Scientists may still be debating the role of viruses in chronic fatigue syndrome, but blood banks aren’t taking any chances. Last summer the AABB, a nonprofit that represents blood-collecting organizations, advised people with the disorder, marked by severe fatigue and aches lasting six months or more, to self-defer from blood donation. Last December the American Red Cross went further, banning people who revealed during a predonation interview that they had the syndrome from ever giving blood at its centers.

The cause for this abundance of caution is XMRV (xenotropic murine leukemia virus–related virus), a retro­virus that has been associated with chronic fatigue syndrome. In a highly publicized 2009 study published in Science, XMRV was found in 67 percent of patients and 3.7 percent of healthy controls. But subsequent studies failed to find the virus in people with or without the syndrome, suggesting to some that XMRV may be a laboratory contaminant that skewed the initial trial.

How worried should one be about XMRV in the blood supply? Not very. There has been no evidence of anyone contracting chronic fatigue syndrome from a blood transfusion, so the risk is hypothetical. And more stringent measures, such as screening potential donors for the syndrome via questionnaire, would take attention away from diseases such as HIV and hepatitis B that are unequivocally blood-borne, says Harvey Klein, chair of the AABB task force examining this issue.

Still, experts are weighing whether or not to test donated blood. The first step in that process is agreeing on a standard method for detecting the virus in the blood. A team at the National Heart, Lung, and Blood Institute is comparing the different nucleic acid tests and blood-sample preparation techniques used by various labs—including the Centers for Disease Control and the Food and Drug Administration—to find the best one.

If the test that comes out on top confirms the results of the 2009 study, that is, if it consistently detects XMRV in blood samples from chronic fatigue syndrome patients and does not detect it in negative controls, “we will have identified sensitive and specific methods to detect XMRV in blood samples,” says Simone Glynn, who chairs the NHLBI working group overseeing the study.

The next step would be to use that test to check for the presence of XMRV in large numbers of blood donor samples. If the virus is prevalent, the team would examine frozen blood samples and check for evidence of transfusion transmission. “Conversely, if we do not find evidence of XMRV in the blood samples from patients with chronic fatigue syndrome who were previously found to be positive, we would conclude that these viruses do not appear to be present in blood,” Glynn says.

For now, excluding people with the syndrome from blood donation is prudent, says Ian Lipkin, director of the Center for Infection and Immunity at the Columbia University Mailman School of Public Health, who is heading the National Institutes of Health–funded investigation into the connection between chronic fatigue syndrome and XMRV. “My sense is that the number of people with the syndrome likely to be sufficiently fit to make blood donations is so few that the Red Cross and AABB have decided for a variety of reasons, scientific and otherwise that it’s just not worth the risk.”