Effect of method of defining the active patient population on measured immunization rates in predominantly Medicaid and non-Medicaid practices.

OBJECTIVE To examine the effect of patient selection criteria on immunization practice assessment outcomes. METHODS In 3 high- (50%-85%) and 7 low- (<25%) Medicaid pediatric practices in urban eastern Virginia, we assessed immunization rates of children 12 and 24 months old comparing the standard criteria (charts in the active files excluding those that documented the child moved or went elsewhere) with 3 alternative criteria for selecting active patients: 1) follow-up: the chart contained a complete immunization record or the patient was found to be active in the practice through follow-up contact by phone or mail; 2) seen in the past year: the chart indicated that the patient was seen in the practice in the past year; 3) consecutive: patients that were seen consecutively for any reason. RESULTS Of the 1823 charts assessed in the high- and low-Medicaid practices, follow-up identified 61% and 83% as active patients; 78% and 95% were ever seen in the past year. At 24 months, mean practice immunization rates were lower for standard (70%) than all 3 alternative criteria (78%-86%). Immunization rate differences between standard and alternative criteria were greater in high- (17%-23%) than low-Medicaid practices (5%-13%). CONCLUSION The standard for practice assessment should be based on a consistent definition of active patients as the immunization rate denominator.