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Clinical Chemistry 12: 379-384, 1966;
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Clinical Chemistry, Vol 12, 379-384, Copyright © 1966 by the American Association for Clinical Chemistry

Elimination of False Negative Results with the FPN Forrest Test for Phenothiazine Derivatives in Urine

Irene S. Forrest 1, Fred M. Forrest 1, and Saul L. Kanter 1

1 Biochemical Research Laboratory, Veterans Administration Hospital, Palo Alto, Calif. 94304.

With the FPN Forrest test for urinary phenothiazine derivatives, omission of drug intake was found to be the most frequent cause of negative color reactions; accordingly, these negative reactions are true negatives. Excessive dilution of urine specimens occasionally yielded "apparent false negatives" by producing traces of drug metabolites, which cannot be detected without modification of the test. When measures to eliminate these conditions are taken, the previously specified reading time (10 sec. after mixing urine and reagent) is reliable. Unspecific color development due to reaction between endogenous urinary constituents and reagent appeared with a delay of 30 sec. or more, and hence recent recommendations of delaying reading times for the test were found to be unsuitable and productive of false positives.

Submitted on November 11, 1965
Accepted on February 28, 1966






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Copyright © 1966 by the American Association for Clinical Chemistry.