APACHE Score: Definition and Calculation
What is the APACHE Score?
The APACHE II (Acute Physiology and Chronic Health Evaluation II) score is a severity-of-illness classification system that predicts ICU mortality by incorporating 12 physiologic variables, age, and chronic health status, with scores ≥8 indicating severe disease and requiring enhanced monitoring protocols. 1, 2
The APACHE II system was introduced in 1985 to reflect both premorbid factors (patient age and chronic medical conditions) and acute physiologic derangements in critically ill patients. 1
Components of APACHE II Calculation
The score incorporates three major categories 1, 2:
1. Acute Physiology Score (12 Variables)
The acute physiology component evaluates the worst values of 12 physiologic measurements during the first 24 hours of ICU admission 3, 4:
- 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
Each variable is assigned points based on deviation from normal values, with greater deviations receiving higher scores. 3
2. Age Points
Age is scored on a scale from 0-6 points 1:
- <44 years: 0 points
- 45-54 years: 2 points
- 55-64 years: 3 points
- 65-74 years: 5 points
- ≥75 years: 6 points
3. Chronic Health Evaluation
Chronic health points (0-5) are assigned based on pre-existing organ insufficiency or immunocompromised state and type of admission (elective surgical, emergency surgical, or medical). 1
Score Interpretation and Clinical Thresholds
An APACHE II score ≥8 is the optimal cut-off for predicting mortality, with 83.3% sensitivity and 91% specificity, and should trigger enhanced monitoring protocols. 1, 2
The score demonstrates a pooled area under the curve (AUC) of 0.81 for mortality prediction, superior to other scoring systems like SOFA (AUC 0.75). 1
Risk Stratification by Score Range:
- APACHE II ≥8: Severe disease requiring intensive monitoring 1, 2
- APACHE II ≥15-17: Consider continuous or extended infusion beta-lactam antibiotics for severe infections 1
- APACHE II ≥20: Continuous beta-lactam administration reduces mortality (RR 0.73) compared to intermittent dosing 1
- APACHE II ≥29.5: Prolonged piperacillin/tazobactam infusions associated with significantly lower mortality (12.9% vs 40.5%) 1
Advantages Over Alternative Scoring Systems
APACHE II outperforms other severity scores in several key aspects 1:
- Superior discriminative power: AUC 0.81 vs SOFA's 0.75 for mortality prediction 1
- Incorporates age and comorbidities: Unlike SOFA, which omits these critical prognostic factors 1
- Highest accuracy for severe conditions: Demonstrated superior performance compared to Ranson, BISAP, and CTSI in acute pancreatitis 5, 2
- Widely validated: Applicable across multiple disease states and ICU populations 2
Practical Clinical Applications
Serial Monitoring
Regular recalculation of APACHE II provides valuable information about disease trajectory 1:
- Daily scoring tracks progression or recovery 1
- Pattern changes indicate treatment response or complications (e.g., sepsis onset) 1
- Particularly valuable in severe acute pancreatitis management 1
Antibiotic Dosing Optimization
For patients with APACHE II ≥15-17 and severe infections, continuous or extended infusion of beta-lactam antibiotics improves clinical cure rates and reduces mortality compared to intermittent dosing. 1
Risk Stratification for Specific Conditions
- Complicated intra-abdominal infections: Calculate within 24 hours of hospitalization or ICU admission 1
- Delirium prediction: APACHE II is incorporated into the PRE-DELIRIC model (AUROC 0.77), with higher scores correlating with increased delirium risk 1
Important Limitations and Caveats
Calculation Complexity
The APACHE II calculation is cumbersome and not all required parameters are routinely collected in standard care. 1, 2
Regional Calibration Issues
The original US APACHE II model showed variable accuracy when applied to UK patients, requiring local recalibration. 1 Different pre-ICU care patterns may create "lead time bias" by affecting physiological measurements used in the calculation. 1
Timing Considerations
While traditionally calculated using worst values in the first 24 hours, admission-only APACHE II scores maintain similar discrimination ability (AUC 83.8% vs 84.6%) and may be logistically easier. 4
Not a Substitute for Clinical Judgment
The positive predictive value is 55.6% after 48 hours, meaning clinical assessment must complement scoring. 1 Pre-ICU care quality significantly impacts physiological measurements and can introduce bias. 1