If you are at a high risk for blood clots, you could be missing your chance to decrease your risk of stroke, according to a study released Tuesday.

More than half of high-risk patients were either not taking any or the right amount of blood-thinning medication at the time of their stroke, according to the study, published in Stroke: Journal of the American Heart Association.

Blood clots can form in the upper chamber of the heart when it beats irregularly or fibrillates. Clots can travel to the brain and block an artery, which can lead to a stroke.

Warfarin, a blood-thinner, reduces the risk of stroke by 70 per cent, and can also reduce the severity of a stroke.

Only 40 per cent of patients who suffered an ischemic (clot-caused) stroke had been taking warfarin prior to their brain attack. Three quarters of those patients getting the drug were not taking the right doses necessary to prevent a stroke.

"These are missed opportunities for stroke prevention," Institute for Clinical Evaluative Sciences (ICES) investigator Dr. David Gladstone said. "Sadly, we frequently see patients admitted to hospital with a devastating stroke who are known to have atrial fibrillation, yet were either not taking warfarin or were taking a dose that is not therapeutic. We consider these to be potentially preventable strokes."

Canadian researchers analyzed data from 2,135 stroke patients and studied 597 with atrial fibrillation, a heart disorder that cause clots, who had suffered an ischemic stroke. The strokes were disabling in 60 per cent of patients and fatal in 20 per cent, they found.

The findings are "particularly troublesome" because the patients were considered to be ideal candidates for warfarin.

"It's a tragedy. On one hand we have an extremely effective and cheap medication for stroke prevention...and on the other hand it remains under-used in people who would benefit most from it," Gladstone said.

Researchers say their findings encourage improving patient care, such as providing more specialized anticoagulation clinics.

"This is a public health priority because atrial fibrillation is one of the most common causes of stroke," Gladstone said. "Many more strokes could be prevented if anticoagulation therapy were optimized in the population at risk."