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Patients find it difficult to adhere to warfarin therapy even when monitored at anticoagulation clinics

Patients find it difficult to always take the anticoagulant warfarin as directed, which can have potentially dire consequences, according to a new study. Patients who missed 1 to 2 doses a week doubled the likelihood that their blood would not be thinned enough to prevent potentially life-threatening clots from forming. Patients taking warfarin must have regular blood tests to make sure that their International Normalized Ratio (INR) is in the target range of 2.0 to 3.0. This range means that the blood is not too thin to cause internal bleeding, but not too thick to form dangerous blood clots.

Ideally, these patients are monitored in specialized anticoagulation clinics, which emphasize the importance of strictly following the warfarin regimen, note researchers at the University of Pennsylvania School of Medicine. They used an electronic bottle cap to monitor when patients took their warfarin for an average of 32 weeks among 136 adults, who visited one of three specialized anticoagulation clinics to initiate warfarin therapy. Overall, 92 percent of the time, patients missed at least one or took one extra dose of warfarin; 36 percent of the time, they missed more than 20 percent of their doses; and 4 percent of the time, they took extra doses.

After accounting for several other factors affecting blood coagulation (for example, vitamin K intake, alcohol consumption, and weight change), there was a significant association between underadherence and inadequate blood thinning (underanticoagulation). For each 10 percent increase in missed pill bottle openings, there was a 14 percent increase in the odds of underanticoagulation.

Participants with more than 20 percent missed bottle openings (1 to 2 missed days each week) had more than a two-fold increased odds of underanticoagulation. Adults who had extra pill bottle openings on more than 10 percent of days had a 73 percent significant increase in overanticoagulation (overly thinned blood that can lead to internal bleeding).

These findings were based on regular INR monitoring and questionnaire responses during clinic visits. The study was supported in part by the Agency for Healthcare Research and Quality (HS11530).

See "The influence of patient adherence on anticoagulation control with warfarin," by Stephen E. Kimmel, M.D., M.S.C.E., Zhen Chen, Ph.D., Maureen Price, R.N., and others, in the February 12, 2007, Archives of Internal Medicine 167, pp. 229-235.

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