False-negative results for urinary phenothiazines and imipramine in Forrest's qualitative assays
James, G.P.; Djang, M.H.; Hamilton, H.H.
Clinical Chemistry 26(2): 345-347
1980
ISSN/ISBN: 0009-9147 PMID: 6243519 Document Number: 157701
When a series of patients' urine samples supplemented in vitro with chlorpromazine or imipramine was assayed with the Forrest qualitative assays, an occasional false-negative result occurred due to interference by ascorbic acid. It interferes with the reagent, not with the analytes, in both assays. This interference was eliminated with the phenothiazine test by using an anion-exchange resin. Eliminating the interference with the assay for imipramine was more difficult; false-negative results can be obtained even after ion-exchange chromatography if the imipramine concentration is < 50 mg/l.