A genetic quirk could help cheating athletes beat drug tests and could unfairly taint fair players.
The genetic variation affects an enzyme that processes testosterone. Testosterone is naturally made in the body by both men and women, although it is primarily known as a male sex hormone. In order to distinguish between naturally present hormone and synthetic testosterone from steroid use, drug tests measure a ratio of two chemicals found in urine.
One chemical, epitestosterone glucuronide (EG), is made at a constant level in the body, regardless of testosterone levels. The other chemical, testosterone glucuronide (TG), is a testosterone by-product.
Testers measure the ratio of TG to EG. Any amount of TG greater than four times the level of EG is considered a red flag for doping.