NEW YORK — Increasingly, American consumers are shopping for healthcare the way they buy a hamburger or milk shake at a fast-food chain: By standing in line at a local store under a menu.

Store-based health clinics — which are staffed mostly by nurse practitioners and offer quick services for routine conditions from colds and bladder infections to sunburn — aren't just a healthcare fad anymore, but fast becoming a serious industry.

About 7 percent of Americans have tried a clinic at least once, according to an estimate by the Convenient Care Association, an industry trade group formed last year. That number is expected to increase, as major pharmacy operators such as Wal-Mart Stores Inc., CVS Corp., Target Corp. and Walgreen Co., partner with miniclinic providers like RediClinic and MinuteClinic to expand operations. The trade group estimated there will be more than 700 by year-end, up from the more than 400 now, and 2,000 by the end of 2008.

Such an express approach to healthcare — which offers a wait time averaging about 15 minutes and evening and weekend hours — is being heralded as serving up a cheaper and quicker alternative than a doctor's office or an emergency room. A physical exam costs on average $60, while a flu shot typically costs about $20. A strep throat test has a price tag of about $15.

"I was frankly very impressed with how thorough (the examination) was," said Susan Anthony, who visited a clinic at a Phoenix, Md., Target for a cough. "And it was fast. I walked in at 10:30 a.m. and was in my car a little after 11 a.m."

The American Medical Association said a growing number of medical practices are extending their office hours or forming their own clinics to compete. But concerns about quality of care are rising among physicians and some experts say the clinics' services need to be more comprehensive if they are going to have a big impact on reducing overall healthcare costs.

The ventures are promising enough that big-shot investors are jumping into the game. RediClinic got an undisclosed cash infusion from Revolution LLC, the investment house launched by AOL founder Steve Case.

Support among health insurance companies is also growing; about 40 percent to 50 percent of clinics accept insurance from providers such as Humana Inc., UnitedHealth Group Inc. and Aetna Inc., according to CCA.

But concerns are rising in the medical industry that these operations are prone to conflicts of interest. Some physicians are also concerned that the clinics could result in underlying health conditions going undetected.

In June, the American Medical Association urged state and federal agencies to look into whether pharmacy chain-owned clinics urge patients to get their prescriptions filled on site. That followed buyouts of miniclinics by two big-name pharmacy operators: Walgreen bought Conshohocken, Pa.-based Take Care Health Systems in June, and CVS acquired Minneapolis-based MinuteClinic last year.

A growing number of states have passed legislation to better define the role of the nurse practitioner at these clinics, but store-based clinic executives say some of the state rules are too burdensome. According to the American College of Nurse Practitioners, approximately 23 states allow nurse practitioners to treat patients on their own. The remainder require some formal relationship with a physician, which varies from supervision to collaboration. Store-based health clinics are held to the same standards as doctors' offices, said Tine Hansen-Turten, executive director of the Convenient Care Association. She pointed out that store clinics are either monitored by a state board of nursing or board of medicine, and sometimes by both.

AMA also wants to ban the practice of insurance companies waiving or lowering co-payments for clinic patients, which it calls a conflict of interest.

Dr. David Plocher, the senior medical officer at Blue Cross Blue Shield of Minnesota, said that "the normal market forces should determine such things." His company has reduced or waived co-payments for 25 member companies who use MinuteClinics and several other store-based clinics.

The savings can be significant. A visit to a store-based clinic averages about $60, but a doctor's visit costs twice as much, according to Barry Barnett, a health care consultant for PricewaterhouseCoopers.

That compares to about $300 for an emergency room visit, he said.

About 40 percent to 50 percent of clinics take insurance. Hansen-Turten estimated 50 percent of clinic customers pay cash, but she noted that the majority have insurance.