What every intensivist should know about prognostic scoring systems and risk-adjusted mortality

In the practice of medicine, multiple scores and prognostic systems have been developed to quantify disease severity, assess prognosis, and guide therapeutic interventions. The Glasgow Coma Scale, the Model for End Stage Liver Disease (MELD), and the American Society of Anesthesiologists Physical Status Classification are but a few examples. Heterogeneity in the practice of intensive care medicine, the high cost of care, the very real chance of death in intensive care units (ICUs), and the desire to make comparisons between ICUs have prompted the development and refinement of ICU-specific prognostic systems.(1-7) Scoring systems may be generic or disease-specific, may be used for cohort analysis or individual patient assessment, can be based on physiologic derangement or resource allocation, and may be simple or complex. In critical care practice, two major categories of scoring systems exist. Organ failure scores (e.g., the Sequential Organ Failure Assessment, SOFA) describe a patient’s physiologic derangements by organ system to provide an objective assessment of the extent and severity of organ dysfunction. The other major category is the severity-of-illness prognostic model, a discussion of which will occupy the majority of this commentary. These systems (e.g., the Acute Physiology and Chronic Health Evaluation, APACHE) use physiologic data, pre-morbid conditions and information regarding the nature of the current illness to predict the likelihood of mortality.