APACHE II Calculator
Reproduce the 1985 adult general-ICU APACHE II severity score from worst first-24-hour physiology, age and defined chronic-health points.
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About
APACHE II is the 1985 adult general medical/surgical ICU severity classification system. It uses the worst eligible value—the value that produces the highest APACHE II points—for each of 12 acute physiology components during the first 24 hours after ICU admission, then adds age and defined severe chronic-health points. The component worst values do not need to occur simultaneously, but FiO₂ and its paired PaO₂ or A–aDO₂ must describe the same oxygenation assessment. [1, 3]
This page calculates the submitted APACHE II score and audit only. It does not read a chart, ABG, laboratory record or GCS examination; it does not diagnose critical illness, calculate a current individual mortality probability, determine ICU placement, choose treatment, or support limits-of-life decisions. APACHE II is not APACHE III/IV, SAPS II, SOFA or MPM. [1, 2, 4, 5]
Formula
Interpretation
A higher total represents more acute physiological derangement and original background burden within this historical model. It does not diagnose critical illness or provide a score-only individual mortality probability: the original outcome work also used admission-diagnosis structure, and modern calibration varies by era, case mix, location and treatment. [1, 5, 6]
| Output element | Formal range | Meaning | What it does not mean |
|---|---|---|---|
| Acute Physiology Score | 0–60 | 12 first-24-hour physiology points | Not an independent diagnosis |
| Age points | 0–6 | Original age category | Not expected lifespan |
| Chronic health | 0, 2, 5 | Original severe chronic-health condition | Not ordinary comorbidity count |
| APACHE II total | 0–71 | Historical severity classification | Not current personal mortality probability |
APACHE II cannot become a treatment threshold, ICU escalation rule, or reason to limit life support by itself. [1, 5]
APACHE II scoring bands, first-24-hour rules, and model limits
First-24-hour model scope
Use an adult general medical/surgical ICU context and the value eligible under the applicable record definition that produces the highest APACHE II points for each item. Those values need not be simultaneous. Oxygenation values must be paired, and data from another admission, ICU stay or time window must not be mixed. This page cannot decide whether a value satisfies local audit or exclusion rules. [1, 3]
Complete acute physiology score table
The APACHE II APS has 12 components: 11 ordinary 0–4 point physiology bands plus one independent GCS contribution.
| Component | Unit or method | 4 points | 3 points | 2 points | 1 point | 0 points | Key limitation |
|---|---|---|---|---|---|---|---|
| Temperature | Core °C | ≥41 or <30 | 39–40.9 or 30–31.9 | 32–33.9 | 38.5–38.9 or 34–35.9 | 36–38.4 | Measurement site and eligibility are record-defined. |
| Mean arterial pressure | mmHg | ≥160 or <50 | 130–159 | 110–129 or 50–69 | — | 70–109 | Use the eligible recorded MAP, not an unpaired or undocumented source. |
| Heart rate | /min | ≥180 or <40 | 140–179 or 40–54 | 110–139 or 55–69 | — | 70–109 | Use the eligible recorded value, not a guessed treatment-adjusted value. |
| Respiratory rate | /min | ≥50 or <6 | 35–49 | 6–9 | 25–34 or 10–11 | 12–24 | Ventilatory support and timing may affect record interpretation. |
| Oxygenation | PaO₂ or A–aDO₂ | PaO₂ <55; A–aDO₂ ≥500 | PaO₂ 55–60; A–aDO₂ 350–499 | A–aDO₂ 200–349 | PaO₂ 61–70 | PaO₂ >70; A–aDO₂ <200 | FiO₂ <0.50 uses PaO₂; FiO₂ ≥0.50 uses A–aDO₂. |
| Arterial pH / serum bicarbonate | pH or mEq/L | pH ≥7.70 or <7.15; HCO₃⁻ ≥52 or <15 | pH 7.60–7.69 or 7.15–7.24; HCO₃⁻ 41–51.9 or 15–17.9 | pH 7.25–7.32; HCO₃⁻ 18–21.9 | pH 7.50–7.59; HCO₃⁻ 32–40.9 | pH 7.33–7.49; HCO₃⁻ 22–31.9 | Use one explicit method; venous pH is not silently relabeled arterial pH. |
| Sodium | mEq/L | ≥180 or <111 | 160–179 or 111–119 | 155–159 or 120–129 | 150–154 | 130–149 | Units must match the published scoring table. |
| Potassium | mEq/L | ≥7.0 or <2.5 | 6.0–6.9 | 2.5–2.9 | 5.5–5.9 or 3.0–3.4 | 3.5–5.4 | A value is scored, not interpreted as an indication for treatment. |
| Creatinine | mg/dL | ≥3.5 | 2.0–3.4 | 1.5–1.9 or <0.6 | — | 0.6–1.4 | Double component points only when the original acute renal failure condition applies. |
| Hematocrit | % | ≥60 or <20 | — | 50–59.9 or 20–29.9 | 46–49.9 | 30–45.9 | Use a documented eligible measurement and its unit. |
| White blood cells | ×10³/µL | ≥40 or <1 | — | 20–39.9 or 1–2.9 | 15–19.9 | 3–14.9 | Do not convert or infer units from a different report convention. |
| Component | Calculation | Formal input | Contribution range | Limitation |
|---|---|---|---|---|
| Glasgow Coma Scale | 15 − documented GCS | Integer GCS 3–15 | 0–12 points | Page does not perform or reconstruct the examination. |
Worked mapping: GCS 15 → 0 points; GCS 14 → 1 point; GCS 3 → 12 points.
The tables are static transcription aids for the published bands; the scorer remains the frozen runtime. [1, 3]
Oxygenation and acid-base branches
FiO₂ 0.4999 uses the PaO₂ branch and FiO₂ 0.50 uses the A–aDO₂ branch. PaO₂ has no 2-point interval. The A–aDO₂ value must already be obtained; this page does not calculate it from altitude, barometric pressure, PaCO₂ or respiratory quotient. Arterial pH is preferred when an ABG is available; serum bicarbonate is the mutually exclusive no-ABG substitute, and changing the method clears the old value. [1, 3]
Age and chronic-health points
Original severe chronic-health eligibility is separate from ordinary comorbidity. The website accepts adults aged 18–120; 18 is a technical entry boundary, not a new APACHE II age band. Qualifying examples include severe liver disease/portal hypertension in the original context, New York Heart Association class IV, severe chronic pulmonary limitation or ventilator dependence, chronic dialysis, or qualifying immunosuppression. The elective/nonoperative or emergency postoperative choice cannot create points without that qualification; uncertain eligibility must remain unresolved. [1, 3]
| Category | Points |
|---|---|
| <45 | 0 |
| 45–54 | 2 |
| 55–64 | 3 |
| 65–74 | 5 |
| ≥75 | 6 |
| Category | Points |
|---|---|
| No qualifying original severe chronic-health condition | 0 |
| Qualifying condition + elective postoperative admission | 2 |
| Qualifying condition + nonoperative or emergency postoperative admission | 5 |
What the calculator actually does
- Confirms the adult general-ICU first-24-hour context.
- Reads the 12 submitted physiology values and selected oxygenation/acid-base branches.
- Applies the original creatinine doubling condition and GCS contribution.
- Adds age and chronic-health points.
- Returns APS, total and a 14-row submitted audit.
It does not read records, judge worst-value eligibility, calculate A–aDO₂ or GCS, diagnose acute renal failure or critical illness, produce a current mortality probability, or choose treatment, ICU placement or life-support limits. [1, 5]
APACHE II and related ICU models
| Model | Time frame | Main inputs | Primary use | Why not interchangeable |
|---|---|---|---|---|
| APACHE II | ICU first 24 hours | 12 physiology items, age, chronic health | Historical severity/case-mix score | This page's 1985 point bands and inputs are not an APACHE III/IV, SAPS, SOFA or MPM conversion. |
| APACHE III/IV | Model-specific ICU windows | Different variables, diagnosis classes and coefficients | Independent updated models | Different variables, diagnosis classes, coefficients and calibration; a total cannot be relabeled as APACHE II. |
| SAPS II | ICU admission-era model | Different physiology, age, disease and type-of-admission inputs | Separate prognostic system | Its predictors and equation are not the APACHE II component bands. |
| SOFA | Baseline and serial organ dysfunction | Six organ systems | Trendable organ-dysfunction description | SOFA describes organ dysfunction over time rather than reproducing APACHE II's first-24-hour point total. |
| MPM | Model-specific ICU time point | Different predictors and equation | Separate mortality model | Its time point, predictors and equation are not interchangeable with APACHE II. |
APACHE IV is an independent updated model, not a conversion of APACHE II. SAPS II and MPM use different predictors and equations; SOFA describes six organ systems and is commonly trended. No score can be relabeled as another model. [2, 4, 5]
Worked examples from the frozen implementation
- Zero vector: temperature 37, MAP 80, HR 80, RR 16, FiO₂ 0.21 with PaO₂ 80, pH 7.4, sodium 140, potassium 4, creatinine 1 without acute renal failure, hematocrit 40, WBC 10, GCS 15, age 44 and no chronic health → APS 0, total 0.
- FiO₂ boundary: FiO₂ 0.4999 with PaO₂ 55 uses PaO₂ and scores 3; FiO₂ 0.50 with A–aDO₂ 200 uses A–aDO₂ and scores 2. One metric is never substituted for the other.
- Acid-base substitute: pH 7.4 scores 0; without ABG, serum bicarbonate 21.9 scores 2. They are mutually exclusive.
- Creatinine doubling: creatinine 2.0 scores 3 when acute renal failure is No and 6 when the explicit original condition is Yes; the page does not diagnose that condition.
- Theoretical high boundary: the frozen high vector totals 71. This demonstrates score arithmetic only, not a mortality percentage or treatment/limits-of-care recommendation.
These examples are generated by the frozen implementation and are not patient-specific predictions. [1, 3]
Calibration and external validation
Discrimination describes ranking ability; calibration describes agreement between predicted and observed outcomes. External validation found APACHE II can retain discrimination while showing poor calibration, and historical Canadian and other ICU cohorts illustrate that era, location and case mix matter. Recalibration is a separate statistical task that this page does not perform. [5, 6, 7, 8]
Important limitations
Do not generalize this page to pediatric, burn-specific, cardiac-surgery/CABG or other specialty ICUs; transfers, readmissions, short-stay observation, treatment-modified physiology, sedation, paralysis, intubation, missing or confounded GCS, mismatched FiO₂/PaO₂ timing, altitude/A–aDO₂ assumptions, evolving renal failure, dialysis, ECMO or mechanical support, unusual laboratory units, pregnancy, physiology after 24 hours, diagnosis category, local calibration and chart-abstraction error can all limit interpretation. A low score does not mean safe, a high score does not automatically mean death, ICU escalation or treatment limitation, and an unstable patient should not wait for an online calculator. APACHE II does not replace continuous clinical assessment. [1, 5, 6]
References
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE. APACHE II: a severity of disease classification system. Crit Care Med. 1985;13(10):818–829. PMID 3928249.
- Knaus WA, Zimmerman JE, Wagner DP, Draper EA, Lawrence DE. APACHE—acute physiology and chronic health evaluation: a physiologically based classification system. Crit Care Med. 1981;9(8):591–597. PMID 7261642.
- Merck Manual Professional Edition. Acute Physiologic Assessment and Chronic Health Evaluation (APACHE) II Scoring System.
- Zimmerman JE, Kramer AA, McNair DS, Malila FM. Acute Physiology and Chronic Health Evaluation (APACHE) IV: hospital mortality assessment for today’s critically ill patients. Crit Care Med. 2006;34(5):1297–1310. PMID 16540951.
- Keegan MT, Soares M. What every intensivist should know about prognostic scoring systems and risk-adjusted mortality. Rev Bras Ter Intensiva. 2016;28(3):264–269. PMID 27737416. PMCID PMC5051184.
- Cox EGM, Wiersema R, Eck RJ, et al. External Validation of Mortality Prediction Models for Critical Illness Reveals Preserved Discrimination but Poor Calibration. Crit Care Med. 2023;51(1):80–90. PMID 36378565. DOI 10.1097/CCM.0000000000005712.
- Wong DT, Crofts SL, Gomez M, McGuire GP, Byrick RJ. Evaluation of predictive ability of APACHE II system and hospital outcome in Canadian intensive care unit patients. Crit Care Med. 1995;23(7):1177–1183. PMID 7600821.
- Chang RWS, Jacobs S, Lee B. Predicting deaths among intensive care unit patients. Crit Care Med. 1988;16(1):34–42. PMID 3338276.
FAQ
Use arterial pH when an arterial blood gas is available. The original APACHE II table permits venous serum bicarbonate as an optional substitute only when no arterial blood gases are available. The page requires one explicit method, never combines the two, and clears the old value when the method changes.
It applies only to the original severe organ-insufficiency or immunosuppression definition that existed before this admission: severe liver disease or portal hypertension with the original prior-failure/encephalopathy/coma context, NYHA class IV heart disease, severely limiting chronic pulmonary disease or ventilator dependence, chronic dialysis, or qualifying immunosuppression. It is not a general comorbidity count, multiple qualifying conditions still count once, and an emergency postoperative category requires immediate surgery to correct a life-threatening condition. If eligibility is uncertain, do not guess to obtain points.
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Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.