Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
APACHE II ("Acute Physiology and Chronic Health Evaluation II") is a severity of disease classification system (Knaus et al., 1985), one of several ICU scoring systems. After admission of a patient to an intensive care unit, an integer score from 0 to 71 is computed based on several measurements; higher scores imply a more severe disease and a higher risk of death.
APACHE II was designed to measure the severity of disease for adult patients admitted to Intensive care units. The lower age is not specified in the original article, but a good limit is to use Apache II only for patients age 15 or older.
This scoring systems is used in many ways:
- Some procedures and some medicine is only given to patients with certain APACHE II score
- APACHE II score can be used to describe the morbidity of a patient when comparing the outcome with other patients.
- Predicted mortalities are averaged for groups of patients in order to specify the group's morbidity.
Even though newer scoring systems, like 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. The resulting point score should always be interpreted in relation to the illness of the patient.
After the initial score has been determined within 24 hours of admission, no new score can be calculated during the hospital stay. If a patient is discharged from the ICU and readmitted, a new APACHE II score can be calculated.
The appendix of the document that originally described the APACHE II score, makes an attempt to describe how to calculate a predicted death rate for a patient. In order to make this calculation of predicted mortality precise, 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 prinicipal diagnosis at admission.
A method to compute a refined score known as APACHE III was published in 1992.
- Knaus WA, Draper EA, Wagner DP, Zimmerman JE (1985). "APACHE II: a severity of disease classification system". Critical Care Medicine. 13: 818–29. PMID 3928249. This is the first published description of the scoring system.
- Knaus WA, Wagner DP, Draper EA, Zimmerman JE, Bergner M, Bastos PG, Sirio CA, Murphy DJ, Lotring T, Damiano A; et al. (1991). "The APACHE III prognostic system. Risk prediction of hospital mortality for critically ill hospitalized adults". Chest. 100 (6): 1619&ndash, 36. PMID 1959406.
- Commonly used website to calculate APACHE II score
- Glasgow Coma Scale (used by APACHE II)