Estimating time-varying drug adherence using electronic records: extending the proportion of days covered (PDC) method

Bijlsma, M. J., Janssen, F. & Hak, E., 1-Mar-2016, In : Pharmcoepidemiology and Drug Safety. 25, 3, p. 325-332 8 p.

Research output: Contribution to journalArticleAcademicpeer-review

Copy link to clipboard


  • Estimating time-varying drug adherence using electronic records extending

    Final publisher's version, 536 KB, PDF document

    Request copy


Purpose: Accurate measurement of drug adherence is essential for valid risk-benefit assessments of pharmacologic interventions. To date, measures of drug adherence have almost exclusively been applied for a fixed-time interval and without considering changes over time. However, patients with irregular dosing behaviour commonly have a different prognosis than patients with stable dosing behaviour. Methods: We propose a method, based on the proportion of days covered (PDC) method, to measure time-varying drug adherence and drug dosage using electronic records. We compare a time-fixed PDC method with the time-varying PDC method through detailed examples and through summary statistics of 100 randomly selected patients on statin therapy. Results: We demonstrate that time-varying PDC method better distinguishes an irregularly dosing patient from a stably dosing patient and demonstrate how the time-fixed method can result in a biassed estimate of drug adherence. Furthermore, the time-varying PDC method may be better used to reduce certain types of confounding and misclassification of exposure. Conclusions: The time-varying PDC method may improve longitudinal and time-to-event studies that associate adherence with a clinical outcome or (intervention) studies that seek to describe changes in adherence over time.
Original languageEnglish
Pages (from-to)325-332
Number of pages8
JournalPharmcoepidemiology and Drug Safety
Issue number3
Publication statusPublished - 1-Mar-2016


  • Adherence, Longitudinal, Methods, Pharmacoepidemiology, Time dependence, controlled clinical trial, controlled study, drug dosage form, exposure, human, information processing, intervention study, major clinical study, medication compliance, randomized controlled trial, statistics

ID: 27654614