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Severity of illness (SOI) is defined as the extent of organ system derangement or physiologic decompensation for a patient. It gives a medical classification into minor, moderate, major, and extreme. The SOI class is meant to provide a basis for evaluating hospital resource use or to establish patient care guidelines.

SOI is most frequently used in conjunction with the APR-DRG system (a variant of diagnosis-related group) which has been developed by 3M.[1] Patients within the are assigned their SOI based on their specific diagnoses and procedures performed during their medical encounter, which is generally an inpatient hospital stay. Patients with higher SOI (e.g. major or extreme) are more likely to consume greater healthcare resources and stay longer in hospitals than patients with lower SOI in the same APR-DRG.

The Centers for Medicare and Medicaid Services, also known as CMS, considered the APR-DRG and SOI system as a potential evolution to the DRG system in 2006, but in March 2007 decided to adopt another internally developed system evolved from the [Diagnosis-related group] DRG [2]. SOI is still commonly used throughout the United States to adjust for patient complexity, so that physicians and other groups can compare resource utilization, complication rates, and length of stay.

See alsoEdit


  • Averill RF, The evolution of case-mix measurement using DRGs: past, present and future, Stud Health Technol Inform. 1994;14:75-83.
  • Averill R, Muldoon J, Vertrees J, Goldfield N, et al., The evolution of case mix measurement using diagnosis-related groups (DRGs), Physician Profiling and Risk Adjustment. In: Goldfield N, ed. 2nd ed. Frederick, MD: Aspen Publishers, Inc; 1999

3M APR DRG Software [1]

RAND Evaluation of DRG Systems[2]

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