Use of ambulatory care services in three provider plans: interactions between patient characteristics and plans

A previous study of low-income enrollees in a closed-panel health maintenance organization (HMO) and a Blue Cross/Blue Shield (BC/BS) plan showed that the effect on the use of health services of the age, sex, health status, previous health care use, race, and family size of the enrollees was different in the two plans. We have replicated this study using the same two provider plans but studying a different group of white collar, middle class enrollees. A third plan, an experimental independent practice association (IPA), was also available for analysis. Utilization was defined as use (yes/no) and the quantity of use for those who used services (in standardized dollars). Significant interactions were detected between plan and all of the independent variables but race. The use of services in the HMO was least affected by enrollees' characteristics (age, sex, race, health status, prior use, family size) and use was most sensitive to patient characteristics in BC. In some respects, the IPA was more like the HMO and in other respects more like the BC/BS plan.