Most GLP-1 programs measure adherence by counting refills. Patient had 8 refills in 12 months? That’s “good adherence.” But it’s wrong — and the gap between refill count and actual adherence is costing programs millions in misallocated interventions.
Refill count ignores timing. A patient who refills monthly for 8 months then stops for 4 months has 8 refills and 0% adherence for the last third of the year. By refill count, they look compliant. By clinical reality, they’ve been off medication for 120 days.
More critically, refill count doesn’t account for:
Proportion of Days Covered (PDC) measures what actually matters: what percentage of days did the patient have medication available?
CMS and NCQA use PDC ≥ 80% as the adherence threshold. This is the metric payers evaluate when deciding whether to continue coverage. It’s the metric CMS will use in the BALANCE Model. And it’s the metric most GLP-1 programs can’t report.
Standard PDC was designed for daily oral medications. GLP-1 programs need adjustments:
1. Weekly injections: Semaglutide and tirzepatide are weekly. A 30-day supply covers ~4 injections. PDC calculation must account for 7-day coverage per injection, not daily.
2. Titration periods: Patients start at low doses (semaglutide 0.25mg → 0.5mg → 1.0mg → 1.7mg → 2.4mg over 16-20 weeks). Each dose change may involve overlapping supplies.
3. Gap tolerance: A 3-day gap between weekly injections is clinically insignificant. A 21-day gap is not. The threshold matters.
4. Measurement windows: 30-day, 60-day, and 90-day PDC windows tell different stories. A patient with 95% PDC at 30 days and 50% PDC at 90 days is declining — but a single refill count hides this.
The operators who report real PDC will win payer contracts. The ones still counting refills will lose them.
Pathriva calculates GLP-1-adjusted PDC automatically from EHR data. See our methodology.