Published: Journal of Analytical Toxicology, Volume 23, Number 2, March/April 1999, pp.125-126.
| Technical Note |
Oxaprozin Interference with Urinary Benzodiazepine Immunoassays
and Noninterference with Receptor Assay
Takashi Nishikawa, Yoshito Kamijo, Hideki Ohtani, and Albert D. Fraser
Immunoassay methods are widely used to screen for urinary benzodiazepines despite several reports of false-positive and false-negative results (1). False-negative results are produced for certain benzodiazepines that have no or low cross-reactivity with the immunoassay used. False-positive results were produced by four commercial immunoassays (Behring EMIT d.a.u., Behring EMIT II, Abbott FPIA, Microgenics/BMC CEDIA, and Biosite Triage) in subjects who had taken oxaprozin (Daypro®) (2–4). Oxaprozin, a nonsteroidal anti-inflammatory drug, is widely prescribed in North America. Oxaprozin and/or its metabolite in urine may directly interfere with these immunoassays because of their cross-reactivities with oxaprozin (4).
Benzodiazepines in body fluids can also be determined by a receptor assay that is based on competitive binding using a benzodiazepine-specific receptor in mammalian brains (5,6). Previous studies of ours and others show that the diagnostic performance of the receptor assay was equal or superior to that of some commercial immunoassays for the urinary screening (7–9). We investigated whether the receptor assay would be interfered with by oxaprozin intake.
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