Coinciding with the popularity of DNA testing kits, companies that claim to be able to tell your “cellular age” from a drop of blood have also attracted quite a few customers.

However, their results can’t always be trusted, according to Science News. Oncologist and Johns Hopkins researcher Mary Armanios told the website that the tests can do more harm than good by sending perfectly healthy customers into a panic. 

“The telomere belongs in the clinic and should not be used as a form of molecular palm reading,” Armanios tells Science News. For instance, Armanios shared the story of one man in his forties who learned he supposedly had the telomeres of an 80-year-old. In hopes of making the most of his remaining time, he quit his job, sold his house, and put off surgery that he believed would further shorten his telomeres.

For a cost of roughly $100, some companies claim to not only be able to tell you your cellular age, but also tell you how to improve your health so that you can live longer. They measure the length of telomeres, the cap at the ends of your chromosomes, in order to determine your biological age. Telomeres shorten with age, but other factors—like diet—can also chip away at them, potentially causing disease and other health-related problems. (On the other hand, telomeres can get longer in outer space, as astronaut Scott Kelly learned.)

However, as Science News notes, this isn’t always the most accurate indicator of health or life span because what’s considered “normal” encompasses a wider range than what those companies would have you believe. Extra-long telomeres may be associated with a higher cancer risk, and on the flip side, shorter telomeres don’t necessarily mean you’ll keel over tomorrow.

Indeed, the tests used by these companies have a 20 percent variability rate, meaning they can produce different results on different days, and not all scientists agree that telomere length can be used as a “biomarker” of age. The National Institute of Aging reached the conclusion that biomarkers for aging could not be scientifically validated, according to WIRED.

Research on telomere length can do a lot of good, though, when done correctly in a lab. These tests can be used to diagnose rare disorders and help patients get the care they need.

[h/t Science News]