A new study published in the New England Journal of Medicine reveals that blood stored in hospitals for more than two weeks can be dangerous for heart surgery patients. The patients who receive blood older than two weeks are more likely to die or suffer health problems than the patients who receive fresher blood.

"We report that the relative risk of postoperative death is increased by 30 percent in patients given blood that has been stored for more than two weeks," the researchers wrote in the New England Journal of Medicine.

The study suggests that blood alters in time and that rules allowing it to be stored for six weeks might actually endanger patients’ life.

The Food and Drug Administration allows hospitals to store the blood for six weeks, so they can maintain supplies of very rare blood.

The study's lead author, Dr. Colleen Gorman Koch of the Cleveland Clinic, is aware that FDA’s rule cannot be easily changed, but she recommends blood centers to use the freshest blood first and to use techniques to reduce the need for a blood transfusion.

The research involved 6,002 patients who received blood transfusions following heart surgery at the Cleveland Clinic Foundation. Nearly half of the patients received blood that was 14 days old or fresher, while the other half received blood that had been stored for more than two weeks.

The results showed that the one-year survival rate was 89 percent for patients who received older blood and almost 93 percent for patients who received fresh blood.

Complication rates were also higher in the older blood group, with patients suffering kidney failure, blood infections or multiple organ failure.

Doctors hope that the six-week rule concerning blood storage would not be changed and suggest that the older blood could be used for health problems less complex than heart surgeries, where complications are less likely to appear.

Currently, about 5 million Americans receive blood transfusions each year, as the National Institutes of Health report.