Tens of thousands of would-be blood donors are turned away each year in case they picked up malaria while traveling abroad, report researchers who uncovered the toll on the nation's already stretched blood supply.

Only a very small portion of those people are thought to be infected. But the U.S. has no way to test donated blood for malaria and so instead safeguards supplies by refusing donations, for at least a year, from travelers to malaria-prone countries.

Monday's study by American Red Cross researchers may spur efforts to create a test that could pump up the blood supply while keeping only true malaria carriers from donating.

"This is an unmet need," said Dr. Jay Epstein, the Food and Drug Administration's blood chief.

Maintaining the blood supply is a balancing act. Some 34,000 pints are needed every day, and demand is rising annually. Yet only about 5 percent of the people eligible to donate blood do so, and scientists must guard the supply against a long list of infections.

FDA rules require blood banks to ask would-be donors if they have lived in or traveled to a malaria-prone country, and postpone donations from those who answer yes in case the parasite is silently incubating in their bodies. Donations from short-term travelers are refused for one year; anyone who spent more than a year in a malaria-prone country or suffered malaria-like symptoms cannot donate for three years.

From 2000 through last year, the Red Cross lost 364,562 donations to malaria deferrals, Red Cross researcher Megan Nguyen told a meeting of the American Society of Tropical Medicine and Hygiene on Monday.

The vast majority were tourists turned away for a year. And while they accounted for only 1 percent of donations during the study, that's a significant loss when supplies are tight — and one that's worsening each year as tourism increases, Nguyen said.

"We're losing 150 to 200 donations a month" in some regions, she said. "That's a pretty big number."

Her study undoubtedly is an underestimate. The Red Cross collects half the nation's blood supply; blood banks that collect the other half also turn away travelers.

Moreover, an unknown number never return when their blood is refused. "People get very upset," Nguyen said.

Parts of Europe allow travelers to donate after a few months if a blood test shows no signs of malaria antibodies, immune cells that attack the parasite.

But those tests detect signs of just two of the four malaria species, FDA's Epstein said. "We don't think that's good enough," he said.

One of the missing species accounts for a significant portion of the transfusion-spread malaria cases — fewer than three a year — that occur in the United States.

FDA scientists are working to create an antibody test that would include that troublesome species, and Epstein said he hopes the new study will "marshal the efforts of others."

The percentage of U.S. travelers diagnosed with malaria when they return home is small — meaning if a good blood test were found, most probably could donate blood. To try to pinpoint how many turned-away donors really pose a risk, Nguyen next year will ask some to volunteer a blood sample for genetic testing to hunt the parasite.