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

  • Catherine S. Parker
  • Zhen Chen
  • Maureen Price
  • Robert Gross
  • Joshua P. Metlay
  • Jason D. Christie
  • Colleen M. Brensinger
  • Craig W. Newcomb
  • Frederick F. Samaha
  • Stephen E. Kimmel
Original Article

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 adherence warfarin medication event monitoring system 

References

  1. 1.
    Chiquette E, Amato MG, Bussey HI. Comparison of an anticoagulation clinic with usual medical care: anticoagulation control, patient outcomes, and health care costs. Arch Intern Med. 1998;158:1641–7.PubMedCrossRefGoogle Scholar
  2. 2.
    Palareti G, Leali N, Coccheri S, et al. Bleeding complications of oral anticoagulant treatment: an inception-cohort, prospective collaborative study (ISCOAT). Italian Study on Complications of Oral Anticoagulant Therapy. Lancet. 1996;348:423–8.PubMedCrossRefGoogle Scholar
  3. 3.
    Kutner M, Nixon G, Silverstone F. Physicians’ attitudes toward oral anticoagulants and antiplatelet agents for stroke prevention in elderly patients with atrial fibrillation. Arch Intern Med. 1991;151:1950–3.PubMedCrossRefGoogle Scholar
  4. 4.
    Bush D, Tayback M. Anticoagulation for nonvalvular atrial fibrillation: effects of type of practice on physicians’ self-reported behavior. Am J Med. 1998;104:148–51.PubMedCrossRefGoogle Scholar
  5. 5.
    Osterberg L, Blaschke T. Adherence to medication. N Engl J Med. 2005;353:487–97.PubMedCrossRefGoogle Scholar
  6. 6.
    Kimmel SE, Chen Z, Price M, et al. The influence of patient adherence on anticoagulation control with warfarin: results from the International Normalized Ratio Adherence and Genetics (IN-RANGE) Study. Arch Intern Med. 2007;167:229–35.PubMedCrossRefGoogle Scholar
  7. 7.
    Howard AF, Frewin DB, Leonello PP, Taylor WB. Compliance with anticoagulant drug therapy: a study on patients with prosthetic heart valves. Med J Aust. 1981;2:274–6.PubMedGoogle Scholar
  8. 8.
    Van der Meer FJ, Briet E, Vandenbroucke JP, et al. The role of compliance as a cause of instability in oral anticoagulant therapy. Br J Haematol. 1997;98:893–900.CrossRefGoogle Scholar
  9. 9.
    Laporte S, Quenet S, Buchmuller-Cordier A, et al. Compliance and stability of INR of two oral anticoagulants with different half-lives: a randomised trial. Thromb Haemost. 2003;89:458–67.PubMedGoogle Scholar
  10. 10.
    Butler JA, Peveler RC, Roderick P, et al. Measuring compliance with drug regimens after renal transplantation: comparison of self-report and clinician rating with electronic monitoring. Transplantation. 2004;77:786–9.PubMedCrossRefGoogle Scholar
  11. 11.
    Byerly M, Fisher R, Whatley K, et al. A comparison of electronic monitoring vs. clinician rating of antipsychotic adherence in outpatients with schizophrenia. Psychiatry Res. 2005;133:129–33.PubMedCrossRefGoogle Scholar
  12. 12.
    Gross R, Bilker WB, Friedman HM, Coyne JC, Strom BL. Provider inaccuracy in assessing adherence and outcomes with newly initiated antiretroviral therapy. AIDS. 2002;16(13):1835–7.PubMedCrossRefGoogle Scholar
  13. 13.
    Pullar T, Kumar S, Tindall H, Feely M. Time to stop counting the tablets? Clin Pharmacol Ther. 1989;46:163–8.PubMedCrossRefGoogle Scholar
  14. 14.
    Cramer JA, Mattson RH, Prevey ML, et al. How often is medication taken as prescribed? A novel assessment technique. JAMA. 1989;261:3273–7.PubMedCrossRefGoogle Scholar
  15. 15.
    Urquhart J. Role of patient compliance in clinical pharmacokinetics. A review of recent research. Clin Pharmacokinet. 1994;27:202–15.PubMedGoogle Scholar
  16. 16.
    Urquhart J. The electronic medication event monitor. Lessons for pharmacotherapy. Clin Pharmacokinet. 1997;32:345–56.PubMedCrossRefGoogle Scholar
  17. 17.
    Wilson SJ, Wells PS, Kovacs MJ, et al. Comparing the quality of oral anticoagulant management by anticoagulation clinics and by family physicians: a randomized controlled trial. CMAJ. 2003;169:293–8.PubMedGoogle Scholar
  18. 18.
    Gallus AS. Towards the safer use of warfarin: an overview. J Qual Clin Pract. 1999;19:55–9.PubMedCrossRefGoogle Scholar
  19. 19.
    Ansell JE, Buttaro ML, Thomas OV, Knowlton CH. Consensus guidelines for coordinated outpatient oral anticoagulation therapy management. Anticoagulation Guidelines Task Force. Ann Pharmacother. 1997;31:604–15.PubMedGoogle Scholar

Copyright information

© Society of General Internal Medicine 2007

Authors and Affiliations

  • Catherine S. Parker
    • 1
  • Zhen Chen
    • 1
  • Maureen Price
    • 1
  • Robert Gross
    • 1
    • 2
  • Joshua P. Metlay
    • 1
    • 3
  • Jason D. Christie
    • 1
  • Colleen M. Brensinger
    • 1
  • Craig W. Newcomb
    • 1
  • Frederick F. Samaha
    • 4
  • Stephen E. Kimmel
    • 1
  1. 1.Center for Clinical Epidemiology and BiostatisticsUniversity of Pennsylvania School of MedicinePhiladelphiaUSA
  2. 2.Division of Infectious DiseasesUniversity of Pennsylvania School of MedicinePhiladelphiaUSA
  3. 3.Department of Medicinethe Veterans Affairs Medical CenterPhiladelphiaUSA
  4. 4.Cardiovascular Divisionthe Veterans Affairs Medical Center and the Hospital of the University of PennsylvaniaPhiladelphiaUSA

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