The genetic tests used by many direct-to-consumer companies are highly inaccurate at genotyping rare pathogenic variants, a large-scale study suggests.

While SNP (single nucleotide polymorphism) chips are a cheap and accurate way of detecting common disease-causing variants, the technology is “extremely unreliable” for picking up rare variants and should not be used alone to guide clinical decisions, researchers say.

A team at the University of Exeter, UK, used data from nearly 50,000 people participating in the UK Biobank study to investigate the accuracy of two commonly used SNP chips for very rare pathogenic variants in the breast cancer genes BRCA1 and BRCA2.

They found that the tests generated far more false positive than true positive results.