About the APACHE II Score Calculator
APACHE II (Acute Physiology and Chronic Health Evaluation II) is the classic intensive care severity-of-illness score published by Knaus and colleagues in 1985. It adds three parts: the Acute Physiology Score from 12 routine measurements, points for age, and points for severe chronic organ insufficiency or immunosuppression. Scores range from 0 to 71, and higher scores were closely associated with hospital death in the original 5,815 ICU admissions from 13 US hospitals.
Enter the worst value of each variable recorded during the first 24 hours after ICU admission — the one furthest from normal. Oxygenation is scored from the alveolar–arterial (A–a) gradient when FiO₂ is 50% or more and from PaO₂ when FiO₂ is below 50%; the calculator works out the A–a gradient from PaO₂, PaCO₂ and FiO₂ at sea level. Creatinine points are doubled in acute kidney injury, and serum bicarbonate can replace arterial pH when no blood gas is available.
The score was validated for adult ICU patients and is mainly used for research, audit and comparing case mix between units. The mortality figure shown is the approximate death rate observed for non-operative patients in that score band; formal risk prediction also needs the admission diagnostic category, and individual prognosis should never rest on a score alone.
With the default inputs, the apache ii score is 10. Change any value above to recalculate instantly.
How to use the apache ii score calculator
- 1Collect the worst value of each variable from the first 24 hours of the ICU stay.
- 2Enter age, vital signs, FiO₂ and arterial blood gas values.
- 3Enter sodium, potassium, creatinine, haematocrit, white cell count and GCS.
- 4Mark acute renal failure and chronic health status if they apply, and pick the admission type.
- 5Read the total score, the point breakdown and the approximate mortality band.
Formula and method
Each of the 12 physiological variables is scored 0 (normal) to 4 (most abnormal) using the worst value in the first 24 hours. Oxygenation uses the A–a gradient when FiO₂ ≥ 0.5 (A–a = FiO₂ × 713 − PaCO₂ ÷ 0.8 − PaO₂ at sea level) and PaO₂ otherwise. Serum creatinine points are doubled for acute renal failure, and the neurological score is 15 minus the Glasgow Coma Scale.
Age adds 0 points for 44 or younger, 2 for 45–54, 3 for 55–64, 5 for 65–74 and 6 for 75 or older. Patients with a history of severe organ insufficiency or immunocompromise receive 5 points if non-operative or emergency post-operative and 2 points if elective post-operative. Band boundaries follow the original Knaus table; values between printed bands are assigned to the band whose lower limit they reach.
- APS
- Acute Physiology Score (sum of 12 variable scores, 0–60)
- A–a
- Alveolar–arterial oxygen gradient (mmHg)
- GCS
- Glasgow Coma Scale (3–15)
Worked examples
Febrile tachycardic patient, age 60
Temperature 38.6 °C (1), heart rate 115 (2), respiratory rate 28 (1), creatinine 1.6 mg/dL (2) and WBC 16 (1) give an APS of 7. Age 60 adds 3, for a total of 10 — the 10–14 band, where non-operative hospital mortality was about 15%.
Septic shock on high FiO₂ with cirrhosis
A–a gradient = 0.6 × 713 − 32 ÷ 0.8 − 70 ≈ 318 mmHg (2 points). Adding temperature 1, MAP 2, HR 3, RR 3, pH 3, K 1, creatinine 3 doubled to 6, Hct 2, WBC 2 and GCS 4 gives APS 29. Age 70 adds 5 and non-operative chronic illness 5, for 39 — above 34, where non-operative mortality was about 85%.
Elective post-operative patient using bicarbonate
With FiO₂ under 50%, PaO₂ 58 scores 3. Serum HCO₃⁻ 19 scores 2, potassium 3.2 scores 1 and GCS 14 adds 1, so APS = 7. Age 50 adds 2 and elective post-operative chronic illness adds 2, for a total of 11.
Frequently asked questions
What is a good or bad APACHE II score?+
Lower is better. In the original study, scores of 0–4 carried about a 4% hospital death rate in non-operative patients, scores of 20–24 about 40%, and scores above 34 about 85%. Outcomes today vary with case mix and era.
Which values should I use for APACHE II?+
Use the worst (most abnormal) value of each variable recorded during the first 24 hours after ICU admission. If a value was not measured, it is conventionally scored as normal (0 points); leave that field blank here and it scores 0.
When is the A–a gradient used instead of PaO₂?+
If the patient is receiving FiO₂ of 0.5 (50%) or more, oxygenation is scored from the alveolar–arterial gradient. Below 50% FiO₂, the PaO₂ itself is scored: above 70 mmHg is 0 points and below 55 mmHg is 4 points.
What counts as chronic health for APACHE II?+
Before admission: biopsy-proven cirrhosis with portal hypertension or prior hepatic failure, NYHA class IV heart failure, severe chronic respiratory disease, chronic dialysis, or immunocompromise from therapy or disease such as leukaemia, lymphoma or AIDS.
Can APACHE II predict an individual patient’s death?+
No. APACHE II was designed to stratify groups of patients for research and quality comparison. Formal predicted mortality also needs the admission diagnosis weight, and even then it describes populations, not a single patient.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.
Sources
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med 1985 (PubMed)
- Mean SOFA score in comparison with APACHE II score in predicting mortality (Table 7 reproduces the APACHE II point table), Cureus 2023 — PMC
- Comparison of critical illness scores in intermediate care units (APACHE II table with mortality bands), 2024 — PMC