Semiquantitation of cannabinoid immunoassays? A reexamination of the EMIT 20-ng/mL assay.

Abstract

Concerns regarding the ability to semiquantitate drugs-of-abuse urine immunoassay results, particularly THC-COOH, prompted us to reexamine EMIT results. The Syva EMIT d.a.u. cannabinoid 20-ng/mL assay was performed on the Cobas Bio centrifugal analyzer, and positive samples were confirmed by Toxi-Lab or GC/MS. Of 39 specimens tested, 17 were confirmed positive. However, four specimens were not accurately semiquantitated by EMIT. These four had very high levels (greater than 100 ng/mL) of the primary metabolite but yielded EMIT results between the low and medium calibrator (20-75 ng/mL). Linearly studies confirmed that the absorbance changes with the EMIT immunoassay plateau near the medium calibrator. In addition, we obtained false positive EMIT results due to sample carryover when samples containing 500 ng/mL (or greater) of THC-COOH preceded a negative specimen. Significant carryover was not observed when concentrations up to 1,300 ng/mL were used with the 100-ng/mL immunoassay, and this effect may be explained by the higher cutoff used. Because EMIT cannabinoid results on the Cobas Bio analyzer can be affected by carryover and a hook effect, they should not be reported even semiquantitatively.