WASHINGTON — Patients' adherence to warfarin therapy should be monitored on a regular basis and not just at the beginning of therapy, Stephen E. Kimmel, M.D., reported in a poster at a conference on cardiovascular disease epidemiology and prevention sponsored by the American Heart Association.
Previous data suggest that about 50% of patient time on warfarin is spent either under- or overcoagulated. Physicians often attribute this to nonadherence, although this has never been rigorously investigated, Dr. Kimmel said at the conference, also sponsored by the National Heart, Lung, and Blood Institute.
The prospective cohort study by Dr. Kimmel and his associates at the University of Pennsylvania, Philadelphia, was conducted at three centers (a university hospital, a veteran's hospital, and an anticoagulation clinic). Electronic monitoring devices (microelectrical mechanical systems, or EMS caps) were used to measure the exact time and date each time a pill bottle was opened. Of 145 patients aged 25–85 years who were newly prescribed warfarin, 70% were male, 55% were African American, and 39% were white.
Over 12 months, patients took the correct dose on 78% of the days, took no pills on 19% of days, and took an extra pill on 3% (it was assumed that one pill was taken per bottle opening). There were no significant differences in adherence behavior by age, race, or clinic site. However, men were more likely than women to be overadherent, they reported.
Patient adherence waned over the first 6 months, from 83% in month 1 to 76% in month 3 to 73% in month 6. After that, however, adherence rebounded up to 78% at month 9 and 82% by 1 year. This may be due to a dropout effect, differential adherence by indication, or perhaps differential clinician patterns of patient counseling, the investigators speculated.
Interestingly, the clinicians' ability to discern whether a patient was adherent was only slightly statistically better than chance. Among the patients who took the correct dose on less than 50% of days, 14% had been labeled “adherent” by the clinician.
On the flip side, they labeled as “nonadherent” 81% of patients who took the correct dose on more than 50% of days, and 59% of those who got it right on more than 70% of days.