Possible Masking Agent: Zinc Found to Produce False Negatives in ELISA [Enzyme-Linked Immunosorbent Assay] Tests

"We conclude that zinc ion (Zn2+) is a potential adulterant in urine samples tested for drugs in routine workplace drug screening under the NIDA-5 panel using ELISA. Its effect in causing potential false-negative results in drug testing is robust and reproducible. This effect appears independent of the mode by which zinc is made available in urine. Although the exact mechanism by which zinc interacts with different components of the ELISA assay is unknown, the enhanced ELISA signal increases in a dose-dependent fashion. As a consequence, it is our conjecture that zinc ion increases the binding of drug conjugate to which the active horseradish peroxidase enzyme is attached, thereby increasing the final output signal.
"Although some urine samples containing added zinc sulfate may show signs of abnormal levels of chromate ions when tested using Adultacheck10 urine test strips, urine samples containing zinc by ingesting zinc supplements do not show any sign of adulteration whatsoever. In addition, such samples do not exhibit turbidity as observed with urine samples to which zinc sulfate has been directly added. Thus, we are aware of no suitable test to determine zinc adulteration in urine and conclude that zinc supplements are effective at subverting routine drug testing and undetectable by standard means."

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Abhishek Venkatratnam and Nathan H. Lents. Zinc Reduces the Detection of Cocaine, Methamphetamine, and THC by ELISA Urine Testing. Journal of Analytical Toxicology (July/August 2011) 35(6):333-340 doi:10.1093/anatox/35.6.333.
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