Beware DNA health tests

It was just a matter of time before the company famous for helping you discover your ethnic roots progressed to predicting what diseases you might get. Utah-based genealogy company Ancestry has been collecting DNA samples from the spit of willing customers for years, amassing a DNA database covering 15 million people.

You might discover that you have Nigerian or Italian heritage and Ancestry will tell you if your DNA matches relatives in the database.

That’s useful for tracking down distant cousins and filling in your family tree, but the big business is testing for genetic predisposition to diseases.

That’s why Ancestry last week launched AncestryHealth with a US$149 one-off test that screens for nine hereditary conditions, including breast cancer, heart disease and blood disorders.

Ancestry’s health screening relies on the same ‘‘SNP-chip’’ genotyping technology used for its ethnicity tracing, which examines 700,000 genetic markers in your DNA. Next year it plans to offer a subscription service for a more sophisticated genetic sequencing service that can identify variants in the genetic code.

The big player in direct-to-consumer DNA health testing to date has been the company 23andMe. The problem, as researchers publishing in The BMJ last week confirmed, is that these types of genetic tests have serious limitations.

They point out that the predictive value of a genetic test is low if you don’t have a family history of a disease. You might test positive for a variant causing colon cancer but any number of other factors – genetic or otherwise – might mean that you never get it.

On the flip side, a negative test may give you false security. The 23andMe test for the BRCA1 and BRCA2 gene mutations that can cause breast cancer only checks for three disease-causing variants mainly relevant for people with Ashkenazi Jewish ancestry.

‘‘This approach would miss in the region of 80 per cent of people with disease-causing BRCA variants in the general population as there are thousands of different disease-causing BRCA variants that the test does not check for,’’ the researchers argue.

AncestryHealth will initiate the tests through doctors rather than just mailing them out as 23andMe does. That’s a good move. The help of clinicians is crucial to interpreting results and understanding the revelations your DNA holds.

You might test positive for a variant causing colon cancer but any number of other factors – genetic or otherwise – might mean that you never get it.