Mechanismof False-Negative Urine CannabinoidImmunoassayScreens by VisineTMEyedrops

To drug-free urine specimens, we added the following drugs of abuse to give concentrations twice the cutoff value for positive test results: 11-nor-9-carboxy-delta-9-tetrahydrocannabinol (9-carboxy-THC), oxazepam, secobarbital, morphine, benzoylecgonine, amphetamine, or phencyclidine (PCP). Visine was then added. Although measured concentrations of several drugs were decreased in the presence of Visine, false-negative results were obtained only for 9carboxy-THC for the EMrr-d.a.u.T and TDxT” urine cannabinoid assays. Visine also decreased 9-carboxy-THC as measured by the Abuscreen assay. At low concentrations of Visine, false-negative cannabinoid results were attnbutable to the benzalkonium chloride ingredient of Visine. The added Visine was not detectable by routine urine analysis and had no effect on the activity of the glucose-6-phosphate dehydrogenase-drug conjugate used in the EMrr-d.a.u. assays. Moreover, analysis by gas chromatography/mass spectrometry showed no chemical modification or loss of 9-carboxyTHC in the Visine-adulterated urine specimens. However, Visine did increase the adhesion of 9-carboxy-THC to the borosilicate glass specimen containers. Results of ultrafiltration studies with Visine suggest that 9-carboxy-THC partitions between the aqueous solvent and the hydrophobic interior of benzalkonium chloride micelles, thereby reducing the availability of 9-carboxy-THC in antibody-based assays.