The shelf life of red blood cells may be shorter than previously believed, according to a new U.S. study, findings that could cut the standard storage period in half and lead to widespread shortages.

Researchers at Johns Hopkins University say that red blood cells stored for longer than three weeks start losing their ability to move freely about the body and, therefore, their ability to deliver oxygen.

According to the findings, red blood cells become less flexible, which makes it difficult or impossible for them to travel through the body’s smallest capillaries. The cells do not recover even after they are transfused into a patient, the study says.

Current standards in both the United States and Canada allow for blood to be stored for up to six weeks.

Lead study author Dr. Steven M. Frank, an associate professor of anesthesiology and critical care medicine at the Johns Hopkins University school of medicine, said current practices may have to change as a result.

"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," Frank said in a statement.

"If I were having surgery tomorrow, I'd want the freshest blood they could find."

The research is published in the journal Anesthesia and Analgesia.

Dana Devine, vice president of medical, scientific and research affairs at Canadian Blood Services, said the latest findings aren’t “the slightest bit surprising.”

“It’s very consistent with probably 20 years worth of red cell literature in this area,” she told in a phone interview.

Devine said it has long been understood that red blood cells become less flexible the longer they are stored in plastic bags, because the plasticizer eventually starts seeping through the cell membranes.

The Canadian Blood Services says red blood cells generally have a shelf life of 42 days when stored in a temperature-controlled environment, between 1 C and 6 C.

Devine noted that one study involving neonatal patients in Ottawa found no difference between the use of “fresh” blood and older blood when it came to mortality rates.

For the Johns Hopkins University study, researchers followed 16 patients who underwent spinal fusion surgery, which typically requires blood transfusions. Six patients received five or more units of blood, while 10 received three units or fewer.

The team took samples from the 53 bags of blood that were used and found that blood older than three weeks generally had red blood cell membranes that were less flexible.

Blood samples from the patients taken three days after surgery found that the damage to the red cells remained and was likely permanent. The life cycle of a red blood cell is about 120 days.

The study noted that the average age of the blood the patients received was more than three weeks, which is likely due to two factors: the freshest blood is often reserved for pediatric patients, and older blood is dispensed first so it can be used before its six-week shelf life is up.

"As a colleague said, it's like how they sell milk in the grocery store; they put the oldest cartons out front so they can sell them before they expire," Frank said.

The researchers noted that two studies currently underway-- one in the U.S. and one in Canada-- to probe the safety of older versus newer blood. Those results are expected sometime next year.

“Those will be the results that determine whether or not we have to change our blood banking practices as far as how old blood is when it’s used and for which kind of patients,” Devine said.

If changes become necessary, the CBS will have to develop a system to ensure that blood is used in hospitals “in a timely manner,” she said.

“Over 90 per cent of our blood is already gone out of our hands before it’s three weeks old. So that means that blood is sitting in the blood banks of hospitals and they are the ones that are going to need to focus on not letting it age to 42 days if we end up in a place where we have to change that rule,” Devine said.