n a suburban strip mall midway between downtown Denver and health-conscious Boulder, there is a place where people can go and order blood tests to detect any number of medical problems, like high cholesterol, diabetes, H.I.V. and prostate and ovarian cancer.

It is neither a doctor's office nor a traditional laboratory that requires a physician's referral for medical tests. It is a retail store, where all that people need is cash, a check or a credit card to find out what ails them.

The store, called QuestDirect and owned by Quest Diagnostics, the largest diagnostic laboratory in the United States, is one of a growing number of direct-to-consumer laboratories that are opening up across the country and on the Internet.

In addition, an estimated 10 to 15 percent of existing commercial and hospital laboratories are now offering testing directly to consumers, without a doctor's referral, said Jondavid Klipp, managing editor of Laboratory Industry Report, a trade publication.

"We're potentially entering a retail era where companies are marketing and selling testing services directly to the consumer," said Dr. Bruce A. Friedman, a pathology professor at the University of Michigan Medical School. "The number of people doing this is minuscule but, in fact, a lot of people are paying attention to it because there is a movement toward these retail labs."

The trend worries many doctors, who question the medical implications of patients' trying to diagnose their own conditions and interpret their own test results. They also question the legality of these direct-to-consumer laboratories.

"Trying to interpret lab tests is a very complex and very specialized thing and requires knowledge far beyond the usual layperson's ability," said Dr. J. Edward Hill, chairman-elect of the American Medical Association's board. "It's unfathomable that people are going to order tests that take years of medical training to understand."

Many states have laws preventing patients from ordering their own tests and receiving results without a doctor's requisition. Direct-to-consumer laboratories in those states sometimes follow the letter of the law but not the spirit, skirting the laws by hiring doctors to sign the requisitions, usually without ever seeing the patients.

To get tested, people can walk into one of the storefront laboratories. Or they can order their tests online at Web sites like HealthcheckUSA.com or Questest.com, and then have their blood drawn at a local traditional laboratory. People can also buy at-home tests, which are then mailed back to the laboratory for analysis.

Popular tests include screening for cholesterol and other heart disease markers, H.I.V., Lyme disease, thyroid problems, liver and kidney function, prostate and ovarian cancer, allergies and sexually transmitted diseases. Some people use self-testing laboratories to screen themselves for recreational drugs before they submit to an employer's drug test.

Dottye Howard, 49, of New York City, says ordering her own blood tests has improved her thyroid problem. The thyroid test she takes every six to eight weeks helps her and her doctor adjust the medications so she has optimal levels in her body. Ms. Howard's managed care company refused to pay for the tests, so she orders them directly from HealthcheckUSA for $79 out of pocket, less expensive than if her own doctor had ordered the tests.

After receiving by fax or mail a doctor- signed requisition from the Web site, she takes a bus or a cab to a local laboratory and gets her blood drawn. Two days later, she can download her results from the Internet, call her doctor and discuss whether to increase or decrease her medications.

"Getting the meds optimized has really improved my quality of life and that could not be done, in my opinion, without the access to these tests," Ms. Howard said.

Sandra Thomas, president of American Hemochromatosis Society, said she advised people with family histories of hereditary hemochromatosis, an iron overload disease, to order their own tests if their doctors refused to screen them for the disease. She said self-testing had probably spared the lives of several people she knew. In those cases, the people learned that they had the disease early enough to be successfully treated.

Proponents of self-testing believe it gives patients more control over their health and may help in the early diagnosis of diseases. Some patients, particularly the uninsured, may never seek health care and may have diseases that go undiagnosed, said Dr. Richard Abrams, an internist in Denver. A simple, inexpensive test can catch common diseases like diabetes, high cholesterol and hepatitis C early, he said.

Self-testing is an extension of the screenings conducted at health fairs, at malls or through corporate wellness programs, Dr. Friedman said. People also test themselves on a limited basis with home health tests, like pregnancy tests and glucometers for diabetes.

But critics worry that even a well-informed consumer outside the medical field is not educated enough to understand the results of many tests. "The problem is you need a doctor to read the tea leaves and put it in the context of your overall health," Dr. Abrams said.

Blood tests are not the only factor in making many diagnoses. "The first thing I try to teach medical students is that about 90 percent of diagnoses can be made accurately by taking a good history and good physical exam," Dr. Hill said. "When you do testing, it is to confirm your suspicions." Direct-to-consumer testing starts the process where it should end, he said.

The College of American Pathologists, whose members work at commercial and other laboratories, takes a cautious position on the issue. "We have concern about the lab tests being ordered by an individual who may not fully understand the consequences of the test and the potential pitfalls in test interpretations," said Dr. Paul Bachner, past president of the organization.

The group's biggest concern centers on false negative results, in which a test does not pick up an existing problem. "Patients may have symptoms, but will conclude that if the test is normal, they don't have that condition and don't seek out medical care," Dr. Bachner said.

A physician caring for such a patient, he said, may recognize discrepancies between the symptoms and the results and repeat the test or order others. On the other hand, false positives can lead people to have more tests or unneeded procedures.

QuestDirect, HealthcheckUSA and other independent laboratories say they encourage customers who have abnormal results to see doctors. When result are significantly out of the normal range, the companies contact the consumers to let them know they should see their doctors immediately.

"We're very careful to let people know that they should use this information in the context of an overall relationship with their personal physician," said Hughes Bakewell, vice president of consumer health for Quest Diagnostics, a $4 billion publicly traded company, which opened eight QuestDirect laboratories across the country, all in states where laws do not prohibit self-testing.

"Typically, people who do this on their own are proactive anyway and it's highly likely they'd act on it," Mr. Bakewell said. No laboratories seem to have done follow- ups on what patients do with the information.

He said typical customers were people who wanted to monitor problemss like high cholesterol and diabetes. But perfectly well people who are health-conscious make up a significant portion of self-testers.

"They're the worried well," said Dr. Henry Soloway, who pioneered direct-to-consumer health testing nearly two decades ago in Las Vegas. "They're wondering how they're doing. They're somewhat sophisticated and may have a specific question they want an answer to."

There are also those who choose self- testing so their results do not show up on their medical records.

Others use the laboratories to save time. Dr. John Halsey, a clinical immunologist in Kansas City, Kan., who started a direct-to- consumer laboratory and then sold it to Quest, said: "Traditionally, if you wanted your cholesterol taken, you'd have to go to the doctor, sit in his waiting room for a half hour. He'd order the test, then you'd have to go to the lab and get the blood drawn. Now, patients can be in and out in 15 minutes."