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.

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