Cancer. Trauma. Sickle cell disease. Surgery. There are many reasons why a child might need a blood transfusion, but they all share a common theme: the need to replace blood or blood products (e.g., red blood cells, platelets) that have been lost or consumed, or make up for defects that keep the body from producing them in adequate amounts.

And though transfusions can be life saving, they come with risks, such as iron overload, inflammation or disease (a very low risk, thanks to improved screening tests). And blood products are expensive and scarce—another reason to be prudent about transfusions.

“There’s little science behind physicians’ current practices when deciding when to transfuse a patient,” says Jenifer Lightdale, MD, MPH, of Boston Children’s Hospital’s Division of Gastroenterology and Nutrition. “Many doctors use criteria their mentors passed down to them, which their mentors passed down to them, and so on. But ideally, the decision should be based on evidence, not tradition.” …