To the Editor:

As a result of work first carried out in this laboratory in 1989, the Syva Co. (Palo Alto, CA) warns in its EMiT” product insert and its EMIT Drug Abuse Urine Assay Cross-Reactivity List that positive results may be ob-tained with the EMiT d.a.u. monoclonal amphetamine assay in patients taking chlorpromazine. This interference is not seen with the EMIT d.a.u. polyclonal amphetamine assay. These observa-tions were confirmed recently (1). Urine samples from patients being prescribed other antipsychotic drugs were tested for amphetamine by both mono- and polyclonal amphetamine EMiT d.a.u. assays. The findings are shown in Table 1, as are the normal doses of the drugs studied. All the urine samples were examined by our amphet-amine confirmatory method, i.e., gas chromatography on an Apiezon/KOH column, and found to be negative. As can be seen from Table 1, there were no thise-positive results with the polyclonal assay. The lack of cross-reac-tivity of thioridazine in the monoclonal assay confirms information supplied by Syva Co. in the aforementioned list. All patients taking chiorpromazine and fluspirilene were positive in the mono-clonal assay. The variable results for pipothiazine treatment may be related to the uncertainty of the timing of the urine sample relative to the once per month depot injection of the drug. Clearly, results obtained with the Syva EMIT monoclonal amphetamine assay shouldbe interpreted with care, especially if the patient is being pre-scribed chlorpromazine, pipothiazine, or fluspirilene