Facing surgery? You could receive blood that's been stored for a week, or three weeks, or nearly six — and there's growing concern that people who get the older blood might not fare as well. It's a question with big implications for the nation's already tight blood supply. Blood is rotated almost like milk on the grocery shelf: The Food and Drug Administration allows red blood cells to be stored for 42 days, and hospitals almost always use the oldest in their refrigerators first to ensure none expires. How old the blood you receive is depends on how much the hospital has of your type that day. The average age of transfused blood is just over 16 days. This summer, hospitals around the country are launching major new research to try to settle if fresher blood really is better for at least some patients. And if so, they're hunting ways to turn back the clock for older blood and offset any deterioration.