Aberrant radioimmunoassay results for cannabinoids in urine

False negative and false positive radioimmunoassay results for cannabinoids in urine are reported. Such results occur infrequently and appear to be due to non-specific binding of the radiolabel by interfering substances, possibly proteins, in the specimens. False negatives arise when these substances and the non-specifically bound radiolabel are precipitated by polyethylene glycol in the separation stage of the assay, increasing the radioactivity in the bound fraction and thus lowering the assay result. False positives occur when the interfering substances and non-specifically bound radiolabel remain in solution during the separation stage, increasing the radioactivity in the free fraction and giving an erroneously high result. False positives could also occur in assays involving solid-phase antibodies or second-antibody separations. Methanol treatment of the samples prior to analysis precipitates the interfering substances and eliminates the aberrant effect. The interfering substances have not been identified through common serum proteins and semen have been excluded. No common factor in the circumstances of the cases that can account for the results has been identified.