Linda Davis describes her life 18 months after bariatric surgery as "amazing."

"I''ve lost about 150 to 155 pounds," says Davis, 42, of St. Louis, "depending on whether you use their scale or mine."

Her health insurance paid for her Roux-en-Y gastric-bypass operation, one of an estimated 140,000 weight-loss surgeries performed each year in this country. Not all insurance companies offer coverage.

Now the field of bariatric surgery is in the midst of a major transition. Some Medicare patients have had to wait six months or more for the operation since the Centers for Medicare and Medicaid Services (CMA) announced changes to coverage in February.

On the surface, the Medicare changes mean that more recipients will qualify for weight-loss surgery. For the first time, patients older than 65 who meet certain criteria will be eligible. Medicare will also cover more variations of weight-loss surgery. Prior to February, it paid for only gastric-bypass operations. Now gastric banding, the Roux-en-Y procedure and other techniques will be allowed.

But Medicare will cover these operations only when they are performed at so-called Centers of Excellence. CMA defines these as high-volume centers that achieve low mortality rates and meet criteria set out by the American College of Surgeons and the American Society of Bariatric Surgery.

Centers of Excellence "have to have the experience, facilities and equipment to handle larger patients," says Dr. J. Chris Eagon, a bariatric surgeon at Washington University School of Medicine. Surgeons must perform at least 150 bariatric operations a year, and nurses and other support staff must meet specific criteria. Even the hospital beds and toilets need to be able to accommodate larger patients.

Washington University''s bariatric surgery program at Barnes-Jewish Hospital recently received its certification, following DePaul Health Center and St. Alexius Hospital. But Eagon says his facility had to stop accepting Medicare patients for a few months until Washington University''s certification became official.

About 20 percent of Eagon''s patients are Medicare recipients, and 5 percent pay for the operation out of pocket. The rest have coverage through an HMO or PPO.

Eagon believes the Centers of Excellence concept may decrease complication rates, which have hovered around 0.4 and 0.5 percent. He also believes more insurance companies may follow Medicare''s lead and begin to pay for the operation.

But he''s worried that it may also mean delays for some patients, and he''s concerned that it may prevent some surgeons, many who may be skilled at bariatric procedures, from performing the operation.

"This is an effective therapy," Eagon says, "a therapy that saves lives, improves the quality of life, and therefore should be offered to all patients."