APACHE II Score Calculator

The APACHE II (Acute Physiology and Chronic Health Evaluation II) score, published by Knaus et al. in 1985, is one of the most widely used ICU severity-of- illness scores. It sums three components: an Acute Physiology Score built from ten physiologic variables (temperature, mean arterial pressure, heart rate, respiratory rate, arterial pH, sodium, potassium, creatinine, hematocrit, and WBC count), a separate Glasgow Coma Scale contribution, age points, and Chronic Health points for pre-existing organ insufficiency or immunocompromise. The full published instrument also scores an eleventh Acute Physiology variable, oxygenation (A-aDO2 or PaO2 depending on FiO2), which this calculator does not collect -- a patient whose main derangement is respiratory failure or ARDS will read as less severe here than their real APACHE II score would show, since that oxygenation penalty (up to 4 points) is simply absent from this total. Nearly every physiologic variable that IS included is scored on a U-shaped curve rather than a straight line: a normal, mid-range value scores 0 points, and points climb whether the value drifts too high OR too low -- a very low sodium and a very high sodium can both score 4 points, for example, which is why moving a variable further from normal in either direction should never be assumed to lower the score. Glasgow Coma Scale is the exception: its 0-12 point contribution (15 minus the GCS total) is a clean linear relationship, and no other single input can contribute as many raw points on its own -- each individual physiologic variable tops out at 4, age tops out at 6, and chronic health points top out at 5 -- reflecting how heavily neurologic status is weighted in real ICU severity assessment.

That said, GCS's advantage is in its point ceiling, not a guarantee that it moves the score the most for every patient: at this calculator's own near-normal defaults, where most physiologic variables already read 0, age or chronic health points can move the total score by a larger fraction than GCS does, simply because the starting total is so low. Age points step up in five bands from 0 (under 45) to 6 (75 or older), and Chronic Health points are entered directly (0, 2, or 5 by published convention) rather than computed from other inputs -- both add on top of the physiology score without interacting with it. Creatinine points double when acute renal failure is present, per the original instrument. The mortality percentage shown here is a simplified educational approximation based on commonly cited score-range correlates (roughly 4% at a score of 4 or below, up to roughly 85% above 34) -- it is NOT the original Knaus logistic-regression equation, which additionally requires a diagnostic-category-specific weight and a term for emergency surgery that this calculator does not collect. Treat the mortality figure as a rough correlate for education, not a patient-specific predicted-mortality calculation a clinician would document.