If you paid $500 for a gene test, would you know what to do with it?

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The FDA didn't tell the three companies to stop selling their tests. But the agency pointed out that it considers them medical devices that need regulatory clearance, and asked for justifications if they believed the clearance wasn't necessary. "The FDA actively regulates genetic tests sold directly to consumers to make sure they are safe and do what they claim to do," spokeswoman Jennifer Dooren said in an email. "Without FDA oversight, the safety and efficacy of the these tests have not been determined and could potentially lead to patient harm."

DNA4Life and DNA-CardioCheck didn't respond to requests for comment. Mark Carbeau, chief executive of Interleukin Genetics, said the company's main product, a test that predicts the risk of gum disease, is sold primarily through dentists. Other products "can be purchased by a consumer, though a medical professional needs to be involved," Carbeau said. "We think, properly done, genetic tests offer real value to the health care community, and we're certainly supportive of the FDA's efforts to protect the public and provide guidance."

The FDA has cracked down on testing companies marketing straight to the public before. In late 2013, it ordered 23andMe to stop selling its personal genome analysis. This past February, the agency approved 23andMe's test for a limited use, to detect whether people carried the gene for Bloom syndrome, a rare genetic disorder. The company has since added other approved uses to determine whether people carry mutations that could be passed down to children.

One of the risks of making genetic tests available directly to consumers is that people may confuse diseases caused by complex risk factors with those caused by a single genetic abnormality. While a single gene mutation can cause cystic fibrosis, "for things like cardiovascular disease that's just not true," said Michael Christman, chief executive of the Coriell Institute, a Camden, N.J., research center focused on genetic medicine.

Unlike some rare hereditary conditions, many pieces of a person's DNA may influence his or her risk for heart disease, cancer, or diabetes, and they're not all well understood. And genetics are only one risk factor, along with environmental exposures and lifestyle, that help determine whether someone will get a disease.

In an ideal world, genetic tests would go through physicians or other experts capable of interpreting results, Christman said. "People joke, there's the $1,000 genome followed by the $1 million interpretation," he said. "The interpretation is complicated, and it needs to be done very well.

Christman said the greatest potential for genetic medicine right now is using DNA profiles to determine how people might react to different drugs or doses, a field called pharmacogenomics. More than 100 drugs have information on their labels about how people with certain genetic markers may respond differently.

The challenge ahead for the FDA is policing the evolving market for genetic testing. "They're trying to figure out, 'How do we help consumers and providers that don't work in genetics all the time to distinguish what tests are useful and what aren't?' " said Joy Larsen Haidle, president of the National Society of Genetic Counselors. " What got some of the labs into trouble before with the FDA was you could send a sample of saliva to a couple of different labs and you would get very different results to the same question."

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