Ambry found that 40 percent were wrong. In addition, some genetic variations classified by second companies as threatening actually were benign.

This tiny sample doesn’t prove that the false-positive rate is 40 percent. But patients like Dr. Clayton are not uncommon, genetic counselors say. And they are increasingly worried about the flip side: false negatives that reassure consumers who actually should be worried.

Dr. Judy Garber, director of the Center for Cancer Genetics and Prevention at the Dana-Farber Cancer Institute in Boston, said one of her patients was told by a consumer testing company she had Li-Fraumeni syndrome, which greatly raises the risk of a number of rare cancers.

Further testing showed that diagnosis was wrong. “It makes you worry about the people who don’t come in,” said Dr. Garber.

“People think they are getting the same kind of genetic testing as they would get from a certified clinical laboratory,” said Stephany Tandy-Connor, a genetic counselor at Ambry. “Nothing could be further from the truth.”

Even some doctors are misled by the reports, she said.

Of course, companies like Ambry have an interest in making sure their business is not usurped by consumer testing firms. But it’s also true that the method used by consumer companies is very different from those used by certified clinical laboratories.

Consumer companies method look for changes in tiny segments of genes, rather than examining the entire gene and looking for alterations. That is cheap but not comprehensive.