Coagulation factor XI boosts clot risk.

Coagulation factors are proteins that guide the thinning and clotting of blood. Their simple names--many are known only by Roman numerals--belie their importance and the specificity of their roles. One of them, factor XI, contributes to the formation of the enzyme thrombin, which in turn helps make a protein called fibrin, a key clotting agent in the blood.

Dutch researchers now report in the March 9 NEW ENGLAND JOURNAL OF MEDICINE that people who have had a major blood clot in a vein--a condition called deep venous thrombosis--are about twice as likely as healthy people to harbor high concentrations of factor XI. Comparing 473 clot patients with 474 healthy participants matched for age and other characteristics, the researchers found that 92 of the patients but only 47 of the volunteers had factor XI concentrations exceeding 121 percent of normal.

Deep venous thrombosis strikes roughly 1 in 100 elderly people, causing pain and swelling, usually in a leg. These clots aren't the fat-based ones that cause heart attacks. But coagulation-based clots sometimes wind up in the heart, get pumped into the lungs, and clog an artery there--a potentially fatal condition.

Having excessive amounts of factor XI itself may not be enough to lead to such a troublesome clot, says study coauthor Joost C. M. Meijers, a biochemist currently at the Academic Medical Center in Amsterdam. Rather, extra factor XI might combine with other risk factors to result in clots, he says.

Drugs that diminish the activity of factor XI might work to ease clotting without causing side effects--such as bleeding--that complicate current anticlotting drug therapy, Meijers says.