WASHINGTON - Every year, thousands of people make a deal with their doctor: I'll pay you a fixed annual fee, whether or not I need your services, and in return you'll see me the day I call, remember who I am and what ails me, and give me your undivided attention.

But this arrangement potentially poses a big threat to Medicare and to the new world of medical care envisioned under President Barack Obama's health overhaul.

The spread of "concierge medicine," where doctors limit their practice to patients who pay a fee of about $1,500 a year, could drive a wedge among the insured. Eventually, people unable to afford the retainer might find themselves stuck on a lower tier, facing less time with doctors and longer waits.

Medicare recipients, who account for a big share of patients in doctors' offices, are the most vulnerable. The program's financial troubles are causing doctors to reassess their participation. But the impact could be broader because primary care doctors are in short supply and the health law will bring in more than 30 million newly insured patients.

If concierge medicine goes beyond just a thriving niche, it could lead to a kind of insurance caste system.

"What we are looking at is the prospect of a more explicitly tiered system where people with money have a different kind of insurance relationship than most of the middle class, and where Medicare is no longer as universal as we would like it to be," said John Rother, policy director for AARP.

Concierge doctors say they're not out to exclude anyone, but are trying to recapture the personal connection shredded by modern medicine. Instead of juggling 2,000 or more patients, they can concentrate on a few hundred, stressing prevention and acting as advocates with specialists and hospitals.

"I don't have to be looking at patient mix and how many are booked per hour," said Dr. Lewis Weiner, a primary care physician in Providence, R.I., who's been in a concierge practice since 2005.

"I get to know the individual," Weiner said. "I see their color. I see their moods. I pick up changes in their lives, new stressors that I would not have found as easily before. It's been a very positive shift."

Making the switch can also be economically rewarding. If 500 patients pay $1,500 apiece, that's gross revenue of $750,000 for the practice. Many concierge doctors also bill Medicare and private insurance for services not covered by their retainer.

Patients and family members say the fee is worth it.

Linda Popkin lives in New York, far from her 97-year-old mother in Florida. With their mother in a concierge practice, Popkin says she and her siblings have direct access to the doctor as needed.

"If one of us calls the doctor, he calls us back," she said. "We are involved in all the decisions. We definitely have peace of mind that mom is seeing a doctor she can speak to if we have any questions. I'm sure you've heard the horror stories about people calling the doctor and they can't get in for three weeks."

Popkin's mother didn't lose her Medicare. She's still covered for medications, specialist visits, hospitalizations and other services. But she has an additional level of personalized attention.

Her doctor is affiliated with a Florida-based management company called MDVIP, a wholly owned subsidiary of consumer products giant Procter & Gamble that represents the largest group of concierge physicians in the country.

MDVIP marketing executive Mark Murrison says its doctors do not sell access, but a level of clinical services above what Medicare or private insurance cover. The cornerstone is an intensive annual physical focused on prevention. About half the patients are Medicare beneficiaries.

Retainer fees range from $1,500 to $1,800 a year, and MDVIP collects $500 of that for legal, regulatory and other support services.

Murrison said the fee is affordable for middle-class households when compared with the cost of many consumer goods and services. "One of our goals is to democratize concierge medicine," he said.

For now, there may be fewer than 2,000 doctors in all types of retainer practice nationally. Most are primary care physicians, a sliver of the estimated 300,000 generalists.

The trend caught the eye of MedPAC, a commission created by Congress that advises lawmakers on Medicare and watches for problems with access. It hired consultants to investigate.

Their report, delivered last fall, found listings for 756 concierge doctors nationally, a five-fold increase from the number identified in a 2005 survey by the Government Accountability Office.

The transcript of a meeting last September at which the report was discussed reveals concerns among commission members that Medicare beneficiaries could face sharply reduced access if the trend accelerates.

"My worst fear - and I don't know how realistic it is - is that this is a harbinger of our approaching a tipping point," said MedPAC chairman Glenn Hackbarth, noting that "there's too much money" for doctors to pass up.

Hackbarth continued: "The nightmare I have - and, again, I don't know how realistic it is - is that a couple of these things come together, and you could have a quite dramatic erosion in access in a very short time."

Another commissioner at the meeting, Robert Berenson, called concierge medicine a "canary in the coal mine."

Several members said it appears to be fulfilling a central goal of Obama's overhaul, enhancing the role of primary care and restoring the doctor-patient relationship.

Yet the approach envisioned under the law is different from the one-on-one attention in concierge medicine. It calls for a team strategy where the doctor is helped by nurses and physician assistants, who handle much of the contact with patients.

John Goodman, a conservative health policy expert, predicts the health care law will drive more patients to try concierge medicine. "Seniors who can pay for it will go outside the system," he said.

MedPAC's Hackbarth declined to be interviewed. But Berenson, a physician and policy expert, said "the fact that excellent doctors are doing this suggests we've got a problem."

"The lesson is, if we don't attend to what is now a relatively small phenomenon, it's going to blow up," he added.

When a primary care doctor switches to concierge practice, it means several hundred Medicare beneficiaries must find another provider.

Medicare declined an interview on potential consequences. "There are no policy changes in the works at this time," said spokeswoman Ellen Griffith.