APACHE II
APACHE II is a severity-of-disease classification system developed in 1985 by William A. Knaus and colleagues to assess the risk of hospital mortality in adult patients admitted to intensive care units (ICUs).[1] It calculates a score ranging from 0 to 71 by summing points from 12 routine acute physiologic measurements, patient age, and chronic health evaluation, using the worst values observed during the first 24 hours of ICU admission.[1] Higher scores correlate with increased mortality risk, enabling standardized prognostic stratification across diverse patient populations and institutions.[1] The system's acute physiology score (APS) is derived from variables including temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation (PaO₂ or A-aDO₂ gradient), arterial pH, serum sodium, serum potassium, serum creatinine, hematocrit, white blood cell count, and the Glasgow Coma Scale, each assigned points based on deviation from normal ranges (0–4 points per variable, except GCS which is inverted).[2] Age contributes additional points (0 for under 45 years, up to 6 for 75 years or older), while chronic health points (0–5) are added for patients with severe underlying conditions, such as immunocompromised states or end-stage organ failure, but only if active at admission.[2] The total APACHE II score is then used with logistic regression equations, derived from a validation cohort of over 5,800 ICU admissions across 13 hospitals, to estimate mortality probability, demonstrating high predictive accuracy.[1] Since its introduction, APACHE II has become a cornerstone for ICU benchmarking, clinical research, and quality assurance, facilitating comparisons of outcomes, resource utilization, and therapeutic efficacy while simplifying the original APACHE system's 34 variables for practical use.[3] It is applied within 24 hours of ICU admission and remains influential despite the development of subsequent systems like APACHE III and IV, due to its simplicity, reliability, and extensive validation in various settings, including medical, surgical, and trauma ICUs.[4]Introduction
Definition and Purpose
The Acute Physiology and Chronic Health Evaluation II (APACHE II) is a severity-of-disease classification system designed specifically for patients in intensive care units (ICUs). Developed to provide a standardized measure of illness severity, it integrates physiological data, age, and chronic health factors to generate a composite score that reflects the overall risk profile of critically ill individuals.[1] The primary purpose of APACHE II is to quantify acute physiological derangements alongside age and chronic health status, enabling clinicians to estimate the probability of hospital mortality based on data collected within the first 24 hours of ICU admission. This prognostic tool supports clinical decision-making, such as identifying high-risk patients for intensified interventions, and facilitates comparisons of outcomes and resource utilization across different ICUs and patient populations. By stratifying patients according to predicted mortality risk, it aids in evaluating the effectiveness of therapies and allocating limited ICU resources more equitably.[1] APACHE II is applicable to adult patients admitted to medical or surgical ICUs, encompassing a broad range of conditions from postoperative recovery to acute organ failures. The total score ranges from 0 (indicating minimal severity) to 71 (indicating extreme severity), with higher scores correlating directly with elevated hospital mortality risk; for instance, scores exceeding 15 are typically associated with mortality probabilities greater than 20%. As the first widely adopted prognostic model in ICUs, APACHE II revolutionized severity assessment by simplifying prior systems while maintaining high predictive accuracy, thereby enabling consistent benchmarking of care quality between institutions.[1][5][6]History and Development
The Acute Physiology and Chronic Health Evaluation (APACHE) system originated from efforts initiated in 1978 by William A. Knaus and colleagues at George Washington University Medical Center to develop a standardized method for assessing severity of illness in intensive care unit (ICU) patients.[3] The original APACHE system was introduced in 1981, drawing on physiological data to classify disease severity and predict outcomes in critically ill individuals.[7] APACHE II emerged as a refined and simplified iteration, published in 1985 in the journal Critical Care Medicine by Knaus, Elizabeth A. Draper, Douglas P. Wagner, and colleagues.[1] Key motivations for its development included addressing the complexity of the initial APACHE model, which relied on 34 physiologic variables, by reducing them to 12 while maintaining predictive power and incorporating chronic health factors more effectively for broader clinical use.[8] This revision was informed by empirical data collected from 5,815 ICU admissions across 13 hospitals in the United States between 1979 and 1982, enabling a more practical tool less dependent on extensive therapeutic details.[1] Validation in the 1985 study demonstrated that APACHE II scores closely correlated with hospital mortality risk, stratifying patients into risk categories with high reliability for outcome prediction.[1] The publication of APACHE II represented a pivotal advancement, as it was the first ICU severity scoring system to balance acute physiology, age, and chronic health evaluations in a concise format suitable for widespread adoption in resource-limited settings.[1]Components
Acute Physiology Score
The Acute Physiology Score (APS) is a core component of the APACHE II system, designed to quantify the degree of acute physiological derangements in critically ill patients by evaluating deviations from normal values across 12 routine physiological and laboratory variables. These variables encompass vital signs, oxygenation status, acid-base balance, electrolyte levels, renal function, hematologic parameters, and neurologic assessment, focusing specifically on reversible changes that occur during acute illness. The APS emphasizes the most abnormal ("worst") measurements recorded during the initial 24 hours of intensive care unit (ICU) admission, capturing the peak severity of organ dysfunction in that period. Each of the 12 variables is scored from 0 (normal or minimal derangement) to 4 points (severe derangement), with points assigned based on predefined thresholds that reflect increasing abnormality, except for the Glasgow Coma Scale which is scored as 15 minus the observed GCS (0 to 12 points). The total APS ranges from 0 to 60 points, providing a summed measure of acute physiological instability without considering age or chronic conditions, which are addressed separately in the full APACHE II score. The variables are: temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation (assessed via PaO₂ or alveolar-arterial oxygen gradient [A-aDO₂] depending on inspired oxygen fraction [FiO₂]), arterial pH, serum sodium, serum potassium, serum creatinine, hematocrit, white blood cell count, and Glasgow Coma Scale (GCS). The following table details the exact scoring thresholds for each variable, as defined in the original APACHE II formulation.[1]| Variable | 4 Points | 3 Points | 2 Points | 1 Point | 0 Points |
|---|---|---|---|---|---|
| Temperature (°C) | ≥41 or ≤29.9 | 39–40.9 or 30–31.9 | 38.5–38.9 or 32–33.9 | 34–35.9 | 36–38.4 |
| Mean Arterial Pressure (mm Hg) | ≥160 or ≤49 | 130–159 or 50–69 | 110–129 | - | 70–109 |
| Heart Rate (beats/min) | ≥180 or ≤39 | 140–179 or 40–54 | 110–139 or 55–69 | - | 70–109 |
| Respiratory Rate (breaths/min) | ≥50 or ≤5 | 35–49 or 6–9 | 25–34 | 10–11 | 12–24 |
| Oxygenation (A-aDO₂ mm Hg, FiO₂ ≥0.5) | ≥500 | 350–499 | 200–349 | - | <200 |
| Oxygenation (PaO₂ mm Hg, FiO₂ <0.5) | <55 | 55–60 | - | 61–70 | >70 |
| Arterial pH | ≥7.7 or <7.15 | 7.6–7.69 or 7.15–7.24 | 7.5–7.59 or 7.25–7.32 | - | 7.33–7.49 |
| Serum Sodium (mEq/L) | ≥180 or ≤110 | 160–179 or 111–119 | 155–159 or 120–129 | 150–154 | 130–149 |
| Serum Potassium (mEq/L) | ≥7 or <2.5 | 6–6.9 or 2.5–2.9 | 5.5–5.9 or 3–3.4 | - | 3.5–5.4 |
| Serum Creatinine (mg/dL)† | ≥3.5 | 2–3.4 | 1.5–1.9 | - | 0.6–1.4 |
| Hematocrit (%) | ≥60 or <20 | - | 50–59.9 or 20–29.9 | 46–49.9 | 30–45.9 |
| White Blood Cells (×10³/μL) | ≥40 or <1 | - | 20–39.9 or 1–2.9 | 15–19.9 | 3–14.9 |
Age Adjustment
The age adjustment in the APACHE II scoring system incorporates patient age as a static demographic factor to account for its independent contribution to mortality risk in critically ill adults. Developed through multivariate logistic regression analysis on a cohort of over 5,800 ICU patients, the age component assigns points based on empirical associations between advancing age and increased hospital mortality, reflecting diminished physiological reserve and comorbidities that impair recovery from acute illness.[1] Age points are determined using the following scale, derived from the original validation dataset where older age groups demonstrated progressively higher observed mortality rates after controlling for acute physiology:| Age (years) | Points |
|---|---|
| ≤ 44 | 0 |
| 45–54 | 2 |
| 55–64 | 3 |
| 65–74 | 5 |
| ≥ 75 | 6 |
Chronic Health Evaluation
The Chronic Health Evaluation component of the APACHE II scoring system assigns points for pre-existing severe organ system insufficiency or immunocompromised states that substantially increase mortality risk in critically ill patients.[1] These points reflect the baseline health burden from chronic conditions, independent of acute physiological derangements.[1] Points are assigned as 0 if no qualifying conditions are present, 2 points for elective postoperative patients with qualifying conditions, and 5 points for non-operative or emergency postoperative patients with qualifying conditions.[1] Regardless of the number of chronic conditions, only the maximum applicable points (0, 2, or 5) are awarded in this category.[1] Qualifying conditions include biopsy-proven cirrhosis with portal hypertension (or episodes of upper gastrointestinal bleeding due to portal hypertension, or prior episodes of hepatic failure, encephalopathy, or coma); New York Heart Association Class IV congestive heart failure; severe chronic obstructive pulmonary disease (COPD) with documented chronic hypoxia, hypercapnia, secondary polycythemia, severe pulmonary hypertension greater than 40 mmHg, or respirator dependency; chronic renal failure requiring dialysis; and immunocompromised states such as leukemia, lymphoma, acquired immune deficiency syndrome (AIDS) with an opportunistic infection, or ongoing immunosuppressive therapy (e.g., high-dose corticosteroids, chemotherapy, or radiation).[1] For non-operative patients, points are assigned only if the chronic condition was present on an ongoing basis or required treatment within the six months preceding ICU admission.[1] For operative patients, points are assigned if the condition was present and required treatment prior to the operation, with emergency postoperative cases treated equivalently to non-operative status for scoring purposes.[1] These chronic health points are added to the acute physiology and age scores to form the total APACHE II score.[1]Calculation
Data Collection
The APACHE II score is computed using the most abnormal ("worst") physiological and clinical values observed during the first 24 hours following a patient's admission to the intensive care unit (ICU). The focus on worst values captures the peak severity of illness during this critical initial phase, providing a reliable snapshot of the patient's condition upon ICU entry.[1][9] Data for the APACHE II score are sourced from multiple clinical inputs to encompass a broad assessment of acute and chronic factors. Vital signs such as temperature, heart rate, mean arterial pressure, and respiratory rate are obtained directly from continuous ICU monitors or bedside recordings. Laboratory values, including arterial pH, serum electrolytes (sodium and potassium), creatinine, hematocrit, and white blood cell count, are derived from routine blood tests and arterial blood gas analyses (serum bicarbonate may be used as an optional substitute for pH if arterial blood gas is unavailable). The Glasgow Coma Scale (GCS) is assessed through standardized neurological examinations, ideally before any sedative administration to avoid confounding influences. Chronic health status is determined from the patient's medical history, reviewing records for pre-existing conditions like severe organ dysfunction or immunosuppression. These diverse sources ensure a holistic evaluation, with the 12 acute physiology variables forming the core of the score.[1][9] Best practices emphasize accuracy and consistency in data gathering to minimize errors in score calculation. Actual measured values should be prioritized over estimates or approximations, as the latter can lead to unreliable severity assessments; for instance, transient or artifactual readings (e.g., a single blood pressure spike) are excluded in favor of sustained abnormalities. For oxygenation, PaO₂ is used when the fraction of inspired oxygen (FiO₂) is less than 0.5, while the alveolar-arterial oxygen gradient (A-aDO₂) is calculated and applied when FiO₂ is 0.5 or greater, selecting the value that yields the highest score to reflect maximal derangement. In cases of elective surgery patients, chronic health points are assigned as 2 if a severe underlying condition exists, but 0 if none, as the admission is considered related to the procedure rather than underlying chronic illness. These guidelines help standardize collection and enhance the score's prognostic validity.[1][9][10] The APACHE II score is primarily calculated at the time of ICU admission using the aforementioned first-24-hour data, though it may be recomputed daily for ongoing monitoring of patient trajectory in some clinical settings. Accurate implementation requires multidisciplinary collaboration, with nurses responsible for real-time vital signs and laboratory logging, physicians contributing to GCS evaluations and historical reviews, and the broader ICU team ensuring data integration to avoid discrepancies. This team-based approach has been shown to improve inter-rater reliability and overall score precision in quality improvement initiatives.[1][11]Variable Scoring
The APACHE II score is computed by aggregating points from three main components: the Acute Physiology Score (APS), age points, and chronic health points, using the worst physiological values observed during the first 24 hours after ICU admission. The APS, which ranges from 0 to 60, is the sum of points assigned to 12 acute physiological variables, each scored from 0 to 4 based on the degree of abnormality relative to normal ranges; these variables include temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, arterial pH, serum sodium, serum potassium, serum creatinine, hematocrit, white blood cell count, and Glasgow Coma Scale (GCS).[1] Specific adjustments apply to certain variables for accuracy. The GCS is scored as 15 minus the patient's actual GCS value to reflect neurological impairment, with the lowest recorded score used. For serum creatinine, the assigned points are doubled in cases of acute renal failure—typically indicated by the need for dialysis or a rapid rise in levels—to account for heightened severity, effectively capping the contribution at 8 points if the base score is 4 (for levels ≥3.5 mg/dL or equivalent). All variables rely on the most abnormal ("worst") measurements within the assessment period to capture peak physiological derangement.[1][12] Age points are added separately, ranging from 0 (for patients ≤44 years) to 6 (for those ≥75 years), with intermediate values such as 2 for ages 45–54, 3 for 55–64, and 5 for 65–74. Chronic health points evaluate pre-existing conditions: 0 if no history of severe organ system insufficiency or immunocompromised state; 2 points if elective postoperative; 5 points if nonoperative or emergency postoperative. The total APACHE II score is then calculated as: \text{Total APACHE II} = \text{APS} + \text{Age Points} + \text{Chronic Health Points} with a possible range of 0 to 71.[1][13] For illustration, consider a hypothetical patient with a temperature of 41°C (scoring 4 points in the APS), age of 70 years (5 age points), and a history of cirrhosis qualifying as severe organ insufficiency under non-operative admission (5 chronic health points); this yields a partial total of 14 points, with additional APS contributions from other variables determining the full score.[1]Total Score Interpretation
The total APACHE II score, ranging from 0 to 71, provides a measure of disease severity that correlates with hospital mortality risk, with higher scores indicating poorer prognosis. In the original 1985 validation study involving 5,815 ICU patients across 13 hospitals, the score demonstrated a strong association with outcomes, yielding approximate mortality risks stratified by score intervals as shown below.[1]| Score Range | Approximate Mortality Risk |
|---|---|
| 0–4 | ~1% |
| 5–9 | ~10% |
| 10–14 | ~20% |
| 15–19 | ~40% |
| 20–24 | ~60% |
| 25–29 | ~80% |
| 30+ | ~95% |