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

M.J. Bijlsma, F. Janssen, E. Hak

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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.


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.


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.


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
JournalPharmacoepidemiology and Drug Safety
Issue number3
Early online date21 Dec 2015
Publication statusPublished - Mar 2016


  • JCR
  • adherence
  • methods
  • longitudinal
  • time dependence
  • Pharmacoepidemiology

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