PDC vs MPR in Specialty Pharma: Which Adherence Measure to Trust, and the Data Rules Behind It

Why two adherence numbers for the same patients can both be right

Ask two analysts for adherence on the same specialty brand and you can get two different numbers, both calculated correctly. One used the medication possession ratio (MPR). The other used the proportion of days covered (PDC). Neither made a mistake. They answered slightly different questions, and the difference matters most for exactly the patients leadership worries about.

MPR adds up the days of supply a patient was dispensed in a period and divides by the days in that period. PDC counts the days on which the patient actually had drug on hand and divides by the same days. The gap appears when fills overlap. A patient who refills early, or receives two shipments close together, can show an MPR above 100 percent while still having gaps later in the year. PDC shifts the overlapping supply forward and caps each day at one, so it cannot exceed 100 percent and it does not let an early refill hide a later gap. That is why PDC has become the more common convention, including in the Medicare Part D Star Ratings adherence measures, which use a PDC threshold of 80 percent.

Choosing PDC is the easy part. In specialty pharma the harder question is whether the fills feeding it are complete and correctly linked. This article explains both measures, where specialty data distorts them, and the rules that make an adherence number hold up in a QBR. It builds on our guides to defining an active patient and discontinuation reasons.

Where specialty data distorts adherence

• One patient, two pharmacies

  A patient moves from one specialty pharmacy to another after a payer change. Unless both records are linked to one patient, each shows a gap and the patient looks non-adherent twice.

  • Different patient IDs per pharmacy
  • Transfers read as gaps
  • Adherence falls after network changes

• Days supply that is not real

  Weight-based doses, injectables dispensed in 28-day packs and dose changes mid-therapy all make days supply a weak guide to when the patient runs out.

  • Default days supply on SP records
  • Dose changes not reflected
  • Loading doses counted as maintenance

• Fills outside the feed

  Bridge and free drug shipments, samples and hospital administration often never reach the specialty pharmacy feed. The patient was covered; the data says they were not.

  • Free drug fills in a separate file
  • Inpatient days look like gaps
  • Samples invisible

• Adherence mixed with persistence

  If a patient stops therapy in month four, a twelve-month PDC counts the remaining months as uncovered. The number then blends stopping with missing doses, two problems with different owners.

  • Stops pull PDC down
  • Holds counted as gaps
  • No separate persistence view

What an adherence measure that holds up looks like

How to put it in place

Questions leaders ask about PDC and MPR