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To the Editor: We report the case of an unexpected positive result on routine urine drug screening for phencyclidine in a patient who assured that she had never used such substance. Because similar results have recently been reported (1, 2) and in order to avoid inappropriate suspicion by medical caregivers, we performed a blood test that confirmed the absence of phencyclidine and the interpretation of the former urine test as a false positive result.
“Ms. A,” a 48-year-old patient with a 31-year history of recurrent depressive disorder, was admitted to the psychiatric hospitalization unit for an acute exacerbation of her mental disorder. She was receiving treatment with venlafaxine 225 mg/day, lamotrigine 100 mg/day, and lormetazepam 2 mg/day. During the prior month she experienced worsening of the depressive symptoms, concurrent with mood-incongruent psychotic symptoms that were not of a clearly depressive nature (thought broadcasting, paranoid and mystic delusions, delusions of control and influence, and grossly disorganized behavior). A routine drug screening by the qualitative immunoassay INSTANT-VIEW Multi-Drug Screen Urine Test (Alpha Laboratories) upon admission was positive for phencyclidine and negative for other drugs. The package insert for the test (3) shows cross-reactivity to methylphenidate, pheniramine, and tenocyclidine, but not for venlafaxine. Given the importance of knowing the origin of the psychotic symptoms, because she would meet DSM-IV criteria for schizoaffective disorder if a toxic etiology was excluded, we performed another urine test 2 weeks later that was also positive for phencyclidine. We then extracted, on the same day, blood and urine samples to check this result by means of a gas chromatography-mass spectrometry analysis. This technique revealed the absence of phencyclidine in blood and in urine (and confirmed that the two prior positive urine tests for phencyclidine were false positives) and confirmed the presence of venlafaxine in blood and of venlafaxine and norvenlafaxine in urine. The urine analysis by gas chromatography-mass spectrometry revealed only prescribed drugs and caffeine. The patient was hospitalized and compliance of the other prescribed drugs was adequate. The diagnosis was changed to schizoaffective disorder, and treatment with amisulpride 100 mg/day yielded a significant improvement of the psychotic symptoms.
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