Max Rady College of Medicine

Term: Severity of Illness

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Glossary Definition

Last Updated: 2011-06-02


The severity of illness has been used as an indicator for estimating a patient's length of stay in hospital and determining a patient's need to acute hospital care.

Brownell et al. (1992) based the definition of severity of illness on the RDRG® software. There are 3 levels of severity for medical patients: those with comorbidities or complications expected to have 1). none or minor impact, 2). moderate impact, or 3). major impact on length of stay. For surgical patients, a fourth class is added for patients having catastrophic comorbidities or complications.

De Coster et al. (1996) defined severity of illness to be one of three sets of InterQual criteria acting as objective, clinical indicators to determine the need for hospitalization and the level of acute care. Severity of illness is grouped by clinical findings, vital signs, imaging, ECG, Haematology, Chemistry, Microbiology, and other.

Related terms 


  • Brownell M, Roos NP. An Assessment of How Efficiently Manitoba's Major Hospitals Discharge Their Patients (Report #92-10). Winnipeg, MB: Manitoba Centre for Health Policy and Evaluation, 1992. [Report] (View)
  • De Coster C, Peterson S, Kasian P. Alternatives to Acute Care . Winnipeg, MB: Manitoba Centre for Health and Evaluation, 1996. [Report] [Summary] (View)

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