A new study has found that direct-to-consumer genetic tests, like those marketed by 23andMe, Ancestry.com, Family Tree DNA, and MyHeritage, can be used to obtain innacurate results.

Data dump: Most of these tests use a technique called genotyping to provide information about a person’s ancestry, risk of developing certain disorders, or status as a carrier of specific diseases. Some companies also make the raw genotyping data available to customers upon request. People can then take that data to third-party companies to interpret for a fee.

Lost in interpretation: Scientists at Ambry Genetics, a diagnostics company that also interprets data from consumer DNA tests, looked at this raw genotyping data from 49 people. They found that 40 percent of the variants noted in the raw data were false positives—that is, they indicated that a particular genetic variant was present when it wasn’t. Most of the false-positive calls were of cancer-linked genes. In eight instances, third-party interpretation services misunderstood the variants present.

Buyer beware: Unlike clinical genetic tests that require a physician’s sign-off, direct-to-consumer tests are not meant to provide a diagnosis, and they offer risk information for only a limited number of conditions. If a consumer DNA kit uncovers a surprising or noteworthy genetic variant, the authors advise people to seek out doctor-ordered genetic tests to confirm the results.