Journal of General Internal Medicine

, Volume 22, Issue 9, pp 1254–1259

Adherence to Warfarin Assessed by Electronic Pill Caps, Clinician Assessment, and Patient Reports: Results from the IN-RANGE Study

Authors

  • Catherine S. Parker
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Zhen Chen
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Maureen Price
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Robert Gross
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
    • Division of Infectious DiseasesUniversity of Pennsylvania School of Medicine
  • Joshua P. Metlay
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
    • Department of Medicinethe Veterans Affairs Medical Center
  • Jason D. Christie
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Colleen M. Brensinger
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Craig W. Newcomb
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
  • Frederick F. Samaha
    • Cardiovascular Divisionthe Veterans Affairs Medical Center and the Hospital of the University of Pennsylvania
    • Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of Medicine
Original Article

DOI: 10.1007/s11606-007-0233-1

Cite this article as:
Parker, C.S., Chen, Z., Price, M. et al. J GEN INTERN MED (2007) 22: 1254. doi:10.1007/s11606-007-0233-1

Abstract

Background

Patient adherence to warfarin may influence anticoagulation control; yet, adherence among warfarin users has not been rigorously studied.

Objective

Our goal was to quantify warfarin adherence over time and to compare electronic medication event monitoring systems (MEMS) cap measurements with both self-report and clinician assessment of patient adherence.

Design

We performed a prospective cohort study of warfarin users at 3 Pennsylvania-based anticoagulation clinics and assessed pill-taking behaviors using MEMS caps, patient reports, and clinician assessments.

Results

Among 145 participants, the mean percent of days of nonadherence by MEMS was 21.8% (standard deviation±21.1%). Participants were about 6 times more likely to take too few pills than to take extra pills (18.8 vs. 3.3%). Adherence changed over time, initially worsening over the first 6 months of monitoring, which was followed by improvement beyond 6 months. Although clinicians were statistically better than chance at correctly labeling a participant’s adherence (odds ratio = 2.05, p = 0.015), their estimates often did not correlate with MEMS-cap data; clinicians judged participants to be “adherent” at 82.8% of visits that were categorized as moderately nonadherent using MEMS-cap data (≥20% nonadherence days). Similarly, at visits when participants were moderately nonadherent by MEMS, they self-reported perfect adherence 77.9% of the time.

Conclusions

These results suggest that patients may benefit from adherence counseling even when they claim to be taking their warfarin or the clinician feels they are doing so, particularly several months into their course of therapy.

KEY WORDS

patient adherencewarfarinmedication event monitoring system

Copyright information

© Society of General Internal Medicine 2007