APACHE II Score

The APACHE II (Acute Physiology And Chronic Health Evaluation II) score is a disease severity classification system, being one of the most established and widely used prognostic tools in Intensive Care Units (ICUs) for adult patients. Developed to be applied within the first 24 hours of ICU admission, its primary objective is to quantify the severity of the patient's acute illness and predict hospital mortality risk for *groups* of patients, serving as a crucial tool for auditing, quality benchmarking between ICUs, and risk stratification in clinical trials. The score is composed of three distinct parts: first, the Acute Physiology Score (APS), which is the most complex component, assessing 12 routine physiological variables (such as temperature, mean arterial pressure, heart rate, respiratory rate, oxygenation [either by A-a gradient or PaO2], arterial pH, serum sodium, serum potassium, creatinine, hematocrit, and white blood cell count), where the most deranged value in the first 24 hours receives a score; second, the patient's age, which adds points incrementally; and third, the Chronic Health Evaluation, which grants additional points if the patient has a documented history of severe pre-existing organ insufficiency (such as liver cirrhosis with portal hypertension, severe COPD, need for chronic dialysis, or an immunocompromised state). The final total score ranges from 0 to 71, and a higher score is directly associated with greater disease severity and an increased risk of death, although it is not precise for predicting an individual patient's outcome.