Interest of hair tests to discriminate a tail end of a doping regimen from a possible unpredictable source of a prohibited substance in case of challenging an anti-doping rule violation.

The presence of letrozole, an aromatase inhibitor, in an athlete's sample constitutes one of the more frequent anti-doping rules violation. It is possible to challenge this violation but it is the athletes who have to demonstrate their innocence. The conditions to evidence/establish the absence of fault or negligence hinge on two points: 1. the athletes or their legal representatives have to present verified circumstances of contamination and the source of contamination has to be identified; and 2. there have to be verified claims by the athlete about the fact that the intake of the prohibited substance was not known, i.e. that the violation was not intentional. This corresponds to the suggested shift terminology from "contaminated product" to "unpredictable source of a prohibited substance". In the recent years, several top athletes challenged their ADRV with a low urine letrozole concentration and requested a hair test. In three cases, letrozole concentration in segmented hair, particularly in the segment corresponding to the urine AAF was significantly lower than 1 pg/mg, which is the limit of quantification of the method. Considering that a ¼ of a 2.5 mg therapeutic dose of letrozole produces a hair concentration of approximately 30 pg/mg, it is easy to establish that the dose that entered in the body of these athletes was incidental. Nevertheless, all three athletes were sentenced a 2-years ban as the source of contamination was not identified. In that sense, the WADA dogma contradicts scientific evidence, and from a forensic perspective, this appears difficult to understand.