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

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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

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