APACHE II Score Calculator (Simplified)

About

The Acute Physiology And Chronic Health Evaluation (APACHE) II score was developed by Knaus, Draper, Wagner, and Zimmerman at George Washington University Medical Center, with the landmark paper published in Critical Care Medicine in 1985. It was designed as a simplified version of the original APACHE system (1981), reducing the number of physiological variables from 34 to 12 to improve clinical practicality while maintaining prognostic accuracy. The APACHE II score comprises three components: the Acute Physiology Score (APS), which assigns points (0-4) to 12 physiological variables based on the worst value within the first 24 hours of ICU admission; age points (0-6 based on age category); and chronic health points (0-5 based on history of severe organ system insufficiency or immunocompromise). The total score ranges from 0 to 71, with higher scores indicating greater severity of illness and higher predicted hospital mortality. The original validation study included 5,815 ICU admissions across 13 hospitals in the United States and demonstrated a strong correlation between rising APACHE II scores and in-hospital mortality. APACHE II has since been validated in hundreds of studies worldwide across diverse ICU populations, including medical, surgical, trauma, and septic patients. It remains one of the most widely used ICU severity scoring systems globally, employed for mortality risk prediction, clinical research stratification, ICU performance benchmarking, and resource allocation. This simplified version presents the physiological variables as categorized dropdown selections for ease of bedside use while preserving the original Knaus scoring system.

Formula

APACHE II = Acute Physiology Score (0-60) + Age Points (0-6) + Chronic Health Points (0-5)

The APACHE II score is the sum of three components: the Acute Physiology Score (APS), age points, and chronic health points. The APS is calculated from 12 physiological variables, each scored from 0 to 4, with higher scores assigned for more extreme deviations from normal. The temperature variable assigns points for hypothermia (<36°C) and hyperthermia (>38.5°C), with extreme values below 30°C or above 41°C scoring 4 points. Mean arterial pressure (MAP) deviation from the normal range of 70-109 mmHg scores points for both hypotension (<70 mmHg) and hypertension (>110 mmHg). Heart rate is scored for both bradycardia and tachycardia, with rates below 40 or above 180 bpm scoring 4 points. Respiratory rate accounts for both bradypnea and tachypnea. Oxygenation is assessed differently depending on FiO2: when FiO2 is 50% or greater, the alveolar-arterial oxygen gradient (AaDO2) is used, and when FiO2 is less than 50%, PaO2 is used directly. Arterial pH is scored for both acidosis and alkalosis. Serum sodium and potassium score for both hypo- and hyper-elevated states. Creatinine scoring is doubled for patients with acute renal failure (defined as a rise in creatinine with oliguria). Hematocrit and white blood cell count complete the 12-variable set. The Glasgow Coma Scale score is subtracted from 15 to derive neurological points (i.e., lower GCS gives higher points). Age points are assigned as follows: <45 years (0), 45-54 (2), 55-64 (3), 65-74 (5), and ≥75 (6). Chronic health points are awarded for severe organ system insufficiency or immunocompromise documented before ICU admission: 2 points for elective postoperative patients and 5 points for nonoperative or emergency surgical patients. The maximum total score is 71. The predicted mortality rate corresponding to APACHE II scores follows a non-linear relationship: a score of 0-4 predicts <5% mortality, 10-14 predicts approximately 15% mortality, 20-24 predicts approximately 40% mortality, 30-34 predicts approximately 70% mortality, and scores above 40 predict >80% mortality. The score is intended to be calculated using the worst physiological values within the first 24 hours of ICU admission, not for serial reassessment.

Score Interpretation

APACHE II is one of the most extensively validated and widely used severity of illness scoring systems in critical care medicine worldwide. It serves multiple important clinical and administrative functions. For individual patient prognostication, the APACHE II score at 24 hours provides an objective estimate of hospital mortality risk that can supplement clinical judgment. The score is particularly useful for stratifying patients in clinical trials, ensuring balanced randomization based on illness severity. APACHE II is also used for ICU performance benchmarking through standardized mortality ratios (SMRs), where the observed mortality of an ICU is compared to the mortality predicted by APACHE II, adjusted for case mix. This allows objective comparisons across different ICUs and over time within the same unit. The score has been incorporated into multiple national ICU audit programs, including the Intensive Care National Audit and Research Centre (ICNARC) in the UK. The Surviving Sepsis Campaign guidelines acknowledge APACHE II as a valid tool for assessing severity in septic patients, though the guidelines note that SOFA is preferred for serial organ dysfunction assessment. The main limitation of APACHE II is that it requires a full 24 hours of data collection before calculation, making it a retrospective rather than real-time assessment tool. It also requires accurate data abstraction of the worst physiological values, which can be resource-intensive and subject to inter-observer variability. The score was developed in 1985, and advances in ICU care since then mean that raw mortality rates for a given APACHE II score have decreased over time, necessitating periodic recalibration. Newer versions (APACHE III and APACHE IV) have been developed with updated coefficients and larger validation databases, but APACHE II remains the most widely used version due to its simplicity and extensive validation. Despite these limitations, APACHE II remains a cornerstone of ICU severity assessment and is routinely used in clinical research, quality improvement, and ICU administration worldwide.

Reference Ranges