Estimates ICU mortality
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.