Donated blood may not have a very long shelf life. At least not as long as we think it might. Nearly 14 million units (or bags) of red blood cells are transfused in the United States every year. Those in charge of collecting and distributing blood need to ensure that enough of this resource is always available, most especially because the national supply depends on donations from volunteers. To preserve this resource, hospital blood banks use the oldest blood first (kind of like moving the oldest dairy products to the front of the shelf in grocery stores). Although preservatives allow plastic bags of red blood cells to be stored up to 42 days on refrigerated shelves, by the end of that time the red cells age. As weeks go by, the chemistry and shape of red cells change so much that they’re much less able to carry life-saving oxygen. That’s certain. Less certain is whether those changes affect patient outcomes. Although donated blood is tested for pathogens, it’s not tested for quality. Some doctors who care for very sick patients who also need a lot of transfusions are convinced that patients who receive older blood don’t do as well. Proving so is extremely difficult because clinical trials need to be large enough to find any differences, and because it’s considered unethical to give any patients only the oldest blood. So far, results of clinical trials have been mixed, with some trials showing no difference, and other trials showing that older blood could be more dangerous. Until recently, there have been very few prospective clinical trials.
