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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 patients 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 The APACHE II total score is designed as a mortality prediction tool. It is not intended to influence the medical management or care of subjects patients during their ICU stay or to be calculated sequentially to show improvement or effect of interventions. A number of physiology measurements are used to calculate the APACHE II total score. The worst values recorded during the initial 24 hours in the ICU is recommended are recommended to be used to calculate the total score but alternatively, the measurements initially recorded during the subject’s patient’s admission can be used for practical reasons. The APACHE II total score is the most widely used ICU mortality prediction score. It differs from the original APACHE score in some ways; the number of physiology measurements is decreased and the weight of some of the physiology measurements 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 total score is not intended to be applied to specific populations such as liver failure or HIV patients and 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 total score must be recalibrated before it can be used in a population other than the general ICU population. ICU prediction scores in general need to be periodically recalibrated to reflect changes and improvements in practice and patient demographics. 

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