Simple rule may allow many women to stop taking blood thinners

August 25, 2008

A Canadian-led international research group has developed a simple rule that may allow hundreds of thousands of women to stop taking blood thinning pills such as warfarin.

The group, led by Ottawa researcher Dr. Marc Rodger, examined 646 people with unexplained blood clots in the legs, arms and lungs (called unprovoked venous thromboembolism). These clots affect up to 2.5 per cent of people in their lifetime and are a major cause of death. Once the initial blood clot is treated, warfarin is prescribed to reduce the risk of further clotting.

“Many patients end up taking warfarin for the rest of their lives, but there has been a lot of debate about this because although blood thinners reduce the risk of clotting, they also increase the risk of major bleeding and we haven’t known how to balance these risks,” said Dr. Rodger, a Senior Scientist at the Ottawa Hospital Research Institute, Hematologist at The Ottawa Hospital and Associate Professor of Medicine the University of Ottawa. “Our study is the first to identify a group of patients who are at low risk of recurrent clotting and therefore may be able to safely stop taking warfarin after six months. If the results are confirmed in further studies and this rule becomes widely used, it could help prevent many cases of major bleeding and death and also reduce the large health care costs associated with chronic warfarin therapy.”

The study is published in the August 26, 2008 edition of the Canadian Medical Association Journal. It is the largest study ever to look at risk factors for recurrent unexplained blood clots, with patients from Canada, the United States, France and Switzerland included. Researchers collected extensive information about each patient and conducted a mathematical analysis to determine which factors were present in those who went on to develop further blood clots. The analysis revealed that the patients at lowest risk were women with zero or one of the following findings after six months of warfarin therapy:
  • Discoloration, redness or swelling of either leg
  • High levels of a clotting marker (D-dimer) in the blood
  • Body mass index of 30 kg/m2 or more
  • Age of 65 years or more
These patients (representing about half of the women), had a 1.6 per cent chance of developing another clot in the next year, compared to a 14.1 per cent chance for women who had two or more of the signs. The researchers could not find any combination of factors that could predict which men were at low risk of developing further blood clots.

This study was funded by the Canadian Institutes of Health Research and bioMérieux.

“The study conducted by Dr. Rodger and his team offers tremendous promise for the thousands of women who have suffered from blood clots and those who are at risk,” said Dr. Peter Liu, Scientific Director of the CIHR Institute of Circulatory and Respiratory Health. “The findings also demonstrate the importance of using knowledge gained from health research to create new treatment options that benefit our health and our health care system.”

About the Ottawa Hospital Research Institute (
The Ottawa Hospital Research Institute (OHRI) is the research arm of The Ottawa Hospital and is an affiliated institute of the University of Ottawa, closely associated with the University’s Faculties of Medicine and Health Sciences. The OHRI includes more than 1,300 scientists, clinical investigators, graduate students, postdoctoral fellows, and staff conducting research to improve the understanding, prevention, diagnosis and treatment of human disease.

About the Canadian Institutes of Health Research (
The Canadian Institutes of Health Research (CIHR) is the Government of Canada's agency for health research. CIHR's mission is to create new scientific knowledge and to catalyze its translation into improved health, more effective health services and products, and a strengthened Canadian health care system. Composed of 13 Institutes, CIHR provides leadership and support to more than 10,000 health researchers and trainees across Canada.

Media contact
Jennifer Paterson
Director, Communications and Public Relations
Ottawa Hospital Research Institute
613-798-5555 x 19691