ATLANTA (AP) -- Heart surgery patients were more likely to die or suffer problems if they received transfusions of blood that is more than two weeks old rather than fresher blood, according to a new study that adds to the debate about the shelf life of blood.

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 may 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.

"However, the results of this study will not settle the debate" because the patients studied were not representative of all transfusion recipients, he added.

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. But Koch said a more rigorous study is already 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 more rigorous testing would be needed before it reviews the policy. Still, the agency said, doctors may 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. All of the patients were treated at the Cleveland Clinic from June 30, 1998, through Jan. 30, 2006.

A little less than half of 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 amount of blood given to patients in both groups was similar.

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.

Complication rates were higher in the older blood group, with higher proportions of those patients suffering kidney failure, blood infections or multiple organ failure, or needing ventilator care more than 72 hours after surgery.

The average age of the "old" blood was 20 days, not the full 42 days allowed by the FDA. The average age of the fresher blood was 11 days.

The median age of patients in the study was 70 years, and many had other illnesses in addition to their heart problems.

The findings were similar to smaller studies of colorectal cancer surgery patients in Denmark, cancer surgery patients in Spain, sepsis patients in Canada and trauma patients in Colorado. Each of those studies found higher rates of various complications in patients who received older transfused blood when compared with patients who got fresher blood.

About half of heart surgery patients get transfusions, typically receiving one or two units, according to the study's authors.

A number of hospitals have re-evaluated long-standing practices and taken steps to minimize transfusions. One example: Duke University Medical Center in North Carolina has reduced its use of transfused blood products by 17 percent in the past 3 1/2 years.

Concern about the safety of older blood for cardiac patients is one reason for the change at Duke, said Dr. Sunil Rao, a Duke assistant professor of medicine who runs the cardiac catheterization labs at the Durham VA Medical Center.

Through the years, scientists have made strides in screening donated blood for HIV and other infections, so it's understandable that some may believe the safety of transfused blood is firmly established. But questions about older blood keep recurring, pointing out a need for additional research, Rao added.

"We are only now starting to realize what happens to blood when it is drawn out of a human body and sent to the blood bank for storage," he said.

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