Patients may be getting blood transfusion too often during surgery, according to a new study by Johns Hopkins researchers.

The study shows wide variation in the use of transfusions, and those who receive blood fare no better, and sometimes do worse.

The problem may be that doctors don’t have clear guidelines about when to use the expensive and scarce resource.

“Over the past five years, studies have supported giving less blood than we used to, and our research shows that practitioners have not caught up,” said Dr. Steven M. Frank, leader of the study published in the journal Anesthesiology.

“Blood conservation is one of the few areas in medicine where outcomes can be improved, risk reduced and costs saved all at the same time,” he said in a statement. “Nothing says it's better to give a patient more blood than is needed.”

Trauma and hemorrhage are exceptions because blood infusions can be lifesaving, said Frank, an associate professor of anesthesiology and critical care medicine in Hopkins School of Medicine.

Frank found that even with blood loss during surgery, if hemoglobin levels didn’t fall below a certain point, a transfusion was usually unnecessary. Sometimes, the patient’s body can develop immune reactions to the foreign substance and can become more prone to infection.

This study isn’t the first to point out the problem. A committee at the federal Department of Health and Human Services said last year that the number of transfusions was excessive and posed dangers to patients, as well as added costs.