When you go to the grocery store, you probably scour the aisles for the ripest produce and read labels to ensure you’re getting the best quality. But did you know that fresh is also best when it comes to blood transfusions?

The Food and Drug Administration allows blood as old as six weeks to be transfused. But a new study published in The New England Journal of Medicine by doctors at the Cleveland Clinic found that storing blood for more than two weeks could be harmful to thousands of heart surgery patients.

They surveyed three-thousand patients who received blood that had been stored for more than fourteen days and three-thousand who received blood stored for fourteen days or fewer.

Those who received the older blood were more likely to die in the year afterward than those who got newer blood. They also were significantly more at risk for kidney failure, blood poisoning and multiple organ failure.

Doctors speculate that red blood cells begin to break down after two weeks, making them more likely to block blood vessels and also reducing their ability to carry oxygen.

While the one-year survival rate was eighty-nine percent for those who got older blood compared with ninety-three percent of those who received the freshest, the news is troubling because heart surgeries are so common. There are more than one-hundred-thousand of the procedures each year in the United States.

The F-D-A says more research is needed before a policy change is warranted, and studies are ongoing.