WASHINGTON (AP) -- The largest U.S. drugstore chain, Walgreen Co., said Wednesday it will hold off selling what was poised to be the first over-the-counter genetic test, after the Food and Drug Administration said the kit has not been proven effective.

Pathway Genomics announced Tuesday that its saliva swab would be on Walgreen's shelves later this month, offering millions of Americans the chance peek into their genetic code for signs of inheritable diseases like Alzheimer's.

But within 24 hours the company's plan was met with stiff response from FDA regulators who said the products may run afoul of federal laws governing medical tests. On Wednesday, the FDA posted a letter to Pathways online, indicating the San Diego-based company never submitted its product for federal review, a requirement for medical devices.

"These kits have not been proven safe, effective or accurate and patients could be making medical decisions based on data from a test that hasn't been validated by the FDA," said agency spokeswoman Erica Jefferson, in an earlier statement Wednesday.

Walgreen said late Wednesday that the Deerfield, Ill., company had decided not to stock the tests until it has "further clarity" on the issue.

Pathway's test would have been the first low-cost, mass-marketed version of kits that screen for genes associated with diseases like prostate cancer, cystic fibrosis and diabetes.

A saliva collection kit plus full genetic analysis from Pathway was slated to cost about $275.

Companies like 23andMe and Navigenics have sold similar kits online for years, with prices ranging between $400 and $1,000.

Those products have never been reviewed by the FDA, even though the agency has the power to regulate all such laboratory-developed medical tests.

But agency officials said Wednesday that Pathway crossed "sanctioned boundaries" for such products by seeking to sell its products in retail pharmacies.

The agency's letter to Pathways, dated May 10, requests a response within 15 days.

"We give them a chance to respond and tell us why do you think that this is, in fact, actually a legal product," said Dr. Alberto Gutierrez, the FDA's director of diagnostic testing.

Pathways said in a statement it is communicating with the FDA and respects Walgreen's decision to hold off stocking the product.

The proliferation of consumer-marketed genetic tests has troubled many public health officials and doctors who worry that the products are built on flimsy data.

"The problem with all of these products is they're based on incomplete, invalidated data and we don't know what the impact on consumers will be," said Dr. Muin Khoury, director of the National Office of Public Health Genomics at the Centers for Disease Control and Prevention.

The biology of how DNA variations actually lead to certain diseases is still poorly understood, although a number of public and private institutions have been racing to find answers.

Khoury said that knowing a patient's medical history -- including whether diabetes or heart disease run in their family -- is actually more useful than current genetic testing. He and other experts worry that increasing prevalence of genomic tests could pressure doctors to order unnecessary tests and treatment.

"I think it's going to be a headache for both primary care physicians and for consumers themselves who are going to get these reports back and not know what to do with the information," said Dr. Peter Kraft, professor of epidemiology at Harvard School of Public Health.

But the prospect of millions of patients walking into their doctor's office with DNA test results may be unrealistic. A CDC survey found that just 22 percent of Americans were aware of genomic testing.

And a sampling of consumers interviewed by The Associated Press found that they have real concerns about the cost and potential misuses of such information.

Deon Green, 36, of Indianapolis, said he might consider getting a genetic test, though the prices seem steep to him.

"I couldn't see myself spending $200," he said. Although, he added, "$100 would be more suitable to my budget."

David Jones, 60, said he sees many good uses for genetic testing, but has no interest in taking one himself because of how the information might be used.

Specifically, he worries that an insurer might cancel coverage because of test results, and he doesn't trust laws that aim to prevent such discrimination.

"I don't care how many laws you have, laws don't stop abuses from happening," he said.

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