Analysis of forensic specimens for cannabinoids. I. Comparison of RIA and GC/MS analysis of blood

Data from five years of radioimmunoassay (RIA) screens and gas chromatographic/mass spectrometric (GC/MS) quantitation of blood, or blood and matched (i.e. concurrently collected) urine specimens for cannabinoids have been used for two distinct evaluations. The first part, which is the subject of this study, considers comparative results from RIA and GC/MS analysis of the specimens. RIA analysis (Abuscreen urine cannabinoids kit) of methanol extracts of blood with extracted blood calibrators (122 specimens) was compared to analysis of nonextracted blood with urine calibrators (108 specimens). RIA of methanol extracts of blood was a reliable assay (limit of detection = 17 ng/mL; interassay coefficients of variation of 2-10%). RIA of nonextracted GC/MS negative blood gave significant displacement of labeled antigen, which precluded accurate quantitation using the urine-based calibrators. Comparison of RIA cannabinoid concentrations to 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid (COOH-THC) concentrations measured by GC/MS yielded correlation coefficients of 0.68 and 0.23 for the methanol extract and nonextract procedures, respectively. Comparison of the qualitative accuracy of the two techniques, however, resulted in similar discrimination of GC/MS positive and negative specimens when the 20-ng/mL extracted blood calibrator was used as the cutoff for methanol-extracted blood specimens, and the 100-ng/mL urine calibrator was used as the cutoff for nonextracted blood specimens. Although qualitatively similar, the methanol extract procedure resulted in quantitatively superior analysis.