It's a nagging worry for anyone about to go under the knife with anesthesia, but a new study suggests that the odds are quite long against having surgery done at the wrong spot.

A review of cases reported to one large medical-malpractice insurer between 1985 and 2004 found just 40 cases of wrong-site surgery out of 1,153 malpractice claims, including 25 that involved locations other than the spine. The study was published Monday in the Archives of Surgery.

A wrong-site surgery includes procedures performed on the incorrect person, organ or limb, or spinal procedures performed on the wrong part of the spine.

The researchers, led by Dr. Mary Kwaan of Brigham and Women's Hospital in Boston and the Harvard School of Public Health, estimated that wrong-site surgery serious enough to result in an insurance claim or lawsuit would occur about once every five to 10 years at a single large hospital.

They calculated that there was a surgery on incorrect limbs or organs once in every 112,994 operations.

The team also considered whether following surgical safety protocols set by a hospital-accrediting agency would have prevented the mishaps. A review of medical records for 13 of the cases suggested that meeting such protocols would have prevented eight of them.

Of the 13 cases, one patient was permanently and significantly injured, two had major but temporary injuries, and 10 had injuries that were temporary and minor.

"No protocol will prevent all cases," Kwaan said. "It will ultimately remain the surgeon's responsibility to ensure the correct site of operation in every case."

Looking at protocols used in the hospitals, the researchers found they had an average of 12 redundant checks on the correct surgical site, involving two to four hospital staff members.

While there is little or no published evidence on which site-verification procedures are the most effective, the researchers suggested that keeping protocols simple would "allow surgical teams to focus their limited time and energy on prevention of more common or harmful errors."

Dr. Carolyn Clancy, director of the federal Agency for Healthcare Quality and Research, which sponsored the study, said, "the good news is that wrong-site surgery is extremely rare and major injury from it even less common.

"The less good news is that although site-verification protocols offer some protection against such errors, they are not foolproof." We have a lot more to do to ensure that wrong-site surgery never happens."

The researchers also collected information about the protocols in place during 2004 to prevent wrong-site surgery at 28 hospitals covered by four malpractice insurers in New England and Texas.

They found that the most common site-verification protocols included:

— Marking the part of the body where surgery is to occur;

— A pre-operative verification checklist by the surgeon and one other health-care staff member;

— Having the patient do an informed-consent form that specifies the site of the surgery.

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