Cells in tissue stay alive because they continually receive oxygen from red blood cells that have to squeeze through the tiniest capillaries in the body to reach them.
Study leader Steven M. Frank, an associate professor of anesthesiology and critical care medicine at the Johns Hopkins University School of Medicine, and colleagues, found that after three weeks, red blood cells aren't flexible enough to squeeze down the tiny blood vessels.
The also found that even after transfusion into patients, red blood cells in blood that has stood on the shelf for more than three weeks fail to regain the flexibility they once had.
They write about their findings in the 28 February online before print issue of the journal Anesthesia & Analgesia.
Frank says in a statement:
"There's more and more information telling us that the shelf life of blood may not be six weeks, which is what the blood banks consider standard."
Shorter storage periods would mean carrying smaller stocks of blood, and blood banks already don't have enough for everyone.
But despite this, Frank suggests blood banks may need to rethink their current practice of keeping blood on the shelf for up to six weeks.
"If I were having surgery tomorrow, I'd want the freshest blood they could find," he adds.
A previous large study published in the New England Journal of Medicine in 2008, found that cardiac surgery patients who received blood transfusions of blood that had been stored for 2 weeks or less had lower rates of complications and death than those who received blood that was older.
For their study, Frank and colleagues examined the blood given to 16 patients undergoing spinal fusion surgery.
Ten of the patients received three or fewer units of blood, while the other six received five or more.
The researchers took a small sample from each bag of blood and measured the stiffness of their red blood cells.
They also took blood samples from patients up to three days after surgery.
They found that the red blood cells in blood that was over three weeks old were more likely to have stiffer cell membranes. Frank says this condition is known to make it more difficult for red blood cells to deliver oxygen.
This was the case even when the blood was in the patient, where the donated red cells would be in the best environment to repair their injury: the correct acidity, oxygen and electrolyte levels.
Frank says once they lose their flexibility, red cells probably stay that way for the rest of their lives, which is about 120 days in total.
Another interesting result of the study is that the researchers found patients who received fewer units of blood had healthier red cells overall, regardless of how old the blood they received was.
This probably means that if the patient doesn't receive too many damaged cells their loss of flexibility is less of a problem than if the patient receives a lot of them, Frank suggests.
Studies are currently under way, one across the US and the other in Canada, to check how safe old blood is compared to new blood. These should be reporting results sometime in 2014.
If these larger, randomized, multi-center studies confirm the findings of this smaller study, Frank says blood banks should probably reconsider reducing the shelf life of the blood they carry.
Funds from the National Institutes of Health's National Institute on Aging and National Heart, Lung, and Blood Institute helped pay for the study.
Written by Catharine Paddock PhD