False-positive RIA for methamphetamine following ingestion of an Ephedra-derived herbal product.

Ephedra species contain a mixture of naturally occurring alkaloids, including ephedrine and pseudoephedrine, norephedrine (phenylpropanolamine), and norpseudoephedrine (cathine or khat) (1,2). None of these species is metabolized to any significant degree by humans (3), and in particular, they are not metabolized to amphetamine and/or rnethamphetamine. These naturally occurring compounds do, however, share common structural features with synthetic phenylisopropylamines, and their presence can result in false-positive immunoassay results for methamphetamine (4-6). All currently marketed point-of-care testing devices use imrnunoassays to detect rnethamphetamine. Presumably, these devices are subjected to the same sorts of cross-reactions that have already been reported with laboratory-based analyzer systems. Indeed, for clinical use, as opposed to workplace testing, the greater the degree of cross-reactivity, the greater the clinical utility of the testing device. Anecdotal reports suggest that false-positive screening immunoassay tests, secondary to ephedra ingestion, are increasingly common. This issue has never been systematically studied. The intent of our investigation was to measure the concentration of ephedrine in the urine of users of a popular dietary supplement (Metabolife 356) and to determine whether the urine concentrations observed after ingestion of the recommended doses of Metabolife 356 were sufficient to cause a positive screening test for rnethamphetamine with a commercially available radioimrnunoassay (RIA) screening technique (Diagnostic Products Corp.). Five healthy subjects interested in weight loss and planning on taking diet supplements participated in the study. All volunteers gave written informed consent prior to participating in the study. Each subject took Metabolife 356 for five days. One subject took one caplet in the early rooming and a second caplet at noon. The other subjects took two caplets in the early rnoming and two Table I. Participants of Study, RIA Screening, and more caplets at noon. According to the label, each caplet Ephedrine Quantitation contained 12 mg of"ephedra group alkaloids." Independent Meth.