Risk of hemorrhage from warfarin higher in clinical practice than clinical trials show

"The rate of hemorrhage in our study is considerably higher than those reported in randomized controlled trials of warfarin therapy, which have ranged between 1% and 3% per person-year," writes lead author Tara Gomes, Institute for Clinical Evaluative Sciences (ICES), Toronto, with coauthors.

Warfarin, a commonly used blood thinner, is used to treat patients with atrial fibrillation to help prevent stroke and blood clots.

The study looked at 125,195 patients in Ontario aged 66 years or older with atrial fibrillation who started warfarin between Apr. 1, 1997 and Mar. 31, 2008. The overall risk of hemorrhage over the 13-year study period was 3.8% per person-year but was 11.8% in the first 30 days of therapy. For people older than 75 years, the overall risk was 4.6% compared with 2.9% for younger people.

Hemorrhage was defined as major bleeding requiring a visit to the emergency department or admission to hospital. Most admissions to hospital involved gastrointestinal hemorrhages (6785, or 63%). Almost 1 in 5 people (1963, or 18%) admitted to hospital for hemorrhages died in care or within a week after discharge.

"There are currently no large studies offering real-world, population-based estimates of hemorrhage rates among patients taking warfarin, which are needed for future comparisons with new anticoagulant agents once they are widely used in routine clinical practice," the authors write.