The Society of Thoracic Surgeons, for example, changed the guidelines for transfusions after coronary artery bypass grafts beginning in April 2012. A normal hemoglobin count is 12 to 16 grams per deciliter of blood, depending on gender. Some doctors automatically ordered a transfusion after surgery; others did so only if the patient’s hemoglobin level fell to 10. Under the new guidelines, transfusion is not recommended until the level falls to seven.

“There’s overwhelming evidence to suggest that blood transfusion is a dual-edged sword,” said Dr. Victor A. Ferraris, a heart surgeon at the University of Kentucky in Lexington, who was the chairman of a committee that wrote the new guidelines. “Some people need it and it saves their lives; other people are harmed by it.”

In another approach, new guidelines emphasize treating patients for anemia in the weeks before surgery to minimize the need for transfusions. Cancer therapies have also changed in a way that reduces transfusion needs. So has surgery: In a total hip replacement, loss of 750 milliliters of blood, about 1.5 pints, was considered standard; now it is just 200 milliliters.

Image Hospitals may pay $225 to $240 a unit of blood, according to executives in the business. Red cells have a life of 42 days. Credit... Emon Hassan for The New York Times

Doctors may be adopting the new guidelines faster because of the recent computerization of medical records, which allows a physician to order transfusions from a computer screen. The computer, though, knows the guidelines, and it will alert a doctor if the order falls outside the norm. The same system can collect data on which surgeons routinely exceed the guidelines.

Insurance plans also discourage transfusions. Some, including Medicaid, pay hospitals a flat fee for a procedure, whether it involves the transfusion of one unit, two, three or none.

And blood is expensive. Nonprofit organizations collect whole blood from unpaid donors, but hospitals may pay $225 to $240 a unit, according to executives in the business, which covers a variety of costs, including testing. If the unit is billed to the patient, the price can be $1,000 or more. Part of the expense is for storage, management and inventory losses; around a million units a year are discarded, mostly because they are not used soon enough.