Be aware of (R)-methamphetamine: Negative immunoassay versus positive confirmation analysis.

The amphetamine-type stimulant methamphetamine exists in two enantiomeric forms, (S)-methamphetamine and (R)-methamphetamine, which are both psychoactive but with the (S)-enantiomer being more potent than the (R)-enantiomer. Illicit methamphetamine encountered in Europe is typically a racemic mixture of both enantiomers and enantiopure (S)-methamphetamine, respectively. However, herein we report two cases with proven enantiopure (R)-methamphetamine consumption with moreover both cases remaining undetected by immunoassay screening. Inconspicuous immunoassay findings can be traced back to a considerably higher sensitivity and concentration-dependent cross-reactivity of the applied drug of abuse assay for the (S)-enantiomer of methamphetamine compared with the (R)-enantiomer, and this limitation should be well known by users of immunoassay drug tests.