False-positive results in the radioimmunoassay detection of stilbene derivatives after administration of fluorinated corticosteroids to animals.

During routine inspections, indications were found that the therapeutic use of fluocortolone or dexamethasone can cause false-positive results in the detection of stilbene derivatives by radioimmunoassay (RIA) in samples of animal origin. After experimental application of the fluorinated corticosteroids to rabbits or calves, the assumed false-positive RIA results were confirmed. The kinetics after application of dexamethasone were concomitant with the elimination of radioactive [3H]dexamethasone. Furthermore, it could be demonstrated that [3H]dexamethasone was transformed in the faeces of rabbits and calves. The metabolites formed were not completely separated by the extraction procedure used for the RIA and seemed to be responsible for the false-positive results.