APACHE II Calculator

APACHE II is a severity-of-disease classification developed for groups of critically ill adult patients. This implementation adds an acute physiology score, age points, and chronic-health points. It shows that breakdown so a trained user can inspect where the total came from.

Use the worst applicable measurements during the first 24 hours in intensive care, following the original method and local clinical protocol. The calculator is not a diagnosis, triage instruction, bedside monitor, or individual mortality prediction. It is not intended for self-assessment.

How the score is constructed

The acute physiology component assigns 0–4 points to 11 measured variables. When acute renal failure is present, the creatinine points double. Neurologic points are calculated directly as GCS points = 15 − actual GCS, producing 0–12 points rather than a coarse category.

Oxygenation uses two different inputs. When FiO₂ is below 50%, enter arterial PaO₂ and score it from the PaO₂ thresholds. When FiO₂ is 50% or higher, enter the alveolar–arterial oxygen difference (A–aDO₂), not a PaO₂/FiO₂ ratio. Confirm units and how the A–a difference was derived before entering it.

Age contributes 0 points below 45 years, 2 at 45–54, 3 at 55–64, 5 at 65–74, and 6 at 75 or older. Qualifying severe organ insufficiency or immunocompromise contributes 2 points for an elective postoperative patient and 5 points for a nonoperative or emergency postoperative patient. Select “none” unless the original chronic-health criteria are actually met.

Worked example

Suppose a 50-year-old adult has all physiological measurements in the zero-point ranges, a GCS of 15, no acute renal failure, and no qualifying chronic-health condition. The acute physiology score is 0, the age score is 2, and the chronic-health score is 0:

APACHE II total = 0 + 2 + 0 = 2

If one input moves outside its zero-point range, only the corresponding points change. For example, a GCS of 12 contributes 15 − 12 = 3 points. The displayed component totals help the clinician find an accidental unit, range, or selection error before recording the result.

How to interpret the result

A higher total reflects greater physiologic derangement, older age, or qualifying chronic illness in the variables represented by APACHE II. It does not, by itself, state why the patient is ill or determine the correct treatment.

The original publication used admission diagnosis and population-level equations when estimating hospital mortality. A raw score-to-percentage lookup applied to every diagnosis is not a valid universal individual prediction, so this calculator intentionally does not display a mortality percentage. Outcome calibration also varies by case mix, location, era, and data collection practice.

Limits and clinical safety

This tool is for a trained clinician or trained clinical data abstractor who can identify the correct observation window, units, qualifying chronic conditions, and effects of treatment or sedation. Glasgow Coma Scale assessment in sedated, paralysed, or intubated patients requires an established clinical protocol; do not invent a value to make the form complete.

The page does not replace the original paper, institutional definitions, electronic validation, or clinical judgment. It is not validated here for people under 16, pregnancy, burns, coronary care populations, or any population excluded or differently handled by the intended deployment. Never delay emergency assessment or change life-sustaining treatment because of this result.

No external medical reviewer participated in this SoupCalc rewrite. Readers should treat it as a transparent software implementation with primary-source links, not as independently reviewed clinical guidance or an implied medical endorsement.

Sources

• Knaus et al., “APACHE II: a severity of disease classification system” (PubMed) — original 1985 publication and method.
• Merck Manual Professional — APACHE II scoring table — accessible parameter thresholds and scoring notes.

Editorial record

Rewritten and implementation-checked by the SoupCalc Editorial Team. Thresholds, GCS arithmetic, oxygen branches, renal multiplier, age points, and chronic-health points were checked against the cited professional table. This is an editorial and software check, not medical peer review.

Last reviewed: August 10, 2026.