Blood Clots More Likely In AB Blood Types

AB blood type is a significant risk factor for venous thromboembolism, or blood clots, a new study suggests.

Researchers from Denmark analyzed looked for blood type and the presence of two genetic mutations, in addition to occurrence of blood clots and heart attacks. Type AB blood, they found, was a significant risk factor for blood clots, accounting for 20 percent of the risk for venous thromboembolism. And the risk was especially high when those with AB blood type also had one or both of two genetic mutations, called V Leiden and prothrombin. The researchers also considered incidence of heart attack but found no consistent increased risk.

But don't panic if your blood type is AB. The study's findings are not necessarily new, nor will they change clinical practice- at least not for now.

Links between blood type, clotting (thrombosis), and its opposite, bleeding, may have to do with a blood component called von Willebrand levels, which are associated with bleeding. People with type O blood have the lowest von Willebrand levels (which make them more likely to bleed); those with AB blood have the highest levels (making them likely to clot); and people with type A and type B blood fall in between.

Many people at risk for thromboembolism are at risk because of underlying disease, age, or the fact that they are in the hospital or just had surgery, and they are probably receiving anti-clotting (anticoagulation) medication without knowing their von Willebrand levels. Is it something doctors might eventually measure to prescribe more or less medication? Maybe, but that's far off.

Although the Danish study is relevant, there's not a lot of data on how to turn it into therapeutic decision-making. Maybe in the future. It adds to a literature that's really just beginning to become to convincing.