Detection of phenothiazine derivatives in urine. An evaluation of 3 methods.

Findings of the present study agree with published results that showed an incidence of approximately 25% “false negative” results with the Forrest FPN test for phenothiazine derivatives in urine. A recently published modification purportedly decreased these “false negatives” to less than 5%. However, our control group of negative urine specimens omitted in Campbell’s study, yielded 100% negative results with the FPN test and 81% positive results with the modified test. Phenistix (Ames) was evaluated for elimination of false negative readings but was found not to have any advantage for this purpose. Data are also presented indicating differing sensitivities of the Forrest FPN test to various phenothiazine derivatives.