Comparison of fluorescence polarization immunoassay, enzyme immunoassay, and thin-layer chromatography for urine cannabinoid screening. Effects of analyte adsorption and vigorous mixing of specimen on detectability.

Four commercial assays for the screening of cannabinoids in urine were compared. Urine specimens from 93 selected subjects were run by fluorescence polarization immunoassay on the Abbott TDx; by enzyme multiplied immunoassay with two Syva EMIT assays; and by thin-layer chromatography with the TOXI-LAB system (Marion Laboratories). The TDx cannabinoid threshold can be set anywhere from 25 to 150 micrograms per L. Twenty-five micrograms per L was chosen for this study. The thresholds for EMIT are fixed at 20 micrograms per L for one assay and 100 micrograms per L for the other. The detection limit for TOXI-LAB, according to the manufacturer, can be anywhere from 5 to 50 micrograms/L, depending on the specimen. Urines, positive by at least one method, were further analyzed by gas chromatography with mass spectrometry (GC/MS), The detection limit for the GC/MS method was 10 micrograms per L. The results showed a few false negatives and unconfirmable positives; in general, correlation was considered acceptable. Dose-response curves comparing TDx and EMIT gave paralell results, with comparable cross-reactivity for the major metabolite, 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid (delta-9-THC-COOH). A dose-response study of TOXI-LAB using delta-9-THC-COOH also gave acceptable results. Adsorption to glass was investigated using spiked urine; a 27 percent reduction in concentration was caused by this phenomenon. Foaming of spiked urine caused by vigorous mixing resulted in a reversible 89 percent apparent reduction in concentration.