The effect of thioridazine on the Automatic Clinical Analyzer serum tricyclic anti-depressant screen.

A patient who had ingested thioridazine and flurazepam was brought to the authors' emergency department. Initial laboratory evaluation included a positive result for a serum screening test for tricyclic anti-depressants performed with the DuPont Automatic Clinical Analyzer. This false positive test result caused considerable unnecessary treatment and expense for the patient. The authors have found that a serum thioridazine concentration of 125 ng/mL (within the usual therapeutic range for this drug) will produce a false positive automatic clinical analyzer serum tricyclic anti-depressant screen result. Because thioridazine is the most widely used phenothiazine and is prescribed more frequently than the most widely used tricyclic anti-depressant, it is important to recognize this cause of a false positive result.