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The Acute Physiology and Chronic Health Evaluation (APACHE II) is a severity of disease measurement and mortality estimation tool for adult patients applied within 24 hours of admission to an intensive care unit (ICU). The instrument's intended use is for newly admitted subjects to the ICU. The instrument consists of individual items that are summed up to create a total score. This instrument is generally collected once at Screening/Baseline and used as a predictor of mortality rather than an outcome measure. The total score is normally collected as part of the instrument with no instructions provided for handling missing data. In the majority of cases, where APACHE II is only assessed once and the collected total score used as a baseline covariate, the SDTM RS domain may be sufficient to support the analysis requirements, as a variable in ADSL copied directly from a result/value in SDTM.

According to the instrument creator, Dr. William Knaus, the APACHE II score was designed as a mortality prediction tool but was not intended to influence the medical management of patients during their ICU stay. A number of variables are used to calculate the APACHE II score. The worst values recorded during the initial 24 hours in the ICU should be used. Alternatively, the variables initially recorded during the patient’s admission can be used for practical reasons. The APACHE II score is calculated at the beginning of the ICU admission to help determine the patient’s mortality risk for the admission. It is not calculated sequentially and is not meant to show improvement or effect of interventions. As such it should not be used to direct medical management. The APACHE II is the most widely used ICU mortality prediction score. It differs from the original APACHE score in some ways; the number of variables is decreased and the weight of some of the variables is adjusted. APACHE III and APACHE IV scores were also developed but are not commonly used because their statistical method is under copyright control. The score was derived in a general ICU population and may be less precise when applied to specific populations such as liver failure or HIV patients. Since APACHE II was studied on patients newly admitted to the ICU, it is not accurate when dealing with patients transferred from another unit or another hospital. This is known as lead time bias and is addressed in APACHE III. The APACHE II score must be recalibrated before it can be used in a population other than the one it was derived in. ICU prediction scores in general need to be periodically recalibrated to reflect changes in practice and patient demographics.

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