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Doctors should remind patients about warfarin compliance, even those who claim to be taking the drug as directed

A substantial number of patients, even those monitored in specialized anticoagulation clinics, do not strictly follow their regimen of warfarin, an anticoagulant drug. Yet, their doctors think they are more compliant with drug therapy than they are, according to a new study.

Patients at three Pennsylvania-based anticoagulation clinics incorrectly took their warfarin medication on one of every five days of intended therapy. This level of nonadherence is enough to adversely alter the coagulation effect of the medication, potentially leading to hemorrhage or blood clots. In this study, patients were six times more likely to miss taking pills than to take extra pills. This would put them at greater risk of blood thickening and thromboembolism (formation of a blood clot that breaks off and can travel to the lungs, heart, or other organs). Not surprisingly, warfarin adherence was associated with anticoagulation control. Drug compliance tended to decrease over the 6 months after starting warfarin treatment, but then rebounded between 6 and 12 months.

The reasons for change in drug compliance over time were unclear, note the University of Pennsylvania researchers. Patient nonadherence was substantially worse when measured by electronic monitoring of pill container openings than by either clinician assessment or patient self-reports. For example, clinicians judged the 145 patients studied to be adherent to warfarin therapy at 82.8 percent of visits. These same visits were categorized as moderately nonadherent using electronic monitoring (over 20 percent of days were nonadherent). Similarly, patients who were deemed moderately nonadherent by electronic monitoring had self-reported perfect adherence 78.5 percent of the time.

The researchers recommend that clinicians emphasize strict compliance with warfarin treatment, even among patients they think are already compliant. The study was supported in part by the Agency for Healthcare Research and Quality (HS11530).

See "Adherence to warfarin assessed by electronic pill caps, clinician assessment, and patient reports: Results from the IN-RANGE study," by Catherine S. Parker, M.S., Zhen Chen, Ph.D., Maureen Price, R.N., and others, in the September 2007 Journal of General Internal Medicine 22, pp. 1254-1259.

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