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According to the instrument creator, Dr. William Knaus, the APACHE II total score was is designed as a mortality prediction tool but was is not intended to influence the medical management or care of patients subjects during their ICU stay. A number of variables physiology measurements are used to calculate the APACHE II total score. The worst values recorded during the initial 24 hours in the ICU should be is used. Alternatively, the variables measurements initially recorded during the patient’s subject’s admission can be used for practical reasons. The APACHE II total 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 total score is the most widely used ICU mortality prediction score. It differs from the original APACHE score in some ways; the number of variables physiology measurements is decreased and the weight of some of the variables 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 score was derived in a general ICU population and may be less precise when total score is not intended to be applied to specific populations such as liver failure or HIV patients . Since APACHE II was studied on patients newly admitted to the ICU, it 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 one it was derived ingeneral 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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