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Health Services Research & Development

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2007 HSR&D National Meeting Abstract

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National Meeting 2007

1056 — Problems in Calculating Medication Adherence from Refill Intervals

Rothendler JA (Center for Health Quality, Outcomes and Economic Research) , Fincke BG (Center for Health Quality, Outcomes and Economic Research), Reisman JI (Center for Health Quality, Outcomes and Economic Research), Berlowitz DR (Center for Health Quality, Outcomes and Economic Research), Christiansen CL (Center for Health Quality, Outcomes and Economic Research), Miller DR (Center for Health Quality, Outcomes and Economic Research), Borzecki AM (Center for Health Quality, Outcomes and Economic Research), Cunningham F (VA Pharmacy Benefits Management Strategic Healthcare Group)

Assessment of medication adherence is an important tool in health services research and clinical care. One common measure of adherence obtainable from administrative records is the “medication possession ratio” (MPR), which reflects the reliability with which patients refill prescriptions. MPR is typically calculated by dividing the days’ supply of medication dispensed by the period over which prescriptions are filled (as determined from refill intervals). Although not usually addressed, there are several factors that can affect computation of MPR and, potentially, its relationship to clinical outcomes. We sought to determine the effect of two such choices that must be made when calculating the MPR.

In VA patients with diabetes mellitus taking oral hypoglycemic medications over approximately 2.5 years, we assessed the effect on MPR of: a) varying the number of intervals between prescription fills used in MPR calculations from 1-4, and b) varying the size of the allowable gap in medication use beyond which an intentional interruption in treatment is assumed (rather than extreme lack of adherence), with the size of such gap or “clean period” ranging from 1-4 times the days’ supply. Analyses were stratified by 30 vs. 90-day medication supplies and whether the prescription was dispensed in-person or by mail. Non-adherence was judged using the commonly employed standard of MPR<0.8.

The percentage of prescriptions (including refills on original prescriptions and renewals) that had MPR<0.8 varied from approximately 5%-40% depending upon choices made in the calculation and sample stratifications specified above. Separately varying the number of fill-fill intervals from 1-4 or the “clean periods” from 1-4 times the days’ supply resulted in up to 2 to 3-fold differences in the percentage of prescriptions with MPR<0.8.

Choices made in calculation of MPR can substantially affect the percentage of patients who are deemed “non-adherent”. Studies should address and articulate the specific methodology utilized so that conclusions can be appropriately evaluated and compared with other studies.

The method by which MPR is calculated may have important implications for reliably assessing the magnitude of medication non-adherence, exploring the association of non-adherence to sub-optimal clinical outcomes, and determining the need for interventions to improve adherence.

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