The Food and Drug Administration is cracking down on 23andMe and other companies that sell genetic tests directly to consumers.

The F.D.A. sent letters this week to five companies involved in that business, saying their tests are medical devices that must receive regulatory approval before they can be marketed.

“Premarket review allows for an independent and unbiased assessment of a diagnostic test’s ability to generate test results that can reliably be used to support good health care decisions,” Alberto Gutierrez, who is in charge of diagnostic test regulation at the F.D.A., wrote in the letters.

The letters, posted on the F.D.A. Web site on Friday, say the companies must apply for approval or discuss with the agency why certain test claims do not require such approval.

But the letters stop short of saying the tests must be taken off the market until they are approved. Dr. Gutierrez said in an interview that it would be unfair to remove the tests from the market because the agency had not, until now, clearly told the companies that the devices needed approval.

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