ATLANTA -- The next time you're at the doctor's office, take a peek at those certificates hanging on the wall. Like gallons of milk, some of them are expiring.

For the first time since leaving medical school, many doctors are having to take tests to renew board certification in their fields -- 147 specialties from dermatology to obstetrics.

Any doctor can deliver a baby, treat cancer, or declare himself a cardiologist. Certification means the doctor had special training in that field and passed an exam to prove knowledge of it.

They used to do this once and be certified for life. That changed in the 1990s -- doctors certified since then must retest every six to 10 years to prove their skills haven't gone stale.

For some specialists, like the doctors who push tubes into heart arteries to unclog blockages, this is the first year many are going through retesting.

Older doctors also are feeling the heat. More than a quarter of a million of them were "grandfathered" with lifetime certificates but are being urged to retest voluntarily to show they still know their stuff.

Most don't want to do this. One who isn't grumbling is Dr. Stephen Mester, 52, a cardiologist at Brandon Regional Hospital in Brandon, Fla.

"I am choosing to renew. It's just an opportunity to maintain my skills and confirm to myself that I can do what I've been trained to do," he said. "Most of what I do today didn't exist, and some of it not even thought of, when I was in medical school."

Research shows that what doctors think they know does not match what they do, and that performance declines with years in practice, two members of the American Board of Internal Medicine wrote recently in the New England Journal of Medicine.

The authors of an opposing essay -- a medical school dean, a professor who teaches primary care and a practicing physician -- complain that testing emphasizes recall of facts rather than making full use of available sources of information, such as electronic references and the Internet, as modern doctors are taught to do.