Measurement of Adherence in Pharmacy Administrative Databases: A Proposal for Standard Definitions and Preferred Measures

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Abstract
Background: A variety of measures have been developed to calculate refill adherence from administrative data such as pharmacy claims databases. These measures have focused on improving the accuracy of adherence measures or clarifying the evaluation time frame. As a result, there are many measures used to assess adherence that may or may not be comparable or accurate. Objective: To compare available refill adherence measures. Methods: A systematic literature review was conducted to identify current or recently used measures of calculating adherence from administrative data. A MEDLINE search (January 1990–March 2006) was undertaken using the search terms adherence or compliance in the title combined with administrative, pharmacy, or records in any field, including subheadings medical, nursing, and hospital records. Non-English articles were excluded. Seven hundred fifteen articles were available for review. Review articles and letters were excluded from measure selection, but were included in the search terms and used to identify additional research articles. Adherence measures were excluded if they were incompletely described, produced non-numeric values, or were duplicates. Eleven refill adherence measures were identified and compared using data from the LOSE Weight (Long-term Outcomes of Sibutramine Effectiveness on Weight) study. Measures compared include Continuous Measure of Medication Acquisition (CMA); Continuous Multiple Interval Measure of Oversupply (CMOS); Medication Possession Ratio (MPR); Medication Refill Adherence (MRA); Continuous Measure of Medication Gaps (CMG); Continuous, Single Interval Measure of Medication Aquisition (CSA); Proportion of Days Covered (PDC); Refill Compliance Rate (RCR); Medication Possession Ratio, modified (MPRm); Dates Between Fills Adherence Rate (DBR); and Compliance Rate (CR). Results: The results suggest that the CMA, CMOS, MPR, and MRA are identical in terms of measuring adherence to prescription refills throughout the study period, each with a value of 63.5%; CMG and PDC are slightly lower (63.0%) and are equivalent to MRA when oversupply is truncated. CR, MPRm, RCR, and CSA result in higher adherence values of 84.4%, 86.6%, 104.8%, and 109.7%, respectively. Conclusions: Five measures produce equivalent results for measuring prescription refill adherence over the evaluation period. Of these, MRA has the fewest calculations, is easily truncated if one desires to exclude surplus medication issues, and requires the least amount of data. MRA is therefore recommended as the preferred measure of adherence using administrative data.