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  • In the last three decades, intensive care units (ICUs) and critical care researchers have amassed a body of pathophysiologic knowledge that has advanced the care of critically ill patients. Severity of illness scoring systems are tools that have been designed both to predict and to evaluate, from multiple perspectives, the outcomes of critically ill patients.
  • Most of these scoring systems have evolved from multivariate regression analysis applied to large clinical databases to identify the most relevant factors for prediction of mortality. Scoring systems are then validated by prospective application to other patient populations.
  • The ideal components of a scoring system are data collected during the course of routine patient management that are easily measured, objective, and reproducible.
  • The most widely applied scoring systems in adults at the present time 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 potential uses of severity-of-illness scoring systems as applied to patient groups include clinical investigation (to standardize or compare study groups), 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 in the 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 provide prediction of outcome of groups of critically ill patients who are 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 is controversial and unresolved.


There are five major purposes of severity-of-illness scoring systems (Table 6-1). First, scoring systems have been used in randomized controlled trials (RCTs) and other clinical investigations.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. The third purpose of these scoring systems is to assess ICU performance and compare the quality of care of different ICUs and within the same ICU over time. For example, severity-of-illness scoring systems could be used to assess the impact on patient outcomes of planned changes in the ICU, such as changes in bed number, staffing ratios, and medical coverage.6 The fourth purpose of these scoring systems is to assess the prognosis of individual patients in order to assist families and caregivers in making decisions about ICU care. Finally, scoring systems are now being used to evaluate suitability of patients for novel therapy (e.g., the use of the APACHE II assessment for prescription of recombinant ...

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