Infection risks after surgery are often dealt with improperly because patients fail to get timely doses of the right medications, U.S. researchers said on Monday.

Out of 30 million operations performed each year in the United States, about 2 percent are complicated by an infection at the site of the surgery, a risk that could be reduced if antimicrobial drugs were administered in the hour before surgery when they can be effective, their report said.

The study, of 34,000 surgical patients at nearly 3,000 U.S. hospitals in 2001, found only 56 percent of patients received prophylactic medications within an hour before surgery.

In 10 percent of cases, the medications were given as long as four hours before surgery, when they offered no prophylactic benefit, Dale Bratzler, of the Oklahoma Foundation for Medical Quality Inc., wrote in the journal Archives of Surgery.

More than half the patients continued to receive medications more than 24 hours after surgery, when infection risks were small, and powerful medications were often given unnecessarily.

Both improper uses can promote development of antibiotic-resistant infections.

In the case of the powerful, broad-spectrum antimicrobial drug vancomycin, which should only be given if the patient has a particular allergy, half the patients who got it did not have the allergy, the report said. The report said older drugs called cephalosporins were effective.

According to the report, surgical-site infections triple patients’ risk of death compared to uninfected patients. The infections lengthen hospital stays by a week on average and add $3,000 or more to the average hospital bill.

“Substantial opportunities remain to improve the use of prophylactic antimicrobials for patients undergoing major surgery,” the report said.