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Management of the critically ill patient is complex. The acuity of illness, multiple organ system derangements, patient heterogeneities such as age, comorbidities, uncharted genomic and epigenetic variability, and the availability of evolving treatments and systems of health care delivery make the prediction of prognosis an elusive, moving target.

Originally created in response to limited ICU resources, an aging population and rising healthcare costs, scoring systems1,2,3 (also called scoring models) are intended to provide objective predictions or probabilities of mortality and long-term outcome to aid clinical decision making and planning. Their role has been expanded to benchmark or compare the performance of ICUs and assess the quality of care provided (eg, observed mortality vs predicted mortality), to predict the resources needed or the appropriateness of ICU admission depending on the severity of illness and finally to assess patients for inclusion in research studies or to compare severity of illness and assess case-mix dissimilarities between patients in various study groups.

Although intensivists will increasingly encounter assessments of their ICU patient population, ICU structure and process and physician performance via scoring systems, the application of scoring systems is not without significant controversy. This chapter discusses the main ICU scoring systems in use today,4,5,6,7 how their performance is assessed, how they are utilized, the limitations and the controversial issues surrounding their use.


The type, timing, and quantity of data collected vary significantly among the numerous scoring systems. Some scores, such as the sequential organ failure assessment, measure the organ dysfunction and thus the severity of the disease at any point of time to monitor the clinical evolution. Others, known as general risk prediction scores focus primarily on survival. These systems were developed on the assumption that the severity of illness and mortality are related to acute physiologic derangements that appear early in the course of the disease.8 The most prevalent are acute physiology score chronic health evaluation (APACHE), mortality probability model (MPM), and simplified acute physiology score (SAPS). They were developed approximately 30 years ago and have since undergone 3 to 4 revisions (Table 63–1). APACHE IV combines a total of 27 variables collected within the first 24 hours of ICU admission, MPM0III and SAPS III combines, respectively, 16 and 20 variables collected within 1 hour of ICU admission. They all depend on acute physiologic variables collected on admission such as vital signs, electrolytes, cell count, blood gas, etc, in addition to chronic health variables and admission diagnosis.

Table 63–1General risk prediction scoring systems and revisions.

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