Evaluation of Minimum Reporting Limits to Determine In-Competition Use of Stimulants

The applicability of urinary minimum reporting limits (MRLs) to determine in-competition use of prohibited substances is an evolving topic. Most stimulants are subject to a universal MRL, despite the wide range of commercially available dosages for commonly used stimulants. Further, it is unknown whether the urinary MRL is reflective of a pharmacological dose ingested after the start of the in-competition period. To evaluate whether urinary MRLs can distinguish between in-competition and out-of-competition use, a controlled administration study was performed with three commonly used stimulants-amphetamine, methylphenidate, and modafinil at relatively low but therapeutically relevant dosages. Four to six volunteers were administered a particular drug once per day for five consecutive days. Urine, serum, dried blood spots (DBS), and oral fluid (OF) were collected during the active administration period and for 48 h after cessation of use. For all participants, urinary concentrations for all target analytes exceeded the MRL even 48 h after cessation of use. In serum and DBS, most volunteers showed detectable amounts at 48 h post use. Peak concentrations were variable between target compounds even with similar administered dosages. Further, there was a reproducible difference between serum and DBS concentrations. Interpretation of results from OF measurements was challenging due to the inability to normalize for hydration status and OF viscosity. Analyte concentrations decreased steadily over the washout period but did not correlate across matrices for all target analytes. The study reiterates the challenges associated with determining in-competition use by relying on urinary concentrations.