Assessment of medication adherence is an important tool in health services research and clinical care. One common measure of adherence is the "medication possession ratio" (MPR), which reflects the reliability with which patients refill prescriptions. Several factors can affect computation of MPR and, potentially, its relationship to clinical outcomes. This study evaluated refill-based adherence measures for three conditions: diabetes mellitus (DM), hypercholesterolemia, and hypertension.
I: Determine how the calculation of MPR is affected by the following decisions: a) Number of prescription fills over which it is measured; b) Duration of time used to identify a therapeutic decision to interrupt treatment versus extreme lack of adherence; c) Method of accounting for medication "excess" presumed to accumulate from early refilling of prescriptions; and d) Means for estimating adherence when more than one drug is used.
II: Optimize calculation of MPR for each selected condition
III: Compare the relationship between optimized measures of adherence and treatment success for the selected conditions
IV. Assess how prescription size differs among racial/ethnic groups and its association with refill-based measures of medication adherence and outcomes.
This observational study, using VA administrative data, included patients receiving outpatient medication for DM, hypercholesterolemia or hypertension. The principal type of analysis was outcomes assessment. For objective I, MPR was assessed while varying the specified parameters. For the other objectives, appropriate regression analyses were performed.
Choices in calculation of MPR can substantially affect the percentage of patients deemed "non-adherent". In those with at least five consecutive prescription fills (4 fill-fill intervals) of the same hypercholesterolemia medication ("statin"), the percentage of prescriptions with MPR<0.8 varied from approximately 5%-21% for strings of fills with 90-day supplies and 10%-47% for fills with 30-day supplies.
In analyses covering both monotherapy and multi-drug regimens, calculating MPR using all includable prescription fill-fill intervals resulted in significant associations between MPR and outcomes for all three conditions, although the explanatory magnitude of MPR (R-squared) remained small in all models. Accounting for the presence and duration of gaps in medication coverage between the last fill and outcome measure improved the model fit for hypercholesterolemia.
In analyses using all includable fill-fill intervals, the odds of reaching "target" outcomes were substantially lower in those with low MPR for all three conditions. Among the conditions, hypercholesterolemia had the steepest gradient of response to adherence.
For patients on oral hypoglycemic medications, statins, or antihypertensive medications, African Americans and Hispanics were significantly less likely than non-Hispanic Whites to receive prescription fills of more than 30-day supplies. The differences were partly explainable by the number and types of apparent changes in the medication regimen, age, and the VA station associated with the prescription. The clinical significance of these differences is uncertain.
Our findings may be useful in optimizing population-based assessments of medication non-adherence based on prescription refills and assessing the impact of levels of adherence on outcomes. While the clinical significance of observed racial/ethnic prescription size differences is uncertain, we believe that these differences should be further explored.
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- Rothendler JA, Fincke BG, Reisman JI, Christiansen CL, Miller DR, Borzecki AM, Berlowitz DR. Diabetes Control in Patients on Oral Monotherapy Regimens: Relationship to Adherence and Medication Dose. Poster session presented at: VA HSR&D National Meeting; 2008 Feb 14; Baltimore, MD.
- Rothendler JA, Fincke BG, Reisman JI, Berlowitz DR, Christiansen CL, Miller DR, Borzecki AM, Cunningham F. Problems in Calculating Medication Adherence from Refill Intervals. Paper presented at: VA HSR&D National Meeting; 2007 Feb 22; Arlington, VA.