What exactly does this APACHE II page calculate?

It reproduces the 1985 APACHE II point score: 12 acute physiological components, age points and defined severe chronic-health points. It does not apply an admission-diagnosis coefficient or calculate a current individual mortality probability.

Which values belong in the first-24-hour assessment?

Use the most abnormal eligible value for each component during the first 24 hours after adult general-ICU admission. The worst components do not need to occur at the same time, but FiO₂ and PaO₂ or A–aDO₂ must come from the same oxygenation assessment.

Why does oxygenation switch between PaO₂ and A–aDO₂?

The published score uses PaO₂ when FiO₂ is below 0.50 and the alveolar–arterial oxygen difference when FiO₂ is 0.50 or higher. The branches are not interchangeable, and this page clears the paired oxygenation value when FiO₂ changes.

When is serum bicarbonate used instead of arterial pH?

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.

How does Glasgow Coma Scale enter APACHE II?

The neurological contribution is 15 minus the documented GCS total. The GCS examination itself should be recorded with its components and confounders; APACHE II does not resolve an untestable or treatment-confounded examination.

When are creatinine points doubled?

The original scoring table doubles the creatinine component when its acute renal failure condition applies. The calculator requires an explicit Yes or No and does not infer this condition from a creatinine value.

What does the chronic-health component mean?

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.

Does an APACHE II score give an individual mortality probability?

No. The original hospital-mortality equation also required emergency postoperative status and an admission diagnostic category weight: logit(R) = −3.517 + 0.146 × APACHE II + 0.603 if post-emergency surgery + diagnostic category weight. Absolute calibration varies by population, era and local care, so a score-only percentage table is not a current patient-specific prediction.

How does APACHE II differ from SOFA, APACHE III/IV, and SAPS II?

APACHE II is a historical first-24-hour ICU severity model containing physiology, age and chronic-health points. SOFA scores six organ systems and is often followed over time; APACHE III/IV and SAPS II use different predictors and equations. These models are not interchangeable.

Is APACHE II validated for children or burn-specific care?

This page is restricted to adults in a general ICU context. It is not a pediatric score, a burn-size or fluid formula, or a dedicated specialty-unit model. Population-specific tools and clinical assessment are needed.

Educational reference only. Not a diagnostic tool. See full disclaimer.

Emergency Medicine & Critical CareAPACHE II

APACHE II Score Calculator

Reproduce the 1985 adult general-ICU APACHE II severity score from worst first-24-hour physiology, age and defined chronic-health points.

Content updated: September 4, 2026View sources

QuickMedCalc is developed and maintained by an independent developer. Medical content is not independently reviewed by a physician.

Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.

Before entering APACHE II values

Use the most abnormal eligible value for each component during the first 24 hours after the same general medical/surgical ICU admission. Component worst values do not need to be simultaneous, but FiO₂ and PaO₂ or A–aDO₂ must come from the same oxygenation assessment. Do not mix records from different admissions, ICU stays, or time windows. Original population and exclusion limits still matter; this calculator cannot establish record eligibility.

Vital physiology

Core temperature

Use the most abnormal eligible core temperature in the window.

°C

Mean arterial pressure

mmHg

Heart rate

/min

Respiratory rate

/min

Oxygenation

FiO₂

Enter 0.21–1.00. Changing FiO₂ clears the paired oxygenation value.

fraction

Arterial oxygen pressure (PaO₂)

Required when FiO₂ is below 0.50; enter the already reported paired PaO₂.

mmHg

Acid-base and electrolytes

Acid-base input method

Use arterial pH when an ABG is available. Serum bicarbonate is an optional substitute only when no ABG is available; methods are never combined.

Serum sodium

mEq/L

Serum potassium

mEq/L

Renal and hematologic data

Serum creatinine

mg/dL

Original acute renal failure condition

Select Yes only when the original APACHE II acute renal failure condition is documented. This page does not diagnose acute renal failure or use modern KDIGO staging; uncertainty must remain unresolved.

Hematocrit

White blood cell count

×10³/µL

Neurologic assessment

Documented Glasgow Coma Scale total

Use a time-recorded reliable GCS. The page does not perform the examination; sedation, paralysis, intubation, drugs, intoxication, hearing/language barriers and baseline neurologic status may confound it. Do not guess.

3–15

Age and chronic health

Age

Enter 16–120. Age 16 is the original lower population boundary; 120 is a technical input guard, not a scoring cutoff. The first age-point threshold remains 45.

completed years

Original severe chronic-health category

Do not select points for ordinary comorbidity. Qualification must pre-date this admission and remain reliable; uncertainty must not be guessed.

Original eligibility examples

Severe liver disease/portal hypertension in the original severe context; New York Heart Association class IV; severe chronic pulmonary limitation, chronic hypoxemia/hypercapnia, severe pulmonary hypertension or ventilator dependence; chronic dialysis; or qualifying immunosuppressive disease/treatment. Emergency postoperative means immediate surgery for a life-threatening condition. Multiple qualifying conditions still count once.

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, 2]

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, 3, 4]

Formula

APACHE II total = Acute Physiology Score + Age points + Chronic Health points. [1, 2]

Acute Physiology Score = sum of 12 physiological component scores; APS range 0–60. [1, 2]

GCS contribution = 15 − documented GCS; age points and original severe chronic-health points are added independently. [1, 2]

FiO₂ <0.50 uses PaO₂; FiO₂ ≥0.50 uses an already reported A–aDO₂. The page does not calculate the gradient. [1, 2]

Use arterial pH when an ABG is available; use serum bicarbonate only as the no-ABG substitute. The methods are mutually exclusive. [1, 2]

Creatinine component points are doubled only when the original acute renal failure condition is explicitly selected; the creatinine value itself is not doubled. [1, 2]

The theoretical total range is 0–71. A complete valid record is required; this page never turns missing information into a normal value or a partial total. [1]

The score is not a current personal mortality probability and does not produce a treatment instruction. [1, 4]

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, 4, 5]

APACHE II output element interpretation
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

Output element: Acute Physiology Score
Formal range: 0–60
Meaning: 12 first-24-hour physiology points
What it does not mean: Not an independent diagnosis

Output element: Age points
Formal range: 0–6
Meaning: Original age category
What it does not mean: Not expected lifespan

Output element: Chronic health
Formal range: 0, 2, 5
Meaning: Original severe chronic-health condition
What it does not mean: Not ordinary comorbidity count

Output element: APACHE II total
Formal range: 0–71
Meaning: Historical severity classification
What it does not mean: Not current personal mortality probability

APACHE II cannot become a treatment threshold, ICU escalation rule, or reason to limit life support by itself. [1, 4]

APACHE II scoring bands, first-24-hour rules, and model limits

First-24-hour model scope

Use a 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, 2]

Complete acute physiology score table

The APACHE II APS has 12 components: 11 ordinary 0–4 point physiology bands plus one independent GCS contribution.

Complete APACHE II acute physiology scoring table
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: Temperature
Unit or method: Core °C
4 points: ≥41 or <30
3 points: 39–40.9 or 30–31.9
2 points: 32–33.9
1 point: 38.5–38.9 or 34–35.9
0 points: 36–38.4
Key limitation: Measurement site and eligibility are record-defined.

Component: Mean arterial pressure
Unit or method: mmHg
4 points: ≥160 or <50
3 points: 130–159
2 points: 110–129 or 50–69
1 point: —
0 points: 70–109
Key limitation: Use the eligible recorded MAP, not an unpaired or undocumented source.

Component: Heart rate
Unit or method: /min
4 points: ≥180 or <40
3 points: 140–179 or 40–54
2 points: 110–139 or 55–69
1 point: —
0 points: 70–109
Key limitation: Use the eligible recorded value, not a guessed treatment-adjusted value.

Component: Respiratory rate
Unit or method: /min
4 points: ≥50 or <6
3 points: 35–49
2 points: 6–9
1 point: 25–34 or 10–11
0 points: 12–24
Key limitation: Ventilatory support and timing may affect record interpretation.

Component: Oxygenation
Unit or method: PaO₂ or A–aDO₂
4 points: PaO₂ <55; A–aDO₂ ≥500
3 points: PaO₂ 55–60; A–aDO₂ 350–499
2 points: A–aDO₂ 200–349
1 point: PaO₂ 61–70
0 points: PaO₂ >70; A–aDO₂ <200
Key limitation: FiO₂ <0.50 uses PaO₂; FiO₂ ≥0.50 uses A–aDO₂.

Component: Arterial pH / serum bicarbonate
Unit or method: pH or mEq/L
4 points: pH ≥7.70 or <7.15; HCO₃⁻ ≥52 or <15
3 points: pH 7.60–7.69 or 7.15–7.24; HCO₃⁻ 41–51.9 or 15–17.9
2 points: pH 7.25–7.32; HCO₃⁻ 18–21.9
1 point: pH 7.50–7.59; HCO₃⁻ 32–40.9
0 points: pH 7.33–7.49; HCO₃⁻ 22–31.9
Key limitation: Use one explicit method; venous pH is not silently relabeled arterial pH.

Component: Sodium
Unit or method: mEq/L
4 points: ≥180 or <111
3 points: 160–179 or 111–119
2 points: 155–159 or 120–129
1 point: 150–154
0 points: 130–149
Key limitation: Units must match the published scoring table.

Component: Potassium
Unit or method: mEq/L
4 points: ≥7.0 or <2.5
3 points: 6.0–6.9
2 points: 2.5–2.9
1 point: 5.5–5.9 or 3.0–3.4
0 points: 3.5–5.4
Key limitation: A value is scored, not interpreted as an indication for treatment.

Component: Creatinine
Unit or method: mg/dL
4 points: ≥3.5
3 points: 2.0–3.4
2 points: 1.5–1.9 or <0.6
1 point: —
0 points: 0.6–1.4
Key limitation: Double component points only when the original acute renal failure condition applies.

Component: Hematocrit
Unit or method: %
4 points: ≥60 or <20
3 points: —
2 points: 50–59.9 or 20–29.9
1 point: 46–49.9
0 points: 30–45.9
Key limitation: Use a documented eligible measurement and its unit.

Component: White blood cells
Unit or method: ×10³/µL
4 points: ≥40 or <1
3 points: —
2 points: 20–39.9 or 1–2.9
1 point: 15–19.9
0 points: 3–14.9
Key limitation: Do not convert or infer units from a different report convention.

APACHE II Glasgow Coma Scale contribution
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.

Component: Glasgow Coma Scale
Calculation: 15 − documented GCS
Formal input: Integer GCS 3–15
Contribution range: 0–12 points
Limitation: 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 runtime source of truth. [1, 2]

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. Use the separate A–a Gradient Calculator when that is the actual task. 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, 2]

Age and chronic-health points

Original severe chronic-health eligibility is separate from ordinary comorbidity. This implementation accepts ages 16–120: 16 reflects the original lower population boundary, while 120 is a technical input guard rather than an APACHE II scoring cutoff. 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, 2]

APACHE II age points
Category	Points
<45	0
45–54	2
55–64	3
65–74	5
≥75	6

Category: <45
Points: 0

Category: 45–54
Points: 2

Category: 55–64
Points: 3

Category: 65–74
Points: 5

Category: ≥75
Points: 6

APACHE II chronic-health points
Category	Points
No qualifying original severe chronic-health condition	0
Qualifying condition + elective postoperative admission	2
Qualifying condition + nonoperative or emergency postoperative admission	5

Category: No qualifying original severe chronic-health condition
Points: 0

Category: Qualifying condition + elective postoperative admission
Points: 2

Category: Qualifying condition + nonoperative or emergency postoperative admission
Points: 5

What the calculator actually does

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 perform a GCS examination, diagnose acute renal failure or critical illness, produce a current mortality probability, or choose treatment, ICU placement or life-support limits. Use the separate Glasgow Coma Scale Calculator to record E/V/M observations when that is the task. [1, 4]

APACHE II and related ICU models

APACHE II and related ICU model comparison
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.

Model: APACHE II
Time frame: ICU first 24 hours
Main inputs: 12 physiology items, age, chronic health
Primary use: Historical severity/case-mix score
Why not interchangeable: This page's 1985 point bands and inputs are not an APACHE III/IV, SAPS, SOFA or MPM conversion.

Model: APACHE III/IV
Time frame: Model-specific ICU windows
Main inputs: Different variables, diagnosis classes and coefficients
Primary use: Independent updated models
Why not interchangeable: Different variables, diagnosis classes, coefficients and calibration; a total cannot be relabeled as APACHE II.

Model: SAPS II
Time frame: ICU admission-era model
Main inputs: Different physiology, age, disease and type-of-admission inputs
Primary use: Separate prognostic system
Why not interchangeable: Its predictors and equation are not the APACHE II component bands.

Model: SOFA
Time frame: Baseline and serial organ dysfunction
Main inputs: Six organ systems
Primary use: Trendable organ-dysfunction description
Why not interchangeable: SOFA describes organ dysfunction over time rather than reproducing APACHE II's first-24-hour point total.

Model: MPM
Time frame: Model-specific ICU time point
Main inputs: Different predictors and equation
Primary use: Separate mortality model
Why not interchangeable: 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. [3, 4]

Worked examples from the 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 high vector totals 71. This demonstrates score arithmetic only, not a mortality percentage or treatment/limits-of-care recommendation.

These examples are educational arithmetic, not patient-specific predictions. [1, 2]

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; era, location and case mix matter. Recalibration is a separate statistical task that this page does not perform. [4, 5]

Important limitations

Do not generalize this page to patients younger than 16, 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, 4, 5]

References

FAQ

Related Calculators

SOFA Score

Calculate the original Classic SOFA-1 score from raw clinical values across six organ systems in one defined 24-hour window, with an optional comparable-baseline change.

GCS

Record standard Glasgow Coma Scale eye, verbal, and motor responses, calculate a complete score when all are testable, and preserve Not Testable components without a fabricated total.

qSOFA

Calculate the original adult Sepsis-3 qSOFA prognostic prompt from respiratory rate, altered mentation, and systolic blood pressure.

Disclaimer

Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.

APACHE II Score Calculator

Before entering APACHE II values

Vital physiology

Oxygenation

Acid-base and electrolytes

Renal and hematologic data

Neurologic assessment

Age and chronic health

About

Formula

Interpretation

APACHE II scoring bands, first-24-hour rules, and model limits

First-24-hour model scope

Complete acute physiology score table

Oxygenation and acid-base branches

Age and chronic-health points

What the calculator actually does

APACHE II and related ICU models

Worked examples from the implementation

Calibration and external validation

Important limitations

References

FAQ

What exactly does this APACHE II page calculate?+

Which values belong in the first-24-hour assessment?+

Why does oxygenation switch between PaO₂ and A–aDO₂?+

When is serum bicarbonate used instead of arterial pH?+

How does Glasgow Coma Scale enter APACHE II?+

When are creatinine points doubled?+

What does the chronic-health component mean?+

Does an APACHE II score give an individual mortality probability?+

How does APACHE II differ from SOFA, APACHE III/IV, and SAPS II?+

Is APACHE II validated for children or burn-specific care?+

Related Calculators