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  • In the last four decades, intensive care units (ICUs) and critical care researchers have amassed a great body of pathophysiologic and clinical knowledge that has advanced the care of critically ill patients. Severity of illness scoring systems are tools that have been designed to both predict and evaluate, from multiple perspectives, the outcomes of critically ill patients.

  • Most scoring systems evolved from multivariate regression analysis applied to large clinical databases of discovery cohorts to identify the most relevant factors for prediction of mortality. Scoring systems are then validated by prospective application to independent validation cohorts.

  • The ideal components of a scoring system are data collected during the course of routine patient management that are easily measured in most or all patients, inexpensive, objective, and reproducible.

  • The most widely applied scoring systems in adults are the Acute Physiology and Chronic Health Evaluation (APACHE), the Mortality Probability Models (MPM), Simplified Acute Physiology Score (SAPS), and Sequential Organ Failure Assessment (SOFA).

  • The uses of severity-of-illness scoring systems for cohorts of patients include clinical investigation (to compare study groups usually at baseline but sometimes over the course of ICU care), ICU administration (to guide resource allocation and budget), and assessment of ICU performance (to compare performance over time or between health care settings).

  • The use of scores to guide decisions about delivery of care to individual patients is controversial; in some studies the accuracy of prediction of outcomes of scoring systems is not greater than that of the individual clinician's judgment.


Severity-of-illness scoring systems were developed to evaluate the delivery of care and predict outcome of groups of critically ill patients admitted to intensive care units (ICUs). The purpose of this chapter is to review the scientific basis for these scoring systems and to make recommendations for their use. While there is a growing recognition that when properly administered, these tools are useful in assessing and comparing patient populations with diverse critical illnesses, their use for predicting individual patient outcome remains controversial and unresolved.

  • Novel propensity scoring systems and case:control matching strategies have also been developed and are now commonly used to simulate clinical trials to assess efficacy and safety of therapeutics in critical care.


There are six major purposes of severity-of-illness scoring systems (Table 13-1). First, scoring systems have been used in randomized controlled trials (RCTs) and other clinical investigations to assess balance of treatment and control groups at baseline.1-5 The second purpose of severity-of-illness scoring systems is to quantify severity of illness for hospital and health care system administrative decisions such as resource allocation and accreditation. The third purpose of scoring systems is to assess ICU performance and compare the quality of care between different ICUs and within the same ICU over time. For example, severity-of-illness scoring systems have been used to assess the impact ...

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