APACHE II ("Acute Physiology and Chronic Health Evaluation II") is a severity-of-disease classification system (Knaus et al., 1985)[1], one of several ICU scoring systems. It is applied within 24 hours of admission of a patient to an intensive care unit (ICU): an integer score from 0 to 71 is computed based on several measurements; higher scores correspond to more severe disease and a higher risk of death.
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APACHE II was designed to measure the severity of disease for adult patients admitted to Intensive care units. The lower age bound is not specified in the original article, but a good limit is to use Apache II only for patients aged 15 or older.
This scoring system is used in many ways:
Even though newer scoring systems, such as SAPS II, have replaced APACHE II in many places, APACHE II continues to be used extensively because so much documentation is based on it.
The point score is calculated from 12 routine physiological measurements, such as blood pressure, body temperature, heart rate, etc., during the first 24 hours after admission, information about previous health status, and some information obtained at admission (such as age). The calculation method is optimized for paper schemas, by using integer values and reducing the number of options so that data fit on a single-sheet paper form. The resulting point score should always be interpreted in relation to the illness of the patient.
The score is not recalculated during the stay—it is by definition an admission score. If a patient is discharged from the ICU and readmitted, a new APACHE II score is calculated.
The appendix of the document (see references) that originally described the APACHE II score, attempts to describe how to calculate a predicted death rate for a patient. In order to improve the accuracy of this calculation of predicted mortality, the principal diagnosis leading to ICU admission was added as a category weight: the predicted mortality is computed based on the patient's APACHE II score and their principal diagnosis at admission.
A method to compute a refined score known as APACHE III was published in 1991. The score was validated on the dataset from 17,440 adult medical/surgical intensive care unit (ICU) admissions at 40 US hospitals. APACHE III scores range from 0 to 299 [2].
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