User Rating 0.0 ★★★★★

Total Usage 0 times

Category Medicine & Tests

Patient Demographics

Age (years)

Admission Type

Severe organ insufficiency or immunocompromised

Glasgow Coma Scale

Eye Response

Verbal Response

Motor Response

Acute Physiology Variables (worst values in first 24h)

Temperature (°C, rectal)

Mean Arterial Pressure (mmHg)

Heart Rate (bpm)

Respiratory Rate (breaths/min)

FiO₂

A-aDO₂ (mmHg)

Arterial pH

Serum Sodium (mEq/L)

Serum Potassium (mEq/L)

Serum Creatinine (mg/dL)

Acute Renal Failure (ARF)

Hematocrit (%)

White Blood Count (×10³/mm³)

Is this tool helpful?

Your feedback helps us improve.

★ ★ ★ ★ ★

About

The APACHE II (Acute Physiology and Chronic Health Evaluation II) system, published by Knaus et al. in 1985, remains the most widely validated ICU severity-of-illness scoring method. It assigns an Acute Physiology Score from 12 routine physiological measurements taken within the first 24 hours of ICU admission, each scored 0 - 4 based on deviation from normal. Age points (0 - 6) and chronic health points (0, 2, or 5) are added to produce a total between 0 and 71. A miscalculated score directly corrupts mortality risk estimates, ICU benchmarking, and triage decisions. Manual tabulation across 12 deviation tables is error-prone under clinical time pressure.

This calculator implements the original Knaus coefficient tables without approximation. Predicted mortality uses the logistic regression model: ln(R ÷ (1 − R)) = −3.517 + (0.146 × Score) with a postoperative correction of −0.603. The model assumes a general mixed ICU population. Disease-specific coefficients are not included. Calibration degrades in burn units, cardiac surgery, and populations with high prevalence of immunosuppression. For APACHE III or IV, a different instrument is required.

Formulas

The APACHE II total score is the sum of three components:

APACHE II = APS + Age Points + Chronic Health Points

Where APS (Acute Physiology Score) is the sum of individual deviation points from 12 physiological variables, each scored 0 - 4 based on worst value in the first 24 hours. The GCS component uses 15 − GCS. Maximum possible total: 71.

Predicted mortality is derived from the logistic regression equation:

ln(R1 − R) = −3.517 + (APACHE II Score × 0.146) + S

Where R = predicted death rate (probability of hospital mortality), S = −0.603 if the patient is a postoperative admission, and 0 otherwise. Solving for R:

R = e^logit1 + e^logit

Where logit = −3.517 + 0.146 × Score + S.

Reference Data

Physiological Variable	+4	+3	+2	+1	0	+1	+2	+3	+4
Temperature °C (rectal)	≥41	39 - 40.9	-	38.5 - 38.9	36 - 38.4	34 - 35.9	32 - 33.9	30 - 31.9	≤29.9
Mean Arterial Pressure mmHg	≥160	130 - 159	110 - 129	-	70 - 109	-	50 - 69	-	≤49
Heart Rate bpm	≥180	140 - 179	110 - 139	-	70 - 109	-	55 - 69	40 - 54	≤39
Respiratory Rate breaths/min	≥50	35 - 49	-	25 - 34	12 - 24	10 - 11	6 - 9	-	≤5
Oxygenation: If FiO₂ ≥ 0.5 use A-aDO₂ mmHg	≥500	350 - 499	200 - 349	-	<200	-
Oxygenation: If FiO₂ < 0.5 use PaO₂ mmHg	-				>70	61 - 70	-	55 - 60	<55
Arterial pH	≥7.7	7.6 - 7.69	-	7.5 - 7.59	7.33 - 7.49	-	7.25 - 7.32	7.15 - 7.24	<7.15
Serum Sodium mEq/L	≥180	160 - 179	155 - 159	150 - 154	130 - 149	-	120 - 129	111 - 119	≤110
Serum Potassium mEq/L	≥7	6 - 6.9	-	5.5 - 5.9	3.5 - 5.4	3 - 3.4	2.5 - 2.9	-	<2.5
Serum Creatinine mg/dL (double if ARF)	≥3.5	2 - 3.4	1.5 - 1.9	-	0.6 - 1.4	-	<0.6	-	-
Hematocrit %	≥60	-	50 - 59.9	46 - 49.9	30 - 45.9	-	20 - 29.9	-	<20
White Blood Count ×10³/mm³	≥40	-	20 - 39.9	15 - 19.9	3 - 14.9	-	1 - 2.9	-	<1
GCS (score 15 − actual GCS)	Points = 15 − GCS (range 0 - 12)
Age years	≤44: 0 pts \| 45 - 54: 2 pts \| 55 - 64: 3 pts \| 65 - 74: 5 pts \| ≥75: 6 pts
Chronic Health (nonoperative or emergency postop)	5 points if organ insufficiency or immunocompromised
Chronic Health (elective postop)	2 points if organ insufficiency or immunocompromised

Frequently Asked Questions

APACHE II uses the worst (most abnormal) value for each of the 12 physiological variables during the first 24 hours after ICU admission. If a variable is not measured, it is assumed normal (scored 0). Using values outside this window or averaging values instead of taking extremes will produce an incorrect score.

Use the alveolar-arterial oxygen difference (A-aDO₂) when FiO₂ is ≥ 0.5. Use PaO₂ when FiO₂ is < 0.5. Using the wrong oxygenation metric for the given FiO₂ will misclassify the respiratory component. If arterial blood gas data is unavailable, assign 0 points.

If the patient has documented acute renal failure (ARF), the creatinine deviation score is doubled. For example, a creatinine of 2.5 mg/dL normally scores 3 points but scores 6 points in the presence of ARF. This modifier reflects the prognostic weight of acute versus chronic renal dysfunction.

The original Knaus criteria include: liver cirrhosis with portal hypertension or prior hepatic encephalopathy, NYHA Class IV heart failure, severe COPD or chronic hypoxia/hypercapnia, chronic dialysis, or documented immunosuppression (chemotherapy, radiation, chronic steroids, leukemia, lymphoma, AIDS). The patient must have had the condition before the current admission.

No. The logistic regression coefficients (−3.517 and 0.146) were derived from a general mixed medical-surgical ICU population in US hospitals circa 1982. Calibration is poor for cardiac surgery, burns, and highly specialized units. Disease-specific coefficients published by Knaus improve accuracy but are not included in the base model. The predicted mortality should be used for benchmarking, not individual clinical decisions.

Nonoperative patients did not have surgery in the 7 days before ICU admission. Emergency postoperative patients had unplanned surgery (e.g., ruptured aneurysm repair). Elective postoperative patients had scheduled surgery. The category affects chronic health points: nonoperative and emergency postoperative patients with qualifying conditions receive 5 points; elective postoperative patients receive 2 points. The postoperative correction factor S = −0.603 applies to both surgical categories in the mortality equation.

The Glasgow Coma Scale ranges from 3 (deep coma) to 15 (fully alert). APACHE II inverts this so that a normal GCS of 15 contributes 0 points and a worst-case GCS of 3 contributes 12 points. This aligns with the deviation-from-normal logic used for all other physiological variables. If the patient is sedated or paralyzed, the pre-sedation GCS or an estimated GCS should be used.