Methods for circumventing positive drug tests continue to evolve and are often spread through internet websites reporting on the proposed effectiveness of various adulteration methods. Recent claims of the use of zinc added directly to urine or ingested prior to urine collection have prompted investigation into the vulnerability of ELISA-based testing, providing interesting but inconclusive results. We investigated the potential interference of zinc used as a direct adulterant and after zinc self-administration for enzyme multiplied immunoassay technique (EMIT)-based drug abuse testing in urine. Negative urine samples and samples collected before and after zinc self-administration were fortified with d-methamphetamine, benzoylecgonine, morphine and 11-nor-9-carboxy-tetrahydrocannabinol prior to analysis by the EMIT. Our data indicate that zinc added directly to urine in concentrations 5,000 times higher than a typical random urine total zinc concentration is capable of producing false-negative results; however, self-administration of oral zinc was unable to generate random urine total zinc concentrations in the required range. Further, no evidence of a secondary interfering substance was observed as a result of oral zinc self-administration. Our results indicate that the total zinc concentrations required to directly interfere with EMIT-based testing are easily distinguishable from routine random urine total zinc concentrations, and that alleged oral ingestion of zinc does not produce total zinc concentrations capable of direct interference.
