APACHE II Score Calculator
The APACHE II (Acute Physiology and Chronic Health Evaluation II) score estimates the severity of illness and predicted hospital mortality for adult ICU patients. Enter the worst values recorded during the first 24 hours of ICU admission. The calculator scores 12 acute physiological variables, age, and chronic health status, then converts the total to a predicted mortality percentage using the original Knaus et al. logistic regression.
What is the APACHE II score?
APACHE II (Acute Physiology and Chronic Health Evaluation II) is a severity-of-illness scoring system developed by William Knaus and colleagues and published in Critical Care Medicine in 1985. It was designed to quantify the degree of acute physiological derangement in critically ill adults during the first 24 hours of ICU admission. The score ranges from 0 to 71 points, though values above 55 are rare in practice. A higher score indicates more severe illness and a greater predicted risk of hospital death. APACHE II is one of the most widely validated and published prognostic tools in critical care medicine, having been tested in millions of patients across multiple countries and care settings.
How is the APACHE II score calculated?
The score has three components. The Acute Physiology Score (APS) sums points from 12 physiological variables recorded during the first 24 hours in the ICU: temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation (A-a gradient when FiO2 >= 50%, or PaO2 when FiO2 < 50%), arterial pH, serum sodium, serum potassium, serum creatinine (doubled if acute renal failure is present), hematocrit, white blood cell count, and the Glasgow Coma Scale contribution (calculated as 15 minus the actual GCS). The Age Score adds 0 to 6 points based on patient age. The Chronic Health Score adds 0, 2, or 5 points depending on whether severe pre-existing organ insufficiency or immunocompromised status is present and, if so, whether admission followed elective surgery (2 points) or emergency surgery or a nonoperative condition (5 points). The three components are simply added together to give the final APACHE II score.
Oxygenation: A-a gradient versus PaO2
The oxygenation parameter switches based on the fraction of inspired oxygen (FiO2). When the patient is receiving >= 50% oxygen (FiO2 >= 50%), the alveolar-arterial (A-a) oxygen gradient is used. The A-a gradient is calculated as the alveolar partial pressure of oxygen (PAO2) minus the measured arterial partial pressure of oxygen (PaO2), where PAO2 = (FiO2/100 x (atmospheric pressure - 47 mmHg)) - (PaCO2 / 0.8). Larger gradients indicate worse gas exchange and carry more points. When FiO2 is less than 50%, the raw PaO2 value is used instead, with lower PaO2 values carrying more points.
Predicting hospital mortality with APACHE II
Knaus and colleagues derived a logistic regression equation to convert APACHE II scores to predicted hospital mortality: ln(R / (1-R)) = -3.517 + (0.146 x APACHE II) + 0.603 (if emergency surgery) + admission diagnosis weight. Because the full set of diagnosis-specific weights is not freely published, this calculator uses the base equation with the emergency surgery adjustment, which is sufficient for most clinical purposes. The resulting probability should be interpreted at the population level - a predicted mortality of 40% means that in a large cohort of patients with that score and admission type, about 40% would be expected to die before hospital discharge. It does not mean the individual patient has a 40% chance of dying.
Limitations and appropriate use
APACHE II was developed in the early 1980s and validated on United States ICU populations. Calibration varies across countries, time periods, and ICU types. The score should never be used as the sole determinant of treatment intensity, ceilings of care, or resource allocation decisions. It does not account for admission diagnosis in this base version, nor does it reflect changes in the patient's condition after the first 24 hours. SAPS II and SOFA are more recently validated alternatives for certain populations. All scoring systems perform better at predicting group outcomes than individual outcomes, and clinical judgment always takes precedence over any prognostic number.
APACHE II score ranges and observed hospital mortality
| APACHE II Score | Non-operative mortality | Postoperative mortality |
|---|---|---|
| 0-4 | 4% | 1% |
| 5-9 | 8% | 3% |
| 10-14 | 15% | 7% |
| 15-19 | 24% | 12% |
| 20-24 | 40% | 30% |
| 25-29 | 55% | 35% |
| 30-34 | 73% | 73% |
| >=35 | 85% | 88% |
Published mortality rates from Knaus et al. (1985) based on 5,815 ICU admissions across 13 hospitals. Individual patient outcomes vary.
Frequently asked questions
When should the APACHE II score be calculated?
APACHE II is calculated using the worst values recorded during the first 24 hours of ICU admission. Using admission values only, or values at any other time point, produces inaccurate predictions. The score is a snapshot tool for the admission period and is not designed to be recalculated daily to track clinical progress.
What does a score above 25 mean?
In the original Knaus validation cohort, nonoperative patients with scores of 25-29 had an observed hospital mortality of about 55%, rising to 73% for scores of 30-34 and 85% for scores of 35 or above. Postoperative patients had slightly different rates. These are group-level statistics from a 1985 cohort; modern ICU care may improve outcomes, but high scores still reliably identify the most critically ill patients.
How is acute renal failure handled in the score?
When acute renal failure (ARF) is present, the creatinine points are doubled. For example, a creatinine of 2.5 mg/dL scores 3 points without ARF but 6 points with ARF. This reflects the additional prognostic impact of acute-on-chronic or de novo renal failure in critically ill patients. Chronic kidney disease alone, without an acute rise in creatinine, does not trigger the doubling.
Can APACHE II be used to decide whether to admit a patient to the ICU?
No. APACHE II was designed for severity stratification after ICU admission, not for triage decisions before admission. The values it requires - arterial blood gas, full chemistry panel, and GCS - may not yet be available at the point of triage. ICU admission criteria are based on clinical assessment of physiological instability and the need for organ support, not on prognostic scores.
What is the difference between APACHE II and APACHE III?
APACHE III, published in 1991, expanded the number of physiological variables to 17, added more precise age weighting, and incorporated hospital-level outcome data to improve calibration. APACHE III is proprietary, requiring a license, whereas APACHE II is in the public domain and free to use, which largely explains why APACHE II remains the most widely cited version in the literature more than 40 years after publication.
Why does the GCS contribute points as "15 minus GCS"?
APACHE II scores physiological derangement: the further a value is from normal, the more points it contributes. A normal GCS is 15, so subtracting the patient's actual score from 15 gives 0 points for a fully alert patient and up to 12 points for a patient with the minimum GCS of 3. This aligns GCS with the directional convention used by all other APS variables.
Is a predicted mortality percentage from APACHE II definitive?
No. The predicted mortality is a statistical estimate based on population-level data from specific ICU cohorts. It should be used alongside the full clinical picture to understand severity, communicate prognosis to families, and benchmark ICU performance. It should not be used in isolation to limit care or predict an individual outcome with precision.
Sources
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: A severity of disease classification system. Critical Care Medicine, 1985;13(10):818-829.
- Zimmerman JE, Kramer AA, McNair DS, Malila FM. Acute Physiology and Chronic Health Evaluation (APACHE) IV: hospital mortality assessment for today's critically ill patients. Critical Care Medicine, 2006.