Knowing the minimal detectable dose can facilitate the interpretation of a hair test result: Case example with chlortalidone.

In case of an adverse analytical finding, a low (estimate) urine concentration can be the consequence of 2 very different situations: it can be the tail end of a drug voluntarily consumed to enhance athletic performance, even by microdosing (which is not effective for all drugs), or it can be the result of a contamination, irrespective of its source. For numerous doping agents, a hair test can allow discriminating doping from contamination based on the measured concentration or even the absence of the target drug. Given hair produces incremental concentrations, its analysis offers the possibility of establishing a pattern of drug use and thus, verifying self-reported histories of exposure. In order to provide a retrospective calendar of drug use, segmental analysis of the hair strand can be performed. In doping, the usual practice is to test the substance in short segments, such as 1 cm to avoid drug dilution when using larger segments. During the last months, seven athletes have returned an adverse analytical finding for the diuretic chlortalidone, with reported urine concentrations in the range 20 to 50 ng/mL. All these athletes submitted, via their legal team, their hair for establishing a pattern of exposure. Results were always consistent with incidental contamination (hair concentration lower than 5 pg/mg), although the source of contamination was never identified. The interpretation of the findings was established in the light of the limited literature, including hair tests after microdosing and therapeutic use.