Severity of illness: how do we measure it and how does it relate to casemix?

Severity of illness (S.O.I.) systems have traditionally been a useful clinical tool for measuring the physical effects of disease on the patient, planning treatment and predicting outcomes. However, they are now becoming a useful management tool to help explain and justify above average treatment costs within casemix classes and are of particular value for large tertiary referral hospitals which tend to treat more severely ill patients. There are three different categories of S.O.I. and several different systems and approaches for measuring it. All have their uses as analytical tools for categorising patients so as to maximise the homogeneity within casemix classes and make any Output or Casemix based Funding System based on such classes more equitable.