How to Calculate the APACHE Score
The APACHE II score is calculated using 12 routine physiological measurements taken during the first 24 hours of ICU admission, combined with the patient's age and chronic health status, yielding a total score ranging from 0 to 71 points. 1
Required Data Components
Physiological Variables (12 Parameters)
The APACHE II system requires measurement of the following 12 physiological variables, using the worst values recorded during the first 24 hours of ICU admission 1, 2:
- Temperature (rectal)
- Mean arterial pressure
- Heart rate
- Respiratory rate
- Oxygenation (A-aDO₂ or PaO₂, depending on FiO₂)
- Arterial pH
- Serum sodium
- Serum potassium
- Serum creatinine
- Hematocrit
- White blood cell count
- Glasgow Coma Scale (15 minus actual GCS)
Each physiological variable is assigned points based on how far the value deviates from normal, with greater deviations receiving higher point values 1.
Age Points
Age is incorporated as a separate component 2, 1:
- Points are added based on the patient's age category
- Older patients receive additional points reflecting increased baseline mortality risk
Chronic Health Evaluation
The chronic health component assesses pre-existing conditions 2, 1:
- Type of admission (elective surgical, emergency surgical, or medical)
- Presence of severe organ insufficiency or immunocompromised state
- Points are added for patients with chronic conditions affecting major organ systems
Calculation Timing and Interpretation
The APACHE II score should be calculated at the end of the first 24 hours of ICU admission using the worst physiological values recorded during that period. 1, 3
Clinical Significance of Scores
- Score ≥8: Optimal cut-off for mortality prediction with 83.3% sensitivity and 91% specificity 4, 2
- Score ≥15: Provides best diagnostic accuracy for predicting mortality in critically ill patients with 85.3% sensitivity and 77.4% specificity 5
- Score 31-40: Associated with extremely high mortality risk 3
- Score 21-30: Associated with very high mortality risk 3
Serial Monitoring
Regular recalculation of the APACHE II score on subsequent ICU days provides valuable information about disease progression or recovery. 2 Daily scoring can track response to treatment, with pattern changes indicating onset of complications such as sepsis 2.
Important Clinical Caveats
Not all required parameters are routinely collected in every clinical setting, which can make calculation cumbersome. 4, 2 Ensure all 12 physiological variables are measured and documented during the first 24 hours of ICU admission to enable accurate scoring 1.
The APACHE II score demonstrates superior discriminative power for mortality prediction compared to other scoring systems, with a pooled area under the curve of 0.81 2. However, the score was developed in 1985 using US patient populations, and regional variations in pre-ICU care quality may affect its predictive accuracy when applied to different healthcare systems 2.
The APACHE III system (introduced in 1991) uses an expanded range of 0-299 points and incorporates additional disease categories, offering refined risk stratification for specific patient populations. 6, 4