The effect of comorbidities on treatment decisions.

Medical decision analyses typically focus on one disease, that is, on one source of risk. In many medical decisions multiple sources of risk co-exist, however. This paper analyzes the effect of such comorbidities on treatment decisions. The effect of comorbidities on treatment decisions depends primarily on the way in which the patient's attitude to health status risks varies with duration. In the QALY model comorbidities do not affect treatment decisions. This property of the QALY model can be used as a diagnostic test of its descriptive and prescriptive validity.