It works like the anti-theft sensors on clothes in retail stores, but this system prevents something even more serious.

Doctors perform tens of thousands of surgeries safely every day, but every once a while, despite being required to mark the spot, they operate on the wrong area or body part. It''s called wrong-site surgery.

Dr. Richard Chole from Washington University said, "The mistake doesn''t happen very often, but in one in 10,000 or 15,000 patients mistakes are made in positioning, in remembering which is right or left, and the mistake is made."

That''s why Dr. Chole invented the check site system. It consists of a patient wristband with a sensor implanted, a marking pen and green sticker, and sensors outside the operating room. The surgeon simply uses the pen to mark the spot, then puts the green sticker on the wristband to deactivate the sensor.

That gets past the detectors outside the O.R. So if the alarm goes off, it''s just a reminder to the surgical team that they missed marking the site and should go back and mark it and proceed from there.

"It''s a simple system, an inexpensive system and we feel it can cut down wrong site surgeries by at least 50 percent...more like 90 percent," he says.

The wristbands with the sensors cost only about $2.50 each. The sensors outside the O.R. have a one-time cost of about $5,000. The "checksite" system was tested at Barnes-Jewish Hospital and goes into use there. Several other hospitals are considering it.