The Problem with the Current Gold Standard for Measuring Adherence

“What gets measured gets managed” is one of the most commonly used business mantras in corporate America and is often attributed to Peter Drucker. Problems arise, however, when we manage to imperfect metrics.

In the medication adherence world, Proportion of Days covered (PDC) and its close cousin, Medication Possession Ratio (MPR) are two such imperfect metrics. Both metrics were created to measure patient adherence—and essentially quantify how many days of a given month a patient had a filled prescription. For instance, let’s assume a patient named Mary is prescribed a daily oral medication. Mary has a prescription that runs out on October 15th, but she doesn’t get around to refilling it until October 17th. Since Mary had a filled prescription for 28 out of 31 days in October, she is 90% adherent based on PDC.

The problem is that PDC is a measurement of whether or not Mary filled her prescription and not her “true adherence,” which would be whether Mary actually took the right dose of her medication at the right time. Maybe Mary often misses taking her medication on the weekends when she goes to visit her grandchildren. Maybe Mary doesn’t fully understand her dosing requirements and is cutting each pill in half as a cost-saving measure. Maybe Mary has a stockpile of old pill bottles sitting in her cabinet from past missed doses.

Hopefully Mary’s story illustrates how PDC may be an inaccurate measurement of true adherence. The industry has adopted PDC as the gold standard for adherence because pharmacy fill data is widely available and can be measured easily. Well-intentioned initiatives to improve healthcare quality and outcomes, such as CMS Medicare Advantage Star Ratings, have adopted PDC as a key metric that gets measured. The revenue of Medicare Advantage plans is heavily dependent on their Star Ratings, and medication adherence is such an important quality goal that it is triple-weighted in the calculation of these ratings. When you combine a significant financial incentive with measurement, you get my corollary to Peter Drucker’s famous business rule: “What gets measured and paid for really gets managed.”

Moving from Improving PDC to Improving True Adherence

Since the industry has been so focused on improving PDC over the past few decades, a tremendous amount of innovation has been made to improve the possession of medications. Pharmacy benefit managers (PBMs) and pharmacies have tried to adopt interventions like switching to 90-day scripts, opting patients into automatic refills, and reminder programs that incessantly bug patients to refill their prescriptions on time.

I recall a digital health company a few years ago touting the fact that they helped patients achieve a “99% adherence rate” in a pilot with a Medicare Advantage client. At face value, these results seemed to be very impressive until I realized that this company was measuring adherence with PDC and that their service included taking all the patients enrolled in the pilot, combining all their pills into daily blister packs, and shipping a month’s supply of these blister packs to patients at the start of every month. Upon realizing this, my main question was: What happened to the other 1% of PDC adherence; did one of the pilot patient’s pills get lost in the mail?

The key question is: Have all these efforts to improve PDC improved true adherence and, therefore, outcomes in healthcare? On a macro level, that is a very difficult question to answer, but one way to approach this issue is to focus on patients that “look” adherent based on PDC, but have poorly controlled disease metrics (such as A1c for diabetics) or are frequent fliers to the hospital. Then, instead of just making sure they fill their prescriptions you should work to motivate them to establish daily habits around care plan adherence, including taking their medications as prescribed. This has been shown to drastically improve end outcomes like hospitalization rates (for instance, see this study).

The metrics and payment models of healthcare are shifting to focus more on value and outcomes—which means we must adopt new modes of management. Payers, providers, and pharmaceutical companies are increasingly being graded and paid based on their ability to produce real-world evidence of better patient outcomes. In order to actually improve these outcomes, all stakeholders need to change the focus from improving medication possession to improving true adherence.

  • Matthew Loper

    Matthew Loper is CEO and Co-founder at Wellth. Prior to founding Wellth, Matt was an investor in publicly traded healthcare technology, medical devices, and services companies at OrbiMed Advisors, a leading healthcare investment firm with $13 billion under management.


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