Over the last year, one of the most common questions I have received is how to best measure medication compliance or adherence for routine program reporting. Given the slowed growth in utilization and the need to differentiate, it seems that most PBMs and health plans have placed a renewed emphasis on improving medication adherence.
While medication possession ratios (MPRs) or versions of MPR tend to dominate the reporting tools, MPR is not, in fact, the best measure of medication adherence. Why? The results of an MPR analysis depend heavily upon the methodological choices made in defining MPR and the quality of the days supply figured provided by the pharmacist; and MPRs allow for little clinical interpretation. Each of these issues is discussed briefly below.
- First as background, MPR is calculated as the sum of the days supply for all claims during a defined period of time divided by the number of days elapsed during the period. MPRs can change significantly based on how the denominator is calculated. In a previously published example in JMCP, a patient’s MPR when the denominator was based on the time between the first and last fill was 0.75; but when the denominator was the entire time period, the MPR was only 0.53. Reason being, in the first approach, the MPR is affected solely by gaps between fills. When the entire calendar period is used, the MPR is affected both by gaps and treatment discontinuation.
- MPRs defined over longer time frames using fixed time periods will, by definition, be lower due to decreases in persistency over time so you cannot do a head-to-head comparison of a vendor who reports MPRs on quarterly basis to another vendor who reports MPR on an annual basis.
- Third, MPRs are highly sensitive to the population included. If the report includes both new and ongoing users, an influx of new patients into the program will artificially lower the MPR when it is based on a fixed time period as the denominator. Reason being, new users have lower persistency rates than ongoing users.
Quality of days supply
MPRs rely on the accuracy of the days supply figure provided by the pharmacist. In the case of inhalers, injectables and liquids, these figures are notoriously unreliable so the reporting of an MPR is simply not appropriate for many medications. For oral pills, the problem is less significant but comes into play when different drugs dosages have price parity and/or pill-splitting is common.
Little clinical interpretation
The most significant limitation of the MPR is the lack of ability to assess the clinical meaning of an observed improvement. When programs claim to improve MPR by 3-5 percentage points, it is simply unknown what clinical impact, if any, will be seen from this increase in MPR. Research examining the relationship between changes in compliance and clinical outcomes, is sorely lacking. While researchers have historically used an MPR of 80% or better as the benchmark for good adherence, it is well-known that this is a somewhat arbitrary cut-off, driven more by precedence than clinical rationale.
Is there a better alternative to MPR? Yes, and I’ll share some thoughts on this alternative later this week.