Calculator Guides
APACHE II Score: Complete Clinical Guide
APACHE II quantifies ICU illness severity using 12 acute physiologic variables, age, and chronic health status (range 0–71). It predicts group mortality — not individual outcome. Most accurate when calculated within the first 24 hours of ICU admission using the worst values.
TL;DR: APACHE II quantifies ICU illness severity using 12 acute physiologic variables, age, and chronic health status (range 0–71). It predicts group mortality — not individual outcome. It's most accurate when calculated within the first 24 hours of ICU admission using the worst values. Don't use it to make individual prognostic decisions; use it to compare populations, benchmark ICU performance, and adjust for case mix in research.
Table of Contents
- What Is the APACHE II Score?
- How to Calculate APACHE II
- Score Interpretation
- Clinical Applications
- Limitations and Considerations
- Evidence and Guidelines
- Frequently Asked Questions
- References
What Is the APACHE II Score?
APACHE II (Acute Physiology and Chronic Health Evaluation II) was published by William Knaus and colleagues in 1985 as a simplified version of the original APACHE system from 1981. The goal: provide a standardized, reproducible severity score that could be calculated at ICU admission to predict hospital mortality across diverse patient populations.
The original APACHE used 34 physiologic variables. APACHE II trimmed this to 12 — the minimum set that maintained predictive accuracy while being practical enough to calculate at the bedside. It also added age and chronic health status, recognizing that baseline reserve matters as much as acute derangement.
APACHE II became the most widely used ICU severity score in the world. It's been cited over 30,000 times and validated across dozens of countries. Despite newer versions (APACHE III, IV), APACHE II remains the standard in most clinical settings because of its simplicity and extensive literature base.
How to Calculate APACHE II
APACHE II has three components:
1. Acute Physiology Score (APS) — 12 Variables
Use the worst value in the first 24 hours of ICU admission. Each variable is scored 0–4 based on deviation from normal.
| Variable | Range Scored |
|---|---|
| Temperature (rectal, °C) | 0–4 |
| Mean arterial pressure (mmHg) | 0–4 |
| Heart rate | 0–4 |
| Respiratory rate | 0–4 |
| Oxygenation: FiO₂ ≥0.5 → A-a gradient; FiO₂ <0.5 → PaO₂ | 0–4 |
| Arterial pH | 0–4 |
| Serum sodium (mEq/L) | 0–4 |
| Serum potassium (mEq/L) | 0–4 |
| Serum creatinine (mg/dL) — doubled for acute renal failure | 0–4 |
| Hematocrit (%) | 0–4 |
| White blood cell count (×1000/µL) | 0–4 |
| Glasgow Coma Scale (15 minus GCS) | 0–12 |
APS total: 0–60 points (though scores above 40 are extremely rare)
2. Age Points
| Age (years) | Points |
|---|---|
| ≤44 | 0 |
| 45–54 | 2 |
| 55–64 | 3 |
| 65–74 | 5 |
| ≥75 | 6 |
3. Chronic Health Points
Assign points only if the patient has a history of severe organ insufficiency OR is immunocompromised:
- Nonoperative or emergency postoperative patient: +5 points
- Elective postoperative patient: +2 points
Qualifying chronic conditions: liver cirrhosis with portal hypertension, NYHA Class IV heart failure, severe COPD, chronic dialysis, or immunocompromised state.
Total APACHE II = APS + Age Points + Chronic Health Points (range: 0–71)
Score Interpretation
| APACHE II Score | Approximate ICU Mortality | Clinical Context |
|---|---|---|
| 0–4 | ~4% | Minimal acute physiologic derangement |
| 5–9 | ~8% | Mild severity |
| 10–14 | ~15% | Moderate severity |
| 15–19 | ~25% | Moderately severe |
| 20–24 | ~40% | Severe |
| 25–29 | ~55% | Very severe |
| 30–34 | ~75% | Critical |
| ≥35 | >80% | Extreme severity |
These are population-level estimates from the original APACHE II validation cohort. Modern ICU care has improved outcomes substantially.
Clinical Applications
ICU Benchmarking
APACHE II is widely used to calculate Standardized Mortality Ratios (SMR = observed mortality / predicted mortality). An SMR <1.0 suggests better-than-predicted performance.
Research Stratification
In clinical trials, APACHE II adjusts for illness severity across randomized groups.
Clinical Communication
An APACHE II of 28 communicates severity to consultants, administrators, and families more precisely than "he's very sick."
Case Vignettes
Case 1: 58-year-old with septic shock from urosepsis. Temp 39.2°C (+1), MAP 62 (+2), HR 124 (+3), RR 28 (+1), pH 7.26 (+3), Cr 2.1 (+3), WBC 22 (+2), GCS 13 (15−13=+2). APS = 17. Age 55–64 (+3). No qualifying chronic conditions (0). APACHE II = 20. Predicted mortality ~40%.
Case 2: 72-year-old post-elective aortic valve replacement. Near-normal physiology. APS = 4. Age 72 (+5). Elective postoperative with NYHA III history (+2). APACHE II = 11. Expected mortality ~15%.
Limitations and Considerations
APACHE II predicts group mortality, not individual fate. A score of 30 means a patient population with that score has ~75% ICU mortality. This specific patient might be in the 25% who survive. Don't use APACHE II to limit care for individuals.
The original calibration data is from 1979–1982. ICU care has fundamentally changed.
Timing matters. APACHE II must be calculated using the worst values in the first 24 hours of ICU admission.
The GCS component is confounded by sedation. A paralyzed, sedated patient scores GCS 3, which adds 12 points to the APS.
APACHE II was not designed for cardiac arrest or burn patients. Disease-specific models outperform APACHE II in those populations.
Evidence and Guidelines
Wong DT and Knaus WA (1991) reviewed the development and applications of the APACHE scoring system. They confirmed that APACHE II retained predictive accuracy while reducing variables from 34 to 12. Can J Anaesth. 1991;38(3):374–383. PMID: 2036699. DOI: 10.1007/BF03007629
Duan et al. (2016) validated the APACHE II score in hypoxemic ICU patients receiving noninvasive ventilation. Intensive Care Med. 2017;43(2):192–199. PMID: 27812731. DOI: 10.1007/s00134-016-4601-3
Frequently Asked Questions
Q: What's the difference between APACHE II and SOFA? APACHE II measures admission severity and predicts overall ICU mortality. SOFA assesses organ dysfunction dynamically — scored daily.
Q: Should I calculate APACHE II daily? No — it's designed as a 24-hour admission score. Use SOFA for daily tracking.
Q: What's the worst possible APACHE II score? Theoretically 71, but scores above 50 are rarely encountered.
Q: Is APACHE II useful in the ED? It wasn't designed for ED use. For ED severity scoring, NEWS2 or qSOFA are more appropriate.
Related Calculators
- SOFA Score — daily organ dysfunction tracking
- qSOFA — rapid sepsis risk screen
- SAPS II — simplified acute physiology score
- Glasgow Coma Scale — GCS component of APACHE II
References
- Wong DT, Knaus WA. Predicting outcome in critical care: the current status of the APACHE prognostic scoring system. Can J Anaesth. 1991;38(3):374–383. PMID: 2036699. DOI: 10.1007/BF03007629
- Duan J, Han X, Bai L, et al. Assessment of heart rate, acidosis, consciousness, oxygenation, and respiratory rate to predict noninvasive ventilation failure in hypoxemic patients. Intensive Care Med. 2017;43(2):192–199. PMID: 27812731. DOI: 10.1007/s00134-016-4601-3
- Knaus WA, Draper EA, Wagner DP, et al. APACHE II: a severity of disease classification system. Crit Care Med. 1985;13(10):818–829. PMID: 3928249.
- Vincent JL, Moreno R, Takala J, et al. The SOFA score to describe organ dysfunction/failure. Intensive Care Med. 1996;22(7):707–710. PMID: 8844239.