Dual-system utilization affects regional variation in prevention quality indicators: the case of amputations among veterans with diabetes.

OBJECTIVE: To determine the impact of dual-system utilization by veterans on regional variation in lower-extremity amputation rates. STUDY DESIGN: Retrospective longitudinal cohort analysis. PATIENTS AND METHODS: Subjects were veterans with diabetes who used Veterans Health Administration (VHA) care and were dually enrolled in Medicare fee for service in fiscal years (FY) 1997--1999. We evaluated the impact of Centers for Medicare and Medicaid Services (CMS) data on prevalence of baseline foot risk factors, medical comorbidities, and amputations in FY 1997--1998, and ranking of 22 regions using risk-adjusted major and minor amputation rates in FY 1999. RESULTS: The addition of CMS data significantly increased the prevalence of amputations and risk factors for the 218,528 dually eligible veterans (all Pvalues <.001). In FY 1999, we identified 3.1 minor and 4.5 major amputations per 1000 patients (VHA data) versus 5.5 minor and 8.6 major amputations per 1000 patients (VHA/CMS data); the prevalence of any peripheral vascular condition in FY 1997--1998 was 5.7% (VHA) versus 13.0% (VHA/CMS). The impact of including CMS data varied across regions for amputation outcomes, ranging from an additional 34.3% to 150.7%. Using observed-to-expected amputation ratios and 99% confidence intervals, the addition of CMS data changed the outlier status for 8 of 22 regions for both major and minor amputations. CONCLUSION: Risk covariates and amputation outcomes were substantially underestimated using VHA data only. Our findings demonstrate the importance of evaluating dual-system utilization when conducting program evaluations for healthcare systems with a substantial number of dual enrollees.