Estimating proportion of days covered (PDC) using real-world online medicine suppliers’ datasets

Abstract

Background The proportion of days covered (PDC) is used to estimate medication adherence by looking at the proportion of days in which a person has access to the medication, over a given period of interest. This study aimed to adapt the PDC algorithm to allow for plausible assumptions about prescription refill behaviour when applied to data from online pharmacy suppliers. Methods Three PDC algorithms, the conventional approach (PDC1) and two alternative approaches (PDC2 and PDC3), were used to estimate adherence in a real-world dataset from an online pharmacy. Each algorithm has different denominators and increasing levels of complexity. PDC1, the conventional approach, is the total number of days between first dispensation and a defined end date. PDC2 counts the days until the end of supply date. PDC3 removes from the denominator specifically defined large gaps between refills, which could indicate legitimate reasons for treatment discontinuation. The distribution of the three PDCs across four different follow-up lengths was compared. Results The dataset included people taking ACE inhibitors (n = 65,905), statins (n = 100,362), and/or thyroid hormones (n = 30,637). The proportion of people taking ACE inhibitors with PDC ≥ 0.8 was 50–74% for PDC1, 81–91% for PDC2, and 86–100% for PDC3 with values depending on drug and length of follow-up. Similar ranges were identified in people taking statins and thyroid hormones. Conclusion These algorithms enable researchers and healthcare providers to assess pharmacy services and individual levels of adherence in real-world databases, particularly in settings where people may switch between different suppliers of medicines, meaning an individual supplier’s data may show temporary but legitimate gaps in access to medication. Accurately identifying problems with adherence provides the foundation for opportunities to improve experience, adherence and outcomes and to reduce medicines wastage. Research with people taking medications and prescribers is required to validate the algorithms’ assumptions.

Overall survival associated with CDK4/6 inhibitors in patients with HR+/HER2– metastatic breast cancer in the United States: A SEER‐Medicare population‐based study

“…PDC was defined as proportion of days with drug coverage between the date a patient filled their first prescription and the date when the last prescription supply was exhausted). This method of assessing PDC does not assume that patients are alive and continue to need the drug of interest for a predetermined length 21 …”

“…This method of assessing PDC does not assume that patients are alive and continue to need the drug of interest for a predetermined length. 21 Primary exposure: The primary exposure was defined at the therapeutic-class level, as a binary measure: ET+CDK4/6 inhibitor versus ET alone. Systemic treatments were identified using Current Procedural Terminology/Healthcare Common Procedure Coding System codes in the medical claims and National Drug Codes in the prescription drug claims.…”

Adherence and treatment patterns of disease-specific drugs among patients with pulmonary arterial hypertension: A nationwide, new-user cohort study

“…First, PDC1 was defined as the number of days of available medication divided by 365 days (1-year follow-up). Second, PDC2 was defined as the number of days of available medication divided by the number of days from the index date to the treatment modification, treatment discontinuation, or end of the last prescription ( Prieto-Merino et al, 2021 ). PDC for the initial combination therapy was based on possession of any medication, which meant days covered by at least one medication in the regimen ( Basak et al, 2014 ).…”

Towards better reporting of the proportion of days covered method in cardiovascular medication adherence: A scoping review and new tool TEN‐SPIDERS

“…The PDC is defined broadly as the proportion (or percentage) of days that an individual has access to medication during a specified observation period, based on the fill dates and days' supply for each dispensing 7 . In the conventional approach of the PDC, the denominator is the number of days between the first prescription fill date and a defined end date, while the numerator is the number of days covered by the prescription fills during the denominator period 9 . This adherence measure is reported to be more precise than the medication possession ratio because overlapping supplies of medications are excluded 7 .…”

“…7 In the conventional approach of the PDC, the denominator is the number of days between the first prescription fill date and a defined end date, while the numerator is the number of days covered by the prescription fills during the denominator period. 9 This adherence measure is reported to be more precise than the medication possession ratio because overlapping supplies of medications are excluded. 7 Hence, the PDC is endorsed by various organizations and authors as the preferred method to measure adherence using administrative drug data.…”