How this instrument works
APACHE II (Acute Physiology and Chronic Health Evaluation II) is a severity-of-illness scoring system for adult patients in intensive care, built by Knaus and colleagues in 1985. It is a clinician-facing instrument for classifying how physiologically deranged a patient is at a given moment — used for illness-severity classification, benchmarking outcomes across ICUs, and risk-adjustment in critical-care research — not a bedside triage tool or a patient-facing self-assessment, and it is not meant to make a treatment decision on its own.
The score has three parts, summed into a single total ranging 0-71. The Acute Physiology Score (APS) sums 12 independently-scored variables — temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation, arterial pH, sodium, potassium, creatinine, hematocrit, white blood cell count, and Glasgow Coma Score — each scored on a two-sided band where both abnormally high and abnormally low values score points and only the normal-range band scores zero — most terms run 0-4, but GCS runs 0-12 (15 minus the actual score) and creatinine can reach 8 points once doubled for acute renal failure, both detailed further below. Age adds 0-6 points across five age bands, and chronic health status adds 0, 2, or 5 points.
Two scoping choices are built into this calculator. Oxygenation is measured one of two ways depending on FiO2: at FiO2 0.5 or higher, the A-aDO2 (alveolar-arterial oxygen gradient) field is scored; below 0.5, the PaO2 field is scored directly instead, and only one of the two ever contributes points to a given calculation. Creatinine points double when acute renal failure is present. Chronic health points require two things at once — a qualifying pre-existing condition and a postoperative-status answer — detailed further below and in the FAQs.
This calculator scores the acid-base term from arterial pH alone. The original 1985 instrument also allowed a fallback venous-HCO3 scoring path for centers without an arterial blood gas, but that path is a secondary substitute in the source itself, and a second full branching subsystem for it was deliberately left out of scope here. On sourcing: the 1985 primary paper sits behind a journal paywall and could not be read directly while building this page; the point table above was instead cross-checked against three independent secondary sources — the Merck Manual Professional Edition, a PROSPECT clinical-trial APACHE II worksheet, and Wikipedia's APACHE II article — which agree exactly on every point value. A handful of boundary figures, particularly the exact A-aDO2 thresholds and the alternate HCO3 path, carry somewhat more uncertainty than the rest of the well-corroborated table.
- Enter the four core vital signs: rectal temperature (temp), mean arterial pressure (map), heart rate (hr), and respiratory rate (rr).
- Enter FiO2 (fio2) first — at 0.5 or higher, enter A-aDO2 (aado2); below 0.5, enter PaO2 (pao2) instead, since only one oxygenation path is scored.
- Enter arterial pH (ph), serum sodium (na), potassium (k), and creatinine (cr), then set acute renal failure (arf) to Yes if present — this doubles the creatinine points.
- Enter hematocrit (hct), white blood cell count (wbc), Glasgow Coma Score (gcs), and patient age in years (age).
- Set chronic to Yes only if a qualifying severe chronic condition predates this admission, then set elective to Yes for planned surgery or No for nonoperative/emergency status — it only matters when chronic is Yes.
- Review the component point breakdown, the Acute Physiology Score, age points, chronic health points, and the final APACHE II total (0-71).
Worked example: respiratory failure with acute kidney injury
A 70-year-old is admitted with temperature 35.0°C (1 point, mild hypothermia), mean arterial pressure 45 mmHg (4 points, severe hypotension), heart rate 145/min (3 points), respiratory rate 32/min (1 point), and FiO2 0.6 — high enough to trigger the A-aDO2 measurement path — with an A-aDO2 of 420 mmHg (3 points). Arterial pH is 7.18 (3 points, severe acidosis), sodium 128 mmol/L (2 points), and potassium 6.2 mmol/L (3 points). Running subtotal so far: 1+4+3+1+3+3+2+3 = 20 points.
Creatinine is 4.0 mg/dL, which alone scores 4 points, but acute renal failure is present, so that doubles to 8 points. Hematocrit is 52% (2 points), white blood cell count is 22 ×1000/mm³ (2 points), and Glasgow Coma Score is 6, giving 15 − 6 = 9 points. Adding these to the running subtotal (20 + 8 + 2 + 2 + 9) gives an Acute Physiology Score of 41. Age 70 adds 5 age points. A pre-existing chronic condition combined with nonoperative/emergency postoperative status adds 5 chronic health points. The APACHE II total is 41 + 5 + 5 = 51.
Questions
What is the APACHE II score used for?
APACHE II classifies how physiologically deranged an ICU patient is within a defined scoring window, most often the first 24 hours of an intensive care admission. It is built for clinicians and researchers — for illness-severity classification, benchmarking outcomes across ICUs, and risk-adjustment in critical-care studies. It is not a bedside triage tool, a treatment-decision rule, or a patient-facing self-assessment; a rising score correlates with higher hospital-mortality risk in the derivation cohort, but any individual patient's course still depends on clinical judgment and factors the score cannot capture.
Why does each variable score points for both high and low abnormal values?
APACHE II's Acute Physiology Score uses a two-sided band for all 12 variables: physiologic derangement in either direction — a temperature too high or too low, a heart rate too fast or too slow — reflects illness severity, so both tails of the range earn points and only the normal middle band scores zero. A potassium of 6.2 mmol/L (high, 3 points) and a potassium of 2.4 mmol/L (low, 4 points) are both markers of instability, just in opposite directions, which is why the table is not a simple one-sided penalty.
Why are there two oxygenation fields, A-aDO2 and PaO2?
The instrument scores oxygenation differently depending on supplemental oxygen delivery. At FiO2 0.5 or higher, the alveolar-arterial oxygen gradient (A-aDO2, in mmHg) is scored, because at high inspired oxygen, PaO2 alone stops reflecting gas-exchange impairment well. Below FiO2 0.5, PaO2 (mmHg) is scored directly instead. This calculator has both fields so you can enter whichever applies, but only one path ever contributes points to a given calculation — the FiO2 value you enter selects it automatically.
Why doesn't this calculator include the venous HCO3 scoring path?
The 1985 instrument includes an alternate way to score the acid-base term from serum bicarbonate when no arterial blood gas is available, as a fallback for centers without ABG access. This calculator scores the acid-base term from arterial pH only and deliberately omits that HCO3 fallback — the source itself treats it as a substitute for a missing measurement, not a parallel primary path, and a second full branching scoring subsystem for a fallback case was out of scope here. Enter arterial pH; if you only have a venous bicarbonate, this calculator is not the right tool.
Why does acute renal failure double the creatinine points?
Serum creatinine is scored on the same 0-4 two-sided band as the other physiology variables, but the instrument doubles that creatinine subscore when acute renal failure is present, since acute kidney failure carries a worse prognosis than a chronic or stable elevation at the same creatinine level. Set arf to Yes only for acute renal failure — the doubling applies to whatever points the creatinine value already earned, so a creatinine that scores 2 points becomes 4 with arf set to Yes, and a value that scores 0 stays 0 either way.
What counts as a qualifying chronic health condition?
Chronic health points require a severe organ insufficiency or immunocompromise that was present and documented before this admission — examples include cirrhosis with portal hypertension, NYHA Class IV heart failure, severe chronic respiratory disease with exercise restriction or ventilator dependency, chronic dialysis, and immunosuppression from cancer, chemotherapy, high-dose steroids, or AIDS. Set chronic to Yes only if one of these applies. The elective field then only matters when chronic is Yes: it adds 2 points for elective postoperative patients or 5 points for nonoperative or emergency postoperative patients. Without a qualifying condition, chronic health points are always 0.
What does the total APACHE II score mean?
The total is the Acute Physiology Score plus age points plus chronic health points, ranging from 0 to 71. There is no single universal cutoff for 'severe' — in the 1985 derivation cohort, higher scores tracked with progressively higher hospital-mortality rates across a large multi-center ICU population, and the score is more commonly used to compare groups of patients or track a unit's case mix over time than to predict one patient's individual outcome. Interpretation should sit alongside diagnosis, trend over time, and the full clinical picture, not the number alone.
How reliable is the point table behind this calculator?
The 1985 primary paper sits behind a journal paywall and could not be read directly while building this page. Instead, the point table was cross-checked against three independent secondary sources — the Merck Manual Professional Edition, a PROSPECT clinical-trial APACHE II scoring worksheet, and Wikipedia's APACHE II article — and all three agree exactly on every value used here. A few boundary figures, most notably the precise A-aDO2 thresholds and the alternate HCO3 scoring path this calculator omits, carry somewhat more uncertainty, since they could not be checked against the original source directly. Treat the total as decision support, not a standalone answer about prognosis.
References
- Knaus et al., APACHE II: a severity of disease classification system, Crit Care Med. 1985;13(10):818-829
- Merck Manual Professional Edition — APACHE II Scoring System table
- PROSPECT clinical trial — APACHE II calculation worksheet
- Wikipedia — APACHE II
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.