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How long should donated blood be stored?

For decades, the Food and Drug Administration (FDA) has limited storage of refrigerated red-blood cells to 42 days. But it has been clear for some time that stored blood degrades in various ways long before that six-week limit, and some research suggests that the changes may be harmful to patients who receive older blood.

Now a study published in the journal Anesthesia & Analgesia has found that after even 21 days, the membranes of stored blood cells have stiffened, apparently the result of damage over time. That’s a problem, because red-blood cells are about the same diameter as small capillaries, and they have to change shape to get through.

“What we showed is that the cell membranes lose their flexibility,” said the lead author, Dr. Steven Frank, an associate professor of anesthesiology at Johns Hopkins. “That makes it more difficult for them to travel through.”

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The study also found that the older blood cells did not recover their flexibility after being transfused into patients.

Still, researchers do not fully understand the importance of such findings to clinical practice. Even permanent changes in cell structure may not matter, according to Dr. Elliott Bennett-Guerrero, a professor of anesthesiology at Duke, who was not involved in the study.

“You constantly have blood cells that are getting old in your body, and they’re filtered out,” he said. “It may be that that same system harmlessly filters out these structurally abnormal blood cells.”

Two large randomized clinical trials have been undertaken to clear up some of the uncertainties about stored red blood cells. In one of them, critically ill patients in Canada will be given transfusions of blood stored for fewer than eight days to see if they experience greater clinical improvement or a reduction in mortality compared with patients transfused with older blood.

Transfused blood has never been tested with the same rigor that goes into testing drugs, noted Dr. Paul Hebert, a professor of medicine at the University of Ottawa and one of the study’s principal investigators.

Historically, he said, “the test was whether the cells would survive in a person for 24 hours. That was the benchmark.” As researchers discovered that blood cells survived for longer periods, the storage limit eventually stretched to 42 days. But the effectiveness of stored blood was not fully evaluated.

“It’s as if we tested whether a drug got into your body, but not whether it actually worked,” Hebert added. “We don’t know whether giving these products old or fresh is just as effective or if both cause harm.”

In the other trial, sponsored by the National Heart, Lung, and Blood Institute, researchers will enroll more than 1,600 patients at 26 hospitals, all undergoing complex cardiac surgery. They will be randomly assigned to transfusion with 10- or 21-day-old blood.

Then researchers will measure and record details of their postoperative health to see if fresher blood results in better outcomes. The study is to be completed by October.

A principal investigator, Dr. Nigel Key, a professor of medicine at the University of North Carolina, said that even if the study showed that newer blood was better for seriously ill cardiac patients, it would not mean that all patients needed the freshest blood.

Dr. Richard Benjamin, chief medical officer at the Red Cross, said the two trials would provide important information. If older blood turns out to be less effective, he said, “there are a couple of ways we can respond.”

However, he said, they would add to the expense.

Whatever the results of the trials may show, experts agree that the lifesaving benefits of transfusion far outweigh any potential risks and that no one should resist transfusions when they are necessary.

“The blood system and transfusions are safer now than they’ve ever been,” Hebert said. “We’re trying to figure out how to make it better.”

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