Logic would seem to favor building lower-cost clinics for simple surgeries.

Not just for the savings, either. Free-standing centers tend to have low infection rates, they typically offer pleasant surroundings in newer construction and parking's rarely a problem.

Yet the proliferation of ambulatory surgery centers -- they now outnumber hospitals statewide -- has some questioning if the lower cost comes at a higher price than people realize.

Foremost among the skeptics, not surprisingly, are traditional acute care hospitals. They say the ambulatory surgery centers, or ASCs, draw healthier patients and patients who are more likely to be insured. That leaves hospitals, already straining to make ends meet with subcost reimbursements from Medicare and Medicaid, bearing a greater share of charity care.

A recent analysis by the Pennsylvania Health Care Cost Containment Council bears that out: On a percentage basis, ambulatory surgery centers are absorbing less than half the uncompensated care as acute care hospitals. Margins for ambulatory surgery centers -- an impressive 24.74 percent -- are more than twice any other kind of health facility. On average, the centers see $806 in revenue per outpatient visit, compared with $296 per outpatient visit at a traditional acute care hospital.

Locally, two centers specializing in endoscopy -- North Shore Endoscopy on Federal Street and Southwestern Endoscopy in Uniontown -- saw patient revenues increase by more than 40 percent between 2004 and 2007. The Lawrence County Surgery Center in New Castle, which specializes in ophthalmic surgery, nearly doubled its revenue, to more than $1.1 million.

Small wonder that the surgery centers have become a growth industry. Statewide, 17 new surgery centers opened between June 2007 and May 2008, for a total of 245 licensed facilities. The eight counties in southwest Pennsylvania have 40 centers now, compared with 15 in 1996, while the remainder of Western Pennsylvania went from two centers to 20 in the same period.

In 2000, about 10 percent of all diagnostic and surgical procedures, such as biopsies or catheterizations, were done in an ambulatory surgery center. By 2007, it was more than 30 percent.

The ambulatory surgery centers' gain, though, has been hospitals' loss: The number of procedures at acute care hospital outpatient units dropped in 2007 for the first time.

Paula Bussard, senior vice president for policy and regulatory services for the Hospital and Healthsystem Association of Pennsylvania, said the data that seem to favor surgery centers don't tell the whole story. More ASCs, she says, has simply resulted in more surgeries, which hardly saves money.

"When you look at the growth in outpatient surgeries, the number of surgical procedures being done, it's phenomenal," she said. "And the number of procedures in hospitals is relatively the same. We're saying those surgeries should be looked at a little more closely."

One inference is that ASCs owned by physicians have a built-in self-referral service, and thus an incentive to recommend more surgeries and scans. It's a notion their professional association disputes.

"I haven't seen any objective study that says unnecessary surgeries are being done in ASCs that aren't being done in hospitals," said Kathy Bryant, president of the Ambulatory Surgery Center Association, based in Alexandria, Va.

It should be no surprise that an aging population would undergo more procedures, she said. Nor is there any puzzle why ASCs see a lower percentage of uninsured patients -- the two biggest categories for uninsured care are patients who come to the emergency room and those delivering babies. Ambulatory surgery centers don't offer either service.

The surgery centers, as a rule, are not physically attached to a hospital. They typically offer minor surgical procedures such as endoscopies or imaging services.

State regulations set out specific parameters for ASCs: Barring some unexpected complication, surgeries can be no longer than four hours, and they can't involve a life-threatening condition or major blood vessels. Nor can infants under 6 months old be treated at an ASC.

Ninety percent of the ASCs are owned by physicians or physician groups, sometimes in joint ventures with hospitals or medical centers such as UPMC. They are privately owned, usually organized as limited liability partnerships, rather than nonprofit entities.

Because of their lower overhead, Medicare reimburses ASCs at 59 percent of what they pay acute care hospitals for the same procedure, Ms. Bryant said, adding that patients have a lower co-payment as a result. "We have taken only a portion of the hospitals' business and done it better than many hospitals can do it."

But while surgery centers profit from caring mainly for the insured, said the hospital association's Ms. Bussard, "the hospitals have to bear the costs that go with the more [medically] complicated patients, Medicare and Medicaid patients, and patients who have no insurance."

Highmark spokesman Michael Weinstein said the insurer does not steer patients to a particular facility.

"It's really a physician and patient choice as to where the treatment is obtained" as long as the facility has a contract with Highmark. But he said there may be a broader cost issue when similar services and procedures are offered in a variety of settings.

There is nothing to limit the number of ASCs -- a state program meant to prevent unnecessary duplication of medical services expired 12 years ago -- and their successful business model seems to assure their continued growth.

Ms. Bussard, though, says there may be an unforeseen side effect. Yesterday, HAP released a survey in which half of Pennsylvania's hospitals reported "a moderate to significant increase" in uncompensated care while 83 percent have seen "significant" declines in their investment income.

It's all putting more and more financial pressure on the state's 170 acute care hospitals, she said, and may put some of them in peril.

"When you do need it, you want that medical center there, don't you?"