Risks of hospital infections were particularly high for patients who already had sepsis, a condition in which the body's immune system overreacts to an infection and may lead to organ failure. Patients with sepsis were twice as likely to develop additional infections when they received more transfusions, they said.

The researchers suggested that for every 38 hospitalized patients considered for a red blood cell transfusion, one patient could potentially be spared an infection if fewer transfusions were used.

"The fewer the red blood cell transfusions, the less likely hospitalized patients were to develop infections," lead author Jeffrey Rohde, assistant professor of internal medicine in the division of general medicine at the University of Michigan Medical School, said in a statement.

"This is most likely due to the patient's immune system reacting to donor blood (known as transfusion-associated immunomodulation or TRIM). Transfusions may benefit patients with severe anemia or blood loss; however, for patients with higher red blood cell levels, the risks may outweigh the benefits," Rohde said.

Jeffrey Carson of the Rutgers Robert Wood Johnson Medical School, in an accompanying editorial, said the study "confirms another potential adverse outcome associated with transfusion: serious infectious disease."

"Clinical trials are needed to establish the optimal transfusion thresholds, to provide additional information about the risks and benefits of RBC transfusion, and to determine how best to use RBC transfusion," Carson wrote.