reports

ACGs: ambulatory care groupings for case-mix categorization

Final report for grant #HS05505, National Center for Health Care Research/Health Care Technology Assessment. Baltimore, MD, USA: Johns Hopkins University.

Johns Hopkins University, Baltimore, MD, USA

This study developed and tested a case-mix measure (ACGs) for application in ambulatory care. The method provides a conceptually simple, statistically valid, and clinically relevant measure that was found useful in predicting the use of ambulatory services. The categorization uses all of the diagnoses made in a clinical setting for each individual patient over a period of time as well as the individual’s age and sex. It requires only data normally collected in insurance claims or encounter records. The system was tested using data from over 160,000 individuals in four HMOs and a state’s Medicaid program. Its application explained over 50% of the variance in concurrent ambulatory care utilization and over 20% of the variance in utilization in a subsequent year. Application of the method also indicated the increased burdens of morbidity among Medicaid enrollees as compared with HMO enrollees, and demonstrated how different types of morbidity are associated with widely different levels of utilization. Even when morbidity is controlled, however, there are remaining differences in utilization among the facilities for specific types of morbidity; these indicate the need to explore reasons for apparent differences in patterns of practice that are not associated with differences in types of morbidity.