What is the APACHE (Acute Physiology and Chronic Health Evaluation) score and how is it calculated?

APACHE Score: Definition and Calculation

What is the APACHE Score?

The APACHE II score is a severity-of-illness scoring system that predicts mortality risk in critically ill patients by incorporating 12 physiologic variables, age, chronic health status, and admission type, with scores ranging from 0 to 71 points. 1

• APACHE II was introduced in 1985 to reflect both premorbid factors (patient age and chronic medical conditions) and acute physiologic derangements in critically ill patients. 1
• The score is calculated using the worst values recorded during the first 24 hours of ICU admission, though admission-only values maintain similar discriminative ability. 2
• APACHE II demonstrates superior mortality prediction compared to other scoring systems, with a pooled area under the curve (AUC) of 0.81. 1

Components of APACHE II Calculation

Acute Physiology Score (APS) – 12 Variables

The acute physiology component assigns 0-4 points for each variable based on deviation from normal:

• Temperature (rectal core temperature) 3
• Mean arterial pressure 3
• Heart rate 3
• Respiratory rate 3
• Oxygenation: Use A-aDO₂ if FiO₂ ≥0.5, or PaO₂ if FiO₂ <0.5 3
• Arterial pH (preferred) or serum HCO₃ if no arterial blood gas available 3
• Serum sodium 3
• Serum potassium 3
• Serum creatinine (double points if acute renal failure present) 3
• Hematocrit 3
• White blood cell count 3
• Glasgow Coma Scale: Score = 15 minus actual GCS 3

Age Points

• <44 years: 0 points
• 45-54 years: 2 points
• 55-64 years: 3 points
• 65-74 years: 5 points
• ≥75 years: 6 points 1

Chronic Health Evaluation

Assign 5 points for nonoperative or emergency postoperative patients, or 2 points for elective postoperative patients if any of the following apply:

• Severe organ insufficiency (liver, cardiovascular, respiratory, renal)
• Immunocompromised state 1

Clinical Interpretation and Risk Stratification

Mortality Prediction Thresholds

• APACHE II ≥8: Optimal cut-off for mortality prediction with 83.3% sensitivity and 91% specificity; triggers enhanced monitoring protocols. 1
• APACHE II ≥15: Indicates severe disease; consider continuous or extended infusion of beta-lactam antibiotics for severe infections, which improves clinical cure rates and reduces mortality. 1
• APACHE II ≥20: Continuous beta-lactam administration shows reduced mortality (RR 0.73) compared to intermittent dosing. 1
• APACHE II score of 15: Provides best diagnostic accuracy for predicting mortality with 85.3% sensitivity and 77.4% specificity. 4

Guideline-Endorsed Applications

• The Infectious Diseases Society of America recommends APACHE II as the preferred severity-of-illness score for risk stratification in adults with complicated intra-abdominal infections when calculated within 24 hours of hospitalization or ICU admission. 5
• The American College of Surgeons recommends APACHE II for acute pancreatitis assessment, with scores ≥8 indicating severe disease and demonstrating the highest accuracy (AUC 0.88) for predicting complications. 6
• Regular recalculation of APACHE II provides critical information about disease progression or recovery, with pattern changes indicating treatment response or onset of complications. 1

Advantages Over Alternative Scoring Systems

• APACHE II outperforms SOFA in discriminative power for mortality prediction (AUC 0.81 vs. 0.75). 1
• Unlike SOFA, APACHE II incorporates age and chronic comorbidities, which are important prognostic factors. 1
• APACHE II can be calculated at any time and used for daily ongoing assessment throughout the ICU stay. 6

Important Caveats and Limitations

• The APACHE II calculation is cumbersome and not all required parameters are routinely collected in every ICU. 1
• Pre-ICU care quality can significantly impact physiological measurements, potentially creating "lead time bias" in mortality predictions. 1
• The original US APACHE II model requires regional recalibration when applied to different healthcare systems, as care patterns before ICU admission vary internationally. 1
• APACHE II was designed for population-level risk stratification and research purposes, not for individual patient triage decisions or to dictate specific treatment choices. 3