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Sena SF, Kazimi S, Wu AH. 
“False-positive phencyclidine immunoassay results caused by venlafaxine and O-desmethylvenlafaxine”. 
Clin Chem. 2002;48(4):676-7.
We recently observed three occurrences of false-positive results for phencyclidine (PCP) on urine samples tested with the Syva® RapidTest d.a.u.® 9 Test Panel (Syva Company, subsidiary of Dade Behring Inc.). The RapidTest d.a.u. 9 device is a single-use, one-step, solid-phase immunochromatographic assay for the qualitative, discrete detection of several drugs/drug metabolites in human urine (1). The urine samples were collected from three different patients who were seen in our Emergency Department within 3–4 weeks after we had implemented the RapidTest device in our laboratory (Danbury Hospital). Because PCP is not a commonly used drug in our locale and the rare positive results have usually been confirmed as false positives, three positive PCP results in such a short time period immediately aroused suspicion.

Venlafaxine, designated (R/S)-1-[2-(dimethylamino)-1-(4-methoxyphenyl)ethyl] cyclohexanol, is a phenethylamine derivative that is chemically unrelated to tricyclic, tetracyclic, and other antidepressants. It is the first antidepressant in a new drug class referred to as the serotonin noradrenergic reuptake inhibitors (SNaRIs) (4). Aside from possessing phenyl and cyclohexyl groups, venlafaxine bears little structural similarity to phencyclidine [1-(1-phenylcyclohexyl)piperidine; see Fig. 1 ]. Given this structural dissimilarity, it is somewhat surprising that venlafaxine or any of its desmethyl metabolites would cross-react with the anti-PCP antibody used in the RapidTest device. However, other examples of unexpected interferences with immunoassays for drugs of abuse have been well documented in the literature, e.g., oxaprozin with the Emit assay for benzodiazepines (5) and efavirenz with the CEDIA test for cannabinoids (6)(7).
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