Thick blood may signal stroke risk.

People with blood that clots readily are slightly more likely to have strokes than people with blood that clots less easily, according to a report in the Aug. 17 CIRCULATION. Most strokes are triggered by blood clots that block oxygen from reaching the brain. Previous research has shown that heart attacks, often caused by blood clots in arteries of the heart, are more common in people with blood prone to clotting.

Researchers took blood samples from 14,700 middle-age people and measured concentrations of two compounds, called factors, that cause platelets to stick together and form clots. Over the next 9 years, people with the highest concentrations of von Willebrand factor were 1.7 times as likely to have a stroke as those with the lowest concentration. People with high factor VIIIc concentrations were 1.9 times as likely to develop strokes as those with lower amounts.

These factors may increase stroke risk by promoting blood clots in the brain, says author Aaron R. Folsom of the University of Minnesota in Minneapolis. However, high blood pressure and smoking are more strongly linked to stroke, so doctors should focus on preventing these risk factors, he says.