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.

Patients 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 DRG.