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APACHE II Calculator

The APACHE II score quantifies illness severity and predicted mortality for patients in intensive care within their first 24 hours of admission. Developed in 1981, this system guides treatment intensity, resource allocation, and prognostic communication. ICU physicians, intensivists, and critical care nurses use it to stratify patient risk and benchmark outcomes across populations.

Last updated:

Creators Małgorzata Koperska, MD
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What Is the APACHE II Scoring System?

The Acute Physiology and Chronic Health Evaluation II (APACHE II) is a validated severity-of-illness classification tool used to predict mortality in critically ill patients. It combines acute physiological derangements, chronic health status, and patient age into a single composite score.

The scoring system evaluates three domains:

  • Patient characteristics: Age and significant comorbidities (organ insufficiency, immunocompromise)
  • Acute physiological parameters: Temperature, blood pressure, heart rate, respiratory rate, oxygenation, and acid-base status
  • Laboratory abnormalities: Electrolytes, renal function, blood counts, and pH

Higher APACHE II scores correlate directly with increased mortality risk. A score of 0–4 predicts approximately 4% mortality, whilst scores above 40 suggest mortality exceeding 80%. The score remains the reference standard for ICU severity assessment across teaching hospitals and research protocols.

APACHE II Score Calculation

The APACHE II score aggregates points from all three domains. Each physiological variable and comorbidity contributes a specific point allocation based on validated thresholds. The formula synthesizes age points, organ insufficiency status, surgical emergency status, and all acute physiological and laboratory derangements.

APACHE II Score = Age Points + Comorbidity Points + Surgery Points

+ Temperature Points + MAP Points + Heart Rate Points

+ Respiratory Rate Points + Oxygenation Points + pH Points

+ Sodium Points + Potassium Points + Creatinine Points

+ Hematocrit Points + WBC Points + (15 − Glasgow Coma Scale)

  • Age Points — 0–6 points based on age brackets; older patients accumulate higher scores
  • Comorbidity Points — 5 points for non-operative or emergency surgery patients with severe organ disease; 2 points for elective postoperative patients
  • Surgery Points — 1 point if surgery was required in the 24 hours prior; 0 otherwise
  • Temperature, MAP, HR, RR, Oxygenation, pH — Each physiological variable earns 0–4 points depending on deviation from normal ranges
  • Electrolytes & Renal Function — Sodium, potassium, and creatinine contribute 0–4 points each; acute renal failure multiplies creatinine weight
  • Hematocrit & WBC — Blood count abnormalities contribute 0–4 points each
  • Glasgow Coma Scale — Subtracts from 15; comatose patients (GCS 3) add 12 points

Key Considerations for Accurate APACHE II Scoring

Precise APACHE II scoring depends on meticulous data collection and correct interpretation of the point-allocation rules.

  1. Use worst values in the first 24 hours — APACHE II requires the single most abnormal value for each parameter during the initial 24-hour ICU stay, not the most recent measurement. A nadir temperature, lowest pH, or peak creatinine within that window must be recorded, even if the patient improves shortly after.
  2. Distinguish elective from emergency postoperative status — The comorbidity points differ significantly: emergency or non-operative patients with organ failure earn 5 points, whilst elective postoperative patients with the same comorbidity earn only 2 points. Misclassification directly distorts the final score and predicted mortality.
  3. Account for renal failure when scoring creatinine — Patients with acute or chronic renal failure have creatinine weighted differently—doubling the maximum points available. Confirm dialysis dependency or measured glomerular filtration rate before assigning creatinine points to avoid underestimating severity in renal patients.
  4. Glasgow Coma Scale requires accurate neurological examination — GCS assessment demands a calm, cooperative patient and experienced examiner. Sedation, intubation, or head trauma may confound scoring. Document the reason for any GCS component that cannot be assessed (e.g., intubated patients receive points only for eye opening and motor response).

Clinical Applications and Outcomes Benchmarking

APACHE II scores enable clinicians to stratify patients into mortality risk bands and guide intensity of care decisions. Hospitals track aggregate APACHE II cohorts to benchmark outcome data against regional and national standards, identifying quality gaps or exceptional performance.

The score facilitates:

  • Prognostic counselling: Communicating realistic mortality estimates to families and patients
  • Triage and resource allocation: Prioritizing invasive monitoring or organ support for highest-risk cohorts
  • Research cohort matching: Ensuring comparable severity between control and intervention groups in clinical trials
  • Quality assurance: Standardized severity adjustment allows meaningful comparison of mortality across ICU populations and institutions

Importantly, APACHE II predicts average population mortality; individual patient outcomes depend on unmeasured factors such as treatment quality, comorbidity severity, and physiological reserve.

Frequently Asked Questions

What is the difference between APACHE II and SAPS II scoring systems?

APACHE II (1981) and SAPS II (1993) both quantify ICU illness severity, but differ in methodology. APACHE II uses 15 physiological variables and includes comorbidity history; SAPS II uses 12 variables and excludes chronic disease points but incorporates admission type. SAPS II was calibrated on more recent cohorts and performs slightly better in European populations. Both remain valid; choice often reflects institutional tradition or local validation studies.

Can APACHE II be calculated on admission to the ICU or must I wait 24 hours?

APACHE II requires the worst physiological values from the initial 24 hours of ICU care. You cannot calculate an accurate score at admission because you lack complete data. Most centres compute the score retrospectively after the first 24 hours have elapsed, capturing the full range of derangements that established the baseline severity. Scores calculated before 24 hours are incomplete and misleading.

What APACHE II score indicates a patient is in critical condition?

APACHE II scores above 25 generally indicate severe illness with predicted mortality exceeding 40%. Scores of 30–34 suggest approximately 50–60% mortality, whilst scores above 40 correlate with mortality exceeding 80% in most cohorts. However, predicted mortality varies by diagnosis—septic patients with a score of 30 may have higher actual mortality than post-operative patients with the same score. Use APACHE II as a summary measure, not as the sole predictor.

How do I score Glasgow Coma Scale if the patient is sedated or paralyzed?

Sedation and paralysis preclude accurate full GCS assessment. Document which components cannot be evaluated and report the reason (e.g., 'sedated,' 'paralyzed'). Many ICUs apply estimated or verbal report-based GCS in these settings, but acknowledge the limitation. Some scoring systems recommend assigning worst-case points for unmeasurable components to avoid artificially lowering severity in non-responsive patients.

Does APACHE II score change as the patient improves, and should I recalculate it daily?

APACHE II is designed as a single point-in-time assessment during the first 24 hours and is not meant to be trended daily. Serial APACHE II calculations lose their predictive power because the score was calibrated for admission severity. Instead, clinicians use daily SOFA or qSOFA scores to track acute changes. A patient's original APACHE II remains a historical marker of admission severity and predicted mortality for that cohort.

What is a normal or healthy APACHE II score?

There is no 'normal' APACHE II score for healthy individuals—the system was developed exclusively for critically ill ICU patients. Non-ICU patients would have scores of 0–5 because they have few physiological derangements or comorbidities. Any patient with an APACHE II score of 0–10 has very low predicted mortality (under 5%) and may not require intensive care resources. The system distinguishes between severe illness groups, not between healthy and ill populations.

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