What's the shelf life of human blood? Like the milk in your fridge, stored donated human blood has an expiration date: currently it's 42 days, set by the FDA. But is fresher blood actually better? As with ordering wine by the glass, should patients about to be transfused blood ask for "whatever was opened most recently"? There's been reason to suspect this is the case. Older stored blood cells are less deformable (stiffer as they pass through tiny capillaries crammed together), are stickier inside blood vessels, and have lower levels of 2,3-diphosphoglycerate and nitric oxide metabolism. All of these factors could decrease oxygen delivery to tissues perfused by older transfused blood. Do any of those factors translate to a difference in clinical outcomes among people receiving older or fresher donated blood? At least 13 randomized trials (including some in