The public is being urged not make health decisions based on genetic tests they do at home.
A team of experts from the University of Southampton warned that the results can be unreliable – either wrongly reassuring or leading to unnecessary worry.
23andMe, one of the companies offering tests, said there were “many cases” where results had
Prof Anneke Lucassen, president of the British Society for Genetic Medicine and a consultant in clinical genetics at University Hospital Southampton, said genetic tests sold online and in shops “should absolutely not be used to inform health decisions without further scrutiny”.
She said finding a health risk via the tests often does not mean a person will go on to develop the health problem in question, while reassuring results might be unreliable.
Some patients’ tests wrongly indicated they had faulty genes suggesting a high risk of certain cancers.
The BMJ paper warns genetic tests often prioritise “breadth over detail”, citing a 23andMe test that checks for a few variants of Brca1 and 2, linked to breast and ovarian cancer risk, when there are actually thousands.
A 23andMe spokesman told BBC News its processes were extremely accurate and it spelt out exactly what its Brca test looked for.
“We are very clear with customers that we test only for certain genetic variants,” he said. “As far as the variants we are testing for, they are some of the most well studied and associated with extremely high risk.”
He added that there have been many cases where customers have taken a 23andMe result to their doctors, been prescribed confirmatory testing and have had preventative treatment as a result.
Dr Helen Wallace, director of GeneWatch UK, said: “We recommend that you do not buy these tests, which are at best a waste of money. Handing your DNA to a private company also raises privacy concerns for you and members of your family.”