Clinics are also exploring new services, such as monitoring chronic diseases in coordination with primary care physicians. MinuteClinic added monitoring services this year for patients with diabetes, asthma, high blood pressure and high cholesterol. In addition, it announced new affiliations, including those with Catholic Healthcare West in Phoenix, Allina Hospitals & Clinics in Minnesota and the Cleveland Clinic.

Greater integration and coordination with doctors and health care systems is important as health care reform unfolds, Golinkin says.

"We've been addressing the issues of access and cost at RediClinic for five years," he says. "I think we're in a good position to be a part of the solution."

Some health insurance systems, meanwhile, are opening their own retail clinics. Sutter Health operates three retail clinics in the Sacramento, Calif., area. The industry is also making inroads into other locations, such as airports. The AeroClinic operates clinics inside international airports in Atlanta, Philadelphia and Charlotte, N.C.

Ill words

But not everyone's a fan of retail-based clinics. Some physician groups argue that nurse practitioners and physician's assistants who staff the clinics may treat conditions beyond their scopes of practice. They also say the clinics could lead to fragmented care if vital information isn't reported back to patients' regular physicians. The American Medical Association says clinics should have on-site physicians to oversee treatment, and it questions whether there's a conflict of interest in joint ventures between clinics and pharmacies.

State laws affecting the industry vary widely, with some states making it easier than others for retail clinics to operate. Much depends on the rules governing the practice of the health care professionals who work in retail clinics. Eleven states, for instance, allow nurse practitioners to practice independently, but most states require some supervision, according to the California HealthCare Foundation. The level of supervision required has a big impact on retail clinics' bottom line.

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Texas, for instance, last year simplified its rules for physician oversight, requiring supervising physicians to be on site at clinics 10% of the time, instead of 20%, and increased the distance allowed between supervising physicians' primary offices and the clinic sites.

Tine Hansen-Turton, the executive director of the Convenient Care Association, says acceptance of retail clinics among the medical community is growing, but the industry still faces resistance from some state-level medical associations that want tighter regulation of retail clinics.

"Some of those battles are not over yet," she says.

This article was reported by Barbara Marquand for Insure.com.