The study adds to the growing body of evidence that inflammation plays a role in strokes and heart attacks, perhaps by weakening blood vessels and causing fatty buildups inside them to break loose and create a blockage.
Women with the highest levels of white blood cells were found to be twice as likely to die from heart disease as women with the lowest levels. High white blood cell counts also were associated with a 40 percent higher risk for nonfatal heart attack and a 46 percent higher risk for stroke.
White blood cells are the body's germ fighters. Their levels rise when the body is fighting infection from viruses and bacteria, and doctors routinely take a white blood cell count to diagnose various illnesses.
Current guidelines for predicting heart disease already direct doctors to test for another marker of inflammation, C-reactive protein, in some patients. But the white blood cell test is cheaper and more widely available. And it is as strong a predictor as C-reactive protein, the researchers said.
The study was published Monday in the Archives of Internal Medicine and was led by Dr. Karen Margolis, associate medical director of the Berman Center for Clinical Research in Minneapolis.
A white blood cell test costs about $25, she said, while a test for C-reactive protein costs about $75.
Cleveland Clinic cardiologist Dr. Steven Nissen, who has done similar research, said the new study might not change medical guidelines but should get doctors' attention.
"It's really a wake-up call for the profession," he said. "Heart disease was not created by cholesterol alone."
Which comes first, inflammation or damaged blood vessels, still is unknown. It is also unclear whether lowering white blood cell counts can lower the risk of heart attacks and strokes, Margolis said.
In the study, a white blood cell count did not have to be off the charts to predict heart disease death. Levels at the upper end of normal, or above 6.7 billion white blood cells per liter of blood, doubled the risk.
The study measured white blood cell counts once, then tracked the women for an average of six years. Researchers took into account other risk factors such as diet, smoking, age and physical activity.
The study is part of the Women's Health Initiative, a research project of the National Institutes of Health that involves thousands of postmenopausal women across the country. Other WHI studies have uncovered the risks of taking hormones.
In an accompanying editorial, Dr. Mary Cushman of the University of Vermont called for more research to perfect inflammation testing. But Cushman cautioned that widespread screening for white blood cell counts could increase health-care costs and needlessly worry some patients.
By Carla K. Johnson