Acute renal failure doubles the creatinine points.
- **Hematocrit (%)** (number): Hematocrit %.
- **WBC (×10³/µL)** (number): White blood cell count ×10³/µL.
- **Glasgow Coma Scale (3–15)** (number): Glasgow Coma Scale total (3–15). Contributes 15 − GCS.
- **Age (years)** (number): Age in years.
- **Chronic Health Evaluation** (select): Severe organ insufficiency or immunocompromise: none = 0, elective postoperative = 2, non-operative or emergency postoperative = 5.

## When to use

- Estimate APACHE II severity using the worst available values from the first 24 hours in the ICU.
- Review how physiology, age, Glasgow Coma Scale, renal failure, and chronic health status contribute to the total score.
- Use a consistent scoring method for clinical review, education, or critical-care research.

## How it works

- Enter temperature, mean arterial pressure, heart rate, respiratory rate, oxygen data, arterial pH, electrolytes, creatinine, hematocrit, WBC count, GCS, age, and chronic health status.
- Enter FiO₂ and provide PaO₂ when FiO₂ is below 0.5, or the A-a gradient when FiO₂ is 0.5 or higher.
- Mark acute renal failure when applicable; the calculator doubles the creatinine points in that case.
- The calculator adds acute physiology, age, and chronic health points, then reports the APACHE II score and base-model mortality estimate.

## Use cases

- ICU admission and severity-of-illness review
- Teaching APACHE II component scoring and oxygenation rules
- Critical-care research or retrospective chart review

## Frequently asked questions

### What does the APACHE II score measure?

It summarizes acute physiologic disturbance, age, and chronic health status in critically ill patients. Higher scores indicate worse prognosis.

### What values should I enter?

Use the worst qualifying values from the first 24 hours in the ICU, interpreted in the appropriate clinical context.

### Which oxygen value is used?

PaO₂ is used when FiO₂ is below 0.5. The A-a gradient is used when FiO₂ is 0.5 or higher.

### How does acute renal failure affect the result?

When acute renal failure is selected, the creatinine component points are doubled.

### Is the mortality estimate an individual prediction?

No. It is a base-model estimate without diagnostic-category weighting and should not replace clinical assessment.

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