Comparison of 3H- and 125I-Radioimmunoassay and Gas Chromatography/Mass Spectrometry for the Determination of 9-Tetrahydrocannabinol and Cannabinoids in Blood and Serum

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Abstract
Blood and serum specimens from 100 subjects in a driving study were tested for the presence of Δ9-tetrahydrocannabinol (THC) by 3H- and 125I-labeled RIA and GC/MS. The specimens were also analyzed for 11-nor-9-carboxy-Δ9-tetrahydrocannabinol by the two RIA methods. Blind quality control specimens containing THC were included with each batch of subject specimens and sent coded to three participating laboratories. The methods were assessed for accuracy, reproducibility, detectability, specificity, and the results correlated. It was found that serum was a better matrix than blood for determination of cannabinoids. The three methods gave parallel but significantly different quantitative results, apparently due to calibration problems. However, each technique was capable of measuring THC concentrations up to three hours after usage.