Although not the final word, the study underscores concerns that blood deteriorates with age and that rules allowing blood to be stored for six weeks might pose a safety risk, at least for certain patients.
The findings bolster the argument of those who believe that older blood should be avoided, wrote Dr. John Adamson, of the University of California at San Diego, in an editorial accompanying the study in this week's New England Journal of Medicine .
The report was limited to heart surgery patients, but similar results have been shown in smaller studies that looked at other types of patients.
The Food and Drug Administration's six-week rule allows blood centers to endure shortages in donations and to maintain supplies of rare blood types.
The study's lead author, Dr. Colleen Gorman Koch, of the Cleveland Clinic, did not call for an immediate change to the FDA rule. She said a rigorous study is under way that could carry the scientific weight to persuade the FDA to reconsider its policy.
In a statement, the FDA said the findings were "provocative," but that rigorous testing would be needed before it reviews the policy. Still, the agency said, doctors might wish to consider the report in making treatment decisions.
It's not clear exactly why blood stored for longer periods is riskier. Some researchers say stored blood becomes depleted of oxygen-carrying chemicals. Red blood cells also become more rigid in storage, impeding their flow through the body.
Researchers examined the records of 6,000 patients who were given blood transfusions during heart-bypass or heart-valve surgery at the Cleveland Clinic from June 30, 1998, through Jan. 30, 2006. A little less than half the patients received blood that had been stored for 14 days or less, and a little more than half got blood that was older.
The study found that the one-year survival rate was 89 percent for those who got older blood, but nearly 93 percent for patients who got fresher blood.