Do Certain Blood Types Boost Stroke Risk?

In an intriguing study, researchers found that people with blood type AB had a 26 percent increased risk for stroke compared with those with blood type O.

And in women with type B blood, researchers noted a 15 percent greater risk for stroke.

Eight blood types

In general, people can have one of eight possible blood types: O positive or negative; A positive or negative; B positive or negative; or AB positive or negative. Type O is the most common blood type. O-negative is also the only blood type that can be given to everyone, no matter what his or her blood type; a person with type O-negative blood is called a universal blood donor.

Researchers at Harvard Medical School looked at data on about 90,000 people who were part of one of two long-term studies: the Nurses' Health Study or the Health Professionals Follow-Up Study. Participants in these studies had been followed for up to 25 years.

"Blood group AB showed the most consistent association with stroke," says study author Lu Qi, M.D.

The study was presented at a recent American Heart Association (AHA) meeting.

Risk by type

Those with type AB had a 26 percent greater risk for stroke than people with type O. Women with type B had a 15 percent greater risk; this increase wasn't seen among men with type B. Type A didn't appear to affect stroke risk.

Although the study showed an association between blood type and stroke, the researchers were cautious to point out that they hadn't proved a cause-and-effect relationship.

Determining the link

"It's hard to know if blood type is a marker for something else or if there's a direct relationship," says Larry Goldstein, M.D., an AHA spokesman. "For example, maybe there is some other genetic factor that is traveling along with the blood type or that's associated with that blood type that may affect stroke risk."

The researchers took into account age, smoking status, and physical activity levels, but not other factors such as cholesterol levels or diabetes, which could also influence stroke risk, Dr. Goldstein says.

Always talk with your health care provider to find out more information.