Whatever the cause, the explosion in the volume of services provided helps explain why Medicare's total payments to doctors per patient rose 51 percent from 2000 to 2008.

A review of physicians' incomes suggests that specialists - who have more opportunities to increase the volume of the services they offer than primary-care doctors - reaped most of the benefit.

On average, primary-care doctors make about $190,000 a year, kidney specialists $300,000, and radiologists close to $500,000, figures that reflect the income doctors receive from both Medicare and non-Medicare patients. The disparity has prompted concern that Medicare is contributing to a growing shortage of primary doctors.

Still, even if primary-care doctors had to rely exclusively on Medicare's lower payment rates their incomes would only drop about 9 percent, according to a recent study co-authored by Berenson, who is also a fellow at the non-partisan Urban Institute.

"The argument that doctors literally can't afford to feed their kids [if they take Medicare's rates] is absurd," said Berenson. "It's just that doctors have gotten used to a certain income and lifestyle."

Regardless of their motivation, if doctors skew their patient base away from Medicare too drastically seniors' access to medical care could be limited.

Is that happening? Again, opinions vary. Based on its studies as well as those done by others others, the Medicare Payment Advisory Commission has concluded the share of affected seniors has been small, and perhaps most significantly, lower than the share of privately insured patients ages 50 to 64 who also report access problems.

But the American Medical Association cites a recent online survey that it commissioned in which nearly one-third of primary-care doctors said they are currently restricting the number of Medicare patients in their practice.

For Michael Trahos, the geriatric specialist in Alexandria, that has meant spacing out routine visits by his Medicare patients such that their share of his weekly appointments has dropped from about half to less than one-third. Trahos said that if a Medicare patient has a serious condition, he will see the person more frequently. But Trahos said it makes him uneasy to push even apparently healthy elderly patients back to twice yearly visits.

"Is it the proper thing to do? Probably not," he said. "These are patients who should be scheduled for proper maintenance every three months."

Adam Weinstein, the kidney specialist in Easton, has taken to supplementing his three-doctor practice by doing medical IT consulting several hours a week. But although the pay is far greater than what he would receive seeing Medicare patients, who make up 70 percent of his practice, the side work means he has less time to serve them. Not only must Weinstein make them wait longer for an appointment, he said, he can no longer afford to answer their phone calls.

"It has definitely made my patients feel more distant from me, and I don't know how to deal with that," Weinstein said.

Financial concerns also prompted Weinstein's group to turn down a request from Chester River Hospital Center in Chestertown, Md., an hour's drive north, to do daily consultations with their mainly elderly patients.

That was disheartening news for the doctor who is currently consulting at Chester River. At 69, David Knutson is semi-retired and said he is satisfied with the payments he has been getting from Medicare. But Knutson grew animated as he spoke of all the ways he would use his time if he could find another doctor to take over coming into the hospital six days a week.

"Gardening, sailing, fishing, hunting, going to the opera," he said. "I'm almost 70. If I'm ever going to do all these things that I've been talking about I better start."

Linda Yao, of the District's Foxhall Internists, has opted for the most extreme response - pulling out of Medicare. The group prides itself on keeping the number of scheduled visits low so that patients who need a last-minute appointment can be accommodated the same day. They also offer half-hour office visits, instead of the 15 minutes on which Medicare reimbursements are predicated. It makes for a white-glove experience for patients, but high overhead for doctors.

After 11 years of serving a patient base of whom as many as half were covered by Medicare, Yao concluded the numbers were no longer adding up. As of April, seniors with Medicare must pay their entire bill out of pocket or through supplemental insurance. So far, only 100 of her approximately 1,750 Medicare patients have elected to stay on.

Could doctors see more Medicare patients if they accepted lower incomes?

Perhaps, said Yao, 42.

But, "the whole system would need to change. ... I graduated medical school $100,000 in debt. I worked 110 hours a week during my residency for $30,000 a year and sacrificed all through my 20s. And even now, you're still seeing people all day, with meetings and paperwork at night, on top of the emotional side of worrying when the patients you care for aren't doing well. This is life-and-death stuff. And I feel like that should be compensated."

aizenmann@washpost.com