At issue - especially with blood supplies chronically low - is whether blood stored over long periods should be avoided. The Food and Drug Administration currently allows blood to be kept for six weeks to help donation centers and hospitals stretch out supplies and endure shortages.

That is important to regions like the Bay Area, where blood often must be brought in from other areas because local donations don't always keep pace with demand.

"If the shelf life was reduced from 42 to 14 days, we just wouldn't have enough blood around," said Dr. Ross Herron, medical director of the American Red Cross Bay Area, which sends blood to hospitals throughout the East Bay and South Bay. "We need to have that shelf life to move blood around."

In some cases, Herron said, blood that's collected locally sits at the Red Cross for seven to 10 days before it's shipped out, depending on the type of blood needed and other circumstances.

Although the findings aren't definitive, they bolster the case of those urging that older blood should be avoided, Dr. John Adamson of the University of California-San Diego wrote in an editorial accompanying the study in this week's New England Journal of Medicine.