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 immunizat ion practice assessment outcomes. Methods. In 3 high- (50%-85%) and 7 low- (< 25%) Medicaid pediatric practic es in urban eastern Virginia, we assessed immunization rates of children 12 and 24 months old comparing the standard criteria (charts in the active fi les excluding those that documented the child moved or went elsewhere) with 3 alternative criteria for selecting active patients: 1) follow-up: the ch art 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 pra ctice in the past year; 3) consecutive: patients that were seen consecutive ly for any reason. Results. Of the 1823 charts assessed in the high- and low- Medicaid practic es, follow-up identified 61% and 83% as active patients; 78% and 95% were e ver seen in the past year. At 24 months, mean practice immunization rates w ere lower for standard (70%) than all 3 alternative criteria (78%-86%). Imm unization rate differences between standard and alternative criteria were g reater in high- (17%-23%) than low- Medicaid practices (5%-13%). Conclusion. The standard for practice assessment should be based on a consi stent definition of active patients as the immunization rate denominator.