How the APACHE II total is assembled

This calculator implements the arterial-pH route rather than the alternative bicarbonate route, so do not enter a serum bicarbonate value in the pH field. It uses the original oxygen split at an FiO2 of 0.50. Creatinine points are doubled only when acute renal failure is clinically present. No other component is doubled.

Worked score of 30

The example patient has a core temperature of 39.0°C for 3 points, MAP 65 for 2, heart rate 145 for 3, and respiratory rate 30 for 1. FiO2 is 0.40, so PaO2 is used; 65 mm Hg contributes 1 point. Arterial pH 7.30 contributes 2. Sodium 128 contributes 2 and potassium 3.2 contributes 1.

Creatinine 1.8 mg/dL ordinarily contributes 2 points. Because the example marks acute renal failure present, those creatinine points double to 4. Hematocrit 28 contributes 2, WBC 18 contributes 1, and GCS 12 contributes 3 because 15 − 12 = 3. These twelve values total an acute physiology score of 25.

Age 68 is in the 65–74 bracket and adds 5 points. No qualifying severe chronic-health condition is selected, so that component adds zero. The displayed total is therefore 25 + 5 + 0 = 30. That total is a severity descriptor, not a 30% mortality estimate.

Three parts answer different questions

Use the correct observation window

The original APACHE II method used the most abnormal qualifying value during the first 24 hours in intensive care. A single convenient set of “current” vital signs is not equivalent. Retrospective abstraction should define ICU admission time, identify all eligible observations, preserve source times, and choose the value that produces the greatest point contribution for each component under the protocol.

Do not mix values from before the ICU period, after the window, or from a different admission merely because they appear more severe. If a local registry, research protocol, or quality program specifies a modified window, name the modification; the result is not strictly the original APACHE II collection.

Treatment may change a value during the window. APACHE II historically scores observed physiology rather than reconstructing untreated physiology. Vasopressor use, ventilation, sedation, dialysis, transfusion, and other interventions remain essential clinical context even though most are not direct fields in this score.

The oxygenation branch is not interchangeable

When FiO2 is below 0.50, the APACHE II table assigns points from arterial PaO2. When FiO2 is 0.50 or higher, it uses the alveolar–arterial oxygen difference, often written A–aDO2. The calculator switches automatically based on the entered FiO2 but cannot calculate an A–a gradient from a blood gas because barometric pressure, PaCO2, and assumptions would be required.

Confirm whether FiO2 was measured or estimated and whether the oxygen value was taken at the same time. Entering PaO2 into the high-FiO2 branch can produce a profoundly wrong point score. Pulse oximetry saturation is not PaO2 and is not an accepted direct substitute in the original table.

For FiO2 below 0.50, PaO2 at least 70 scores zero, 61–69 scores 1, 55–60 scores 3, and below 55 scores 4. For the A–a branch, below 200 scores zero, 200–349 scores 2, 350–499 scores 3, and 500 or more scores 4.

Thresholds create deliberate steps

APACHE II is a point system, so two nearby values can fall on opposite sides of a threshold. Rounding 69.6 mm Hg to 70 before scoring might change the oxygen point. Preserve the laboratory or monitor precision supported by the source, follow the governing abstraction policy, and document any rounding rule.

A zero-point band does not mean clinically normal for every patient. It means the value falls in the original APACHE II reference band. Conversely, a four-point extreme does not reveal the cause. Fever, shock, arrhythmia, ventilatory failure, metabolic disturbance, kidney injury, bleeding, marrow disease, medication effect, and many other conditions require their own assessment.

The maximum theoretical score is 71, but totals are not evenly distributed and the scale is not a percentage. A difference of five points is not automatically a fixed relative increase in risk across diagnoses, hospitals, eras, or patient groups.

Glasgow Coma Scale needs clinical judgment

The neurologic contribution is 15 minus the observed GCS. The original method sought a neurologic value reflecting the patient’s condition, but sedation, paralysis, intubation, intoxication, aphasia, trauma, and preexisting neurologic disease can make assessment difficult. This calculator cannot infer an unmedicated GCS or decide how a local protocol handles an untestable verbal response.

Record eye, verbal, and motor components, not only the total. Note the time, airway status, sedatives, neuromuscular blockers, and limitations. Do not casually substitute a pre-sedation score unless the applicable clinical or registry rules authorize it.

A low GCS is an emergency finding that demands evaluation independent of APACHE II. The score must never delay airway management, neurologic workup, glucose testing, imaging, antidote consideration, or other time-sensitive care.

Chronic-health points are narrowly defined

The original definitions concern severe organ-system insufficiency or an immunocompromised state present before the current hospital admission. Examples in the original framework included biopsy-proven cirrhosis with portal hypertension, advanced functional heart disease, severe chronic respiratory disease, chronic dialysis, and immunosuppression from disease or treatment. A common stable condition does not automatically qualify.

The category also depends on operative status: 5 points for a qualifying nonoperative or emergency postoperative patient and 2 for a qualifying elective postoperative patient. That distinction should come from the admission and operative record, not from guessing after seeing the score.

If eligibility is uncertain, calculate and document both scenarios rather than silently choosing the higher number. A registry data dictionary, investigator, or experienced clinician should resolve the final classification.

Why this calculator does not report mortality

APACHE II outcome prediction uses more than the raw total. The original work combined score information with the principal diagnosis leading to ICU admission and considered emergency surgery. Published diagnosis coefficients and equations were calibrated in a particular 1980s hospital cohort. A table that maps every score to one mortality percentage discards that structure.

Case mix, treatment, ICU organization, coding, discharge practices, and survival have changed. External studies find different calibration in different units and eras. A model can rank severity reasonably while overpredicting or underpredicting absolute mortality. Local recalibration and validation are necessary before using a model for benchmarking.

For an individual or family, a population estimate is not destiny. Prognosis should come from the treating team, who can integrate diagnosis, response to treatment, goals, function, frailty, complications, and uncertainty. Never use this calculator alone for limitation-of-treatment decisions.

Appropriate and inappropriate uses

Reasonable uses include describing severity in a defined research cohort, risk adjustment after validation, quality measurement with a prespecified protocol, and teaching how the point table works. Inappropriate uses include home symptom checking, diagnosing sepsis, deciding whether someone “needs ICU,” comparing clinicians without case-mix controls, or converting the total to a universal survival percentage.

Data audit checklist

1. Confirm patient and ICU encounter.
2. Record exact ICU admission time.
3. Use the protocol’s observation window.
4. Verify core or rectal temperature basis.
5. Calculate or confirm MAP correctly.
6. Match oxygen value to FiO2 and time.
7. Use arterial pH, not bicarbonate, here.
8. Confirm U.S. creatinine units in mg/dL.
9. Double creatinine points only for acute renal failure.
10. Document GCS components and limitations.
11. Apply chronic-health definitions narrowly.
12. Record operative category.
13. Retain component values and points.
14. Do not infer a mortality percentage.

It has no role in ICU scoring, diagnosis, prognosis, billing, or medical decision-making.

Frequently asked questions

References

• Knaus et al., APACHE II: a severity of disease classification system
• Review of APACHE II development, variables, and use
• CMS discussion of ICU severity measurement and APACHE methods
• NCBI Bookshelf overview of the Glasgow Coma Scale