The blood thinner warfarin should be used cautiously in some heart patients with kidney failure, say doctors who found the drug was linked to an increased risk of stroke in this group.

Wafarin is prescribed to reduce the risk of strokes in people with atrial fibrillation, the most common type of abnormal heart rhythm.

Few studies have looked at whether warfarin also helps prevent strokes in atrial fibrillation patients who are on kidney dialysis. The combination of the two conditions has a prevalence of nine per cent, according to researchers.

To find out, Dr. Kevin Chan of Fresenius Medical Care North America in Waltham, Mass., and his colleagues studied 1,671 end-stage renal disease (ESRD) patients diagnosed with atrial fibrillation.

The patients were in their early 70s on average, and they were monitored for an average of 1.6 years after dialysis began, the team said in Thursday's online issue of the Journal of the American Society of Nephrology.

Dialysis patients whose blood was not monitored for warfarin levels after they received the drug had 2.79 times higher risk of stroke than those taking no drugs, the researchers found, after taking factors such as age into account.

Affected by extent of thinning

The increased risk seemed to relate to the degree of blood thinning that occurred in response to the drug, the researchers found.

Dialysis patients taking two other types of blood thinners, acetylsalicylic acid (Aspirin) and clopidogrel, showed no increased risk of stroke.

The findings need to be confirmed through randomized controlled trials, the researchers noted.

"Until then, physicians should be cognizant of the possible risks associated with warfarin use for atrial fibrillation in ESRD patients with careful evaluation of the risks and benefits of intervention at the individual patient level," they concluded.

The team also called for more research to determine why warfarin had the opposite effect on stroke risk in kidney disease patients on dialysis, compared with others.

Since bleeding is a well-known complication of kidney failure, adding blood thinners may boost the risk of stroke from bleeding in the brain, the researchers speculated.