A study on medical mistakes found operating room teams around the country leave sponges, clamps and other tools inside about 1,500 patients every year, largely because of stress from emergencies or complications discovered during surgery.

Both the researchers and several other experts agree the number of such mistakes is small compared with the roughly 28 million operations a year in the United States. However, they say there is room to improve.

"It shows the system works. It just doesn't work perfectly," said Verna Gibbs, a surgeon at the University of California-San Francisco who has done separate research on medical mistakes.

Dr. Sidney Wolfe, health research director of the public-interest lobby group Public Citizen, was more critical. He said the real number of lost instruments may be even higher, because hospitals are not required to report such mistakes to public agencies.

He also pointed to the study's finding that surgical teams failed to count equipment before and after the operation, in keeping with standard practice, in one-third of cases where something was left behind. It tended to happen during emergencies.

"It's not something that takes a lot of time," Wolfe said. "I just don't think it's excusable."

The study, which was published Thursday in The New England Journal of Medicine (news - web sites), was done by researchers at Brigham and Women's Hospital and Harvard School of Public Health, both in Boston. It is the biggest and most reliable study yet on such mistakes.

The researchers checked insurance records from about 800,000 operations in Massachusetts for 16 years ending in 2001. They counted 61 forgotten pieces of surgical equipment in 54 patients. From that, they calculated a national estimate of 1,500 cases yearly. A total of $3 million was paid out in the Massachusetts cases, mostly in settlements.

Most lost objects were sponges, but also included were metal clamps and electrodes. In two cases, 11-inch retractors — metal strips used to hold back tissue — were forgotten inside patients. In another operation, four sponges were left inside someone.

The lost objects were usually lodged around the abdomen or hips but sometimes in the chest, vagina or other cavities. They often caused tears, obstructions or infections. One patient died of complications, but the researchers withheld details for reasons of privacy.

Most patients needed additional surgery to remove the object, but sometimes it came out by itself or in a doctor's office. In other cases, patients were not even aware of the object, and it turned up in later surgery for other problems.

The study found that emergency operations are nine times more likely to lead to such mistakes, and operating-room complications requiring a change in procedure are four times more likely. A rise of one point in body-mass index, a measure of weight relative to height, raises the chances of such a mistake by 10 percent. Researchers say big patients simply provide more room and more fat in which to lose track of objects.

The length of the operation or the hour of day does not appear to make a difference, suggesting that fatigue isn't at fault for the mistakes. "It tends to be in unpredictable situations," said lead author Dr. Atul Gawande of Brigham and Women's Hospital.

However, some other researchers say fatigue could promote such mistakes in a way undetected by this study.

The Boston research team suggested more X-ray checks be done right after those operations where such errors are most likely. Metal instruments and radiologically tagged sponges show up in such checks.

Eventually, wands similar to supermarket bar-code readers might be developed to detect missing equipment, researchers said.

However, some others said such mistakes are so rare — occurring about 50 times in 1 million operations — that figuring out how to prevent them could be difficult.

"Something has to be done about this. It's just a very tough balance to decide. Do we really want to add this hoop for every patient to jump through?" said Dr. Kaveh Shojania, author of a 2001 federal study on medical mistakes.

Lori Bartholomew, research director at the Physician Insurers Association of America, said: "I find it's going to be difficult to make much more improvement, because some of the risk factors are things that are hard to control." The Rockville, Md., group represents medical malpractice insurers.