A new study has found that at-home DNA testing kits are wrong 40% of the time.

For those who haven’t heard of them, direct-to-consumer genetic tests are genetic tests available that offer information about one’s ancestors, risks of certain diseases, and other traits, such as eye color. The demand for this kind of genetic tests has increased recently, along with the number of people interested in personalized healthcare. However, at-home DNA tests shouldn’t be taken as diagnostic because they only offer risk information for a small number of conditions.

Companies like 23AndMe and DNA Direct sell genetic tests that allegedly determine descendence or diagnose genetic predispositions in a person’s genetic makeup, but their results are highly questionable.

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A new paper written by researchers at diagnostics company Ambry Genetics emphasizes that false positives are one of the greatest weaknesses of these kinds of at-home tests.

Stephanie Mlot from Geek.com made an excellent analogy: one’s genome is like a book about a person, with each gene representing a different chapter, and their DNA sequence serving as the letters that constitute the words. A genetic test performed in a professional lab will be able to read each word in specific chapters, checking if large sections aren’t missing or duplicated. Direct-to-consumer (DTC) tests, however, use a method called SNP array, reading only specific letters but not whole chapters.

“Many of these DTC labs also release raw data to the consumer,” wrote Stephany Tandy-Connor in a blog post. She is a genetic counselor for Ambry Genetics.

After a thorough look, researchers at Ambry Genetics reported that at-home DNA tests have a 40 percent false-positive.

“Our results demonstrated a 40% false positive rate highlighting the importance of confirming DTC raw data alterations in a clinical laboratory that is experienced in complex alteration detection and classification, especially prior to making any medical management recommendations,” added Tandy-Connor.

There is nothing wrong with wanting to be aware of your vulnerabilities, but DTC tests might not be the best way to discover them. Tests that provide incomplete genetic data are potentially harmful, and might lead to inappropriate changes in customer’s healthcare, researchers say.

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