Harold Black (middle) suffered a life-threatening blood clot soon after being diagnosed with cancer. He survived the clot and today, people like Harold have a much better chance of avoiding blood clots, thanks to research led by Dr. Philip Wells (left) and Dr. Marc Carrier (right).

As a regular member of a church choir, Harold Black was used to standing and singing for long periods. But in September 2018, he had trouble catching his breath while singing in the congregation. Two days later he went to The Ottawa Hospital’s emergency department and was quickly diagnosed with a life-threatening blood clot in his lung, called a pulmonary embolism. The doctors also found a serious clot in his leg.

“It was a scary experience, but I feel very lucky because I was told that the first sign of a pulmonary embolism is often death,” said Harold, 76.

Harold had two days of intensive treatment and monitoring at The Ottawa Hospital. The clots have now cleared up, but he continues to take daily shots in his belly to prevent new ones.

Blood clots can occur in anyone, but they are particularly common in people with cancer, like Harold. In fact, blood clots are the second-leading cause of death in people with cancer, tied with infections.

“Cancer itself may increase the risk of blood clots, but chemotherapy, surgery and other factors can contribute as well,” explained Dr. Marc Carrier, who treated Harold’s blood clots at The Ottawa Hospital. 

This problem inspired Dr. Carrier and fellow blood clot specialist Dr. Philip Wells to design a clinical trial to see if a low dose of a blood thinner pill could prevent blood clots in people newly diagnosed with cancer. The results, recently published in the New England Journal of Medicine, are now changing practice around the world. 

The study included 563 people newly diagnosed with cancer who had a higher risk of developing blood clots (based on blood test results and other clinical factors). Half received a blood thinner pill twice a day and half received a placebo. Those who received a placebo were more than twice as likely to develop blood clots compared to those who received the blood thinner (10.2 percent with placebo compared to 4.2 percent with the blood thinner).

“Blood thinners are commonly used to prevent blood clots in other high-risk groups, but the traditional thinking has been that these drugs would cause too much bleeding in people with cancer,” said Dr. Carrier, who is also a senior scientist at The Ottawa Hospital and associate professor at the University of Ottawa. “Our study shows that if you select the right patients and use a relatively low dose of a certain blood thinner, the benefits easily outweigh the risks.”

With about 1.9 million people diagnosed with cancer every year in Canada and the U.S., the researchers estimate that about half, or 950,000 could be considered for the blood clot prevention strategy tested in the study. In this population, the strategy would be expected to prevent clots in six percent, or 57,000 people.

“Blood clots are not only life-threatening, but they can also cause pain, reduce quality of life and can be expensive to treat,” said Dr. Wells, who is also senior scientist and Chief of Medicine at The Ottawa Hospital and the University of Ottawa. “We expect that this study will change practice and help many people with cancer avoid blood clots.”

Black continues his cancer treatment, but he is back to singing in his church choir.

“If this research prevents people like me from developing blood clots, that will make a big difference for a lot of people,” he said.

The blood clot (thrombosis) program at The Ottawa Hospital and the University of Ottawa is the largest and the most research-intensive in the world. With four publications in the New England Journal of Medicine since 2015, their research is transforming lives both in Ottawa and around the world.

“I want to thank the outstanding physicians, nurses, research coordinators and other members of our thrombosis team,” said Dr. Wells. “But above all, I want to thank our patients for participating in our research and helping us improve care for them and others around the world.”

This trial was sponsored by the Ottawa Hospital Research Institute, with most of the funding from the Canadian Institutes of Health Research. The BMS-Pfizer Alliance also provided funding but had no role in designing the study or analyzing the results. The study was also supported by the CanVECTOR research network and the Ottawa Methods Centre. Research at The Ottawa Hospital is possible because of generous donations to The Ottawa Hospital Foundation.