Four reagent formulations (three provided by a manufacturer; one prepared in-house by mixing equal volumes of two commercial reagents) are used for the assay of phencyclidine (PCP) in urine samples. Performance characteristics evaluated included assay precision and sensitivity at and near the assay cutoff concentration. Data resulting from the reagent prepared in-house are better than those using then commercially available formulations, and are comparable with those obtained using the recently available new commercial formulation.
Apparent analyte concentration (equivalent of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid [9-THC-COOH]) obtained by radioimmunoassay (RIA) for cannabinoids using reagents manufactured at four different periods (from the same manufacturer) and specific 9-THC-COOH concentration as determined by GC/MS are examined for the significance of their correlation. The resulting regression equations are then used to estimate the apparent RIA analyte concentrations of reagents manufactured at different time periods that are equivalent to a specific 9-THC-COOH concentration. Correlation coefficients of the regression analysis improve from approximately 0.4 to 0.7 in parallel with the increasing reagent specificity. The apparent RIA analyte concentrations that correspond to 15 ng/mL 9-THC-COOH decrease from about 110 to 50 ng/mL again in parallel with the increasing reagent specificity. These findings empirically demonstrate that reagent specificity is the determining factor of the resulting apparent RIA analyte concentration when testing samples that contain 9-THC-COOH and other metabolites (derived from marijuana exposure). Thus, if the 9-THC-COOH concentration as determined by GC/MS is of primary concern, the initial test cutoff concentration should be adjusted in accordance with the specificity of the reagent used.
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