Why patients need to be vigilant about blood clots

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The Question: I take warfarin for atrial fibrillation. Every three weeks, I go to a clinic to get my blood checked and, if needed, my warfarin dose is adjusted. Today, I woke up feeling ill with a runny nose, a fever and exhaustion. Should I stay home and wait until my next clinic appointment, come to the hospital or call the clinic?

The Answer: A cold or flu can increase the effect of your warfarin and may warrant an adjustment in the dose. The best thing to do is to call the clinic and describe your symptoms. Clinic staff may tell you to come to hospital and have a blood tested sooner than scheduled. Depending on the severity of your illness, the staff may also suggest the warfarin be changed.

“Although anticoagulants are life-saving, they’re potentially dangerous drugs given for dangerous diseases,” said Dr. Bill Geerts, a thrombosis specialist. “Over the age of 70, about 10 per cent of people have atrial fibrillation and most of these patients should be taking an anticoagulant.”

Not every patient who could benefit from the drug receives it. In fact, it requires so much monitoring and effort, that some patients are not given the option. Others are put on aspirin, even though, as Dr. Geerts points out, it is much less effective at preventing stroke in atrial fibrillation, a disorder of the heart’s electrical system.

Anticoagulants include heparin, low molecular weight heparin, warfarin and new oral anticoagulants dabigatran and rivaroxaban. There are two groups of patients typically prescribed them: those with atrial fibrillation and patients with deep vein thrombosis/pulmonary embolism – abnormal blood clots that develop in a leg vein or travel to the lung.

Anticoagulants should also be prescribed prophylactically to many surgical and medical patients in hospitals – but that isn’t always the case.

In fact, Dr. Geerts estimates more than 20,000 Canadians die each year after being struck down by these silent killers. [This rate is an extrapolation based on data from the United States].

“Clots are one of the commonest causes of death associated with hospitalization,” Dr. Geerts said in an interview.

Some institutions do audits on the use of drugs to prevent blood clots associated with hospitalization. At Sunnybrook, October figures show 91 per cent of inpatients received appropriate thrombosis prevention. According to Dr. Geerts, that figure represents an increase over the past few years.

Prevention of blood clots in hospitals is seen as such an important patient safety practice that Canadian hospitals are now required as part of their accreditation to take steps to provide patients with anticoagulants, audit how well they meet that objective and provide education to staff about complications.

When patients are prescribed warfarin, for example, they require a regular blood test every week to every four to six weeks. If they receive too much of the drug, they are at risk of bleeding, including a particularly devastating complication of bleeding into the brain. If they don’t receive enough, they are at increased risk for developing another blood clot or stroke.

The medication dose varies considerably by patient and by week. Weight does not necessarily affect the dose but genetic factors, diet, activities and other medications do.

“There are some patients in our clinic who take only half a milligram of warfarin a day, while others take 25 mg per day,” said Dr. Geerts. “To take warfarin safely, both the patient and the supervising health professional have to be obsessive about using it properly and monitoring it.”

Here is a link of anticoagulant clinics across the country for those patients wanting more information. They typically require a referral from a family physician.

Lisa Priest is Sunnybrook’s Manager of Community Engagement & Patient Navigation. Her blog Personal Health Navigator provides advice and answers questions from patients and their families, relying heavily on medical and health experts.  Her blog is reprinted on with the kind permission of Sunnybrook Health Sciences Centre.  Send questions to

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