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Try out PMC Labs and tell us what you think. Learn More. Its short-term effects are seen for approximately 1 h after ingestion and may include hallucinations, disinhibition, euphoria and agitation. Long-term use can lead to symptoms resembling psychotic disorders such as schizophrenia. Its detection time in urine is approximately 8 days. Patient A was a year-old male with paranoid schizophrenia, admitted to an acute psychiatric ward under Section 2 of the Mental Health Act because of deterioration in mental state following medication nonadherence and a history of illicit drug use. He was transferred to the rehabilitation centre under Section 3 of the Act 5 months later, exhibiting mainly negative symptoms of schizophrenia.
He was receiving treatment with venlafaxine mg twice daily, lithium carbonate mg once daily and clozapine mg in the evening; he also had lactulose 10 ml twice daily.
A urine drug screen was performed after staff found cannabis in his room. The test was repeated and were positive for PCP only. Patient B was a year-old male with paranoid schizophrenia admitted under Section 2 of the Mental Health Act after being arrested for wielding knives in public. He was receiving treatment with risperdal consta 50 mg IM twice weekly, venlafaxine 75 mg twice daily, clonazepam 0.
A urine drug screen was performed since he had become increasingly guarded and irritable, despite good adherence to medication. The result was positive for PCP and benzodiazepines. The benzodiazepines could be explained by clonazepam but the patient again denied taking any PCP. The same were obtained when the test was repeated.
Given that both patients denied taking PCP our suspicion was aroused. None of the other patients on the unit who had urine drug screens tested positive for PCP. Venlafaxine was the only medication taken by both patient A and B. A review of the literature revealed several case reports of false positive urine immunoassay for PCP in patients taking venlafaxine of various doses. In one case series, three patients in an emergency department in Danbury Hospital, Connecticut, USA, were found to have false positive urine assay for PCP due to venlafaxine. This effect is thought to be due to cross-reactivity between venlafaxine and the active metabolite O-desmethylvenlafaxine with the PCP assay reagent, although they are not structurally related.
Based on this information, the urine assay showing PCP for patients A and B were determined to be false positives due to cross-reactivity with venlafaxine.
For patient B, we were able to exclude illicit drug use as a cause for his altered mental state. Increased awareness of the cross-reactivity between PCP and venlafaxine is important for all healthcare professionals to avoid inappropriate suspicion of illicit drug use.
National Center for Biotechnology InformationU. BJPsych Bull. Gabriella L. Landy and Mukesh Kripalani.
Author information Copyright and information Disclaimer. Landy is a core trainee CT3 psychiatry, : ku. Copyright Royal College of Psychiatrists. This article has been cited by other articles in PMC. References 1. Urine drug screening: practical guide for clinicians. Mayo Clin Proc ; 83 : 66— False-positive phencyclidine immunoassay caused by venlafaxine and O-desmethylvenlafaxine. Clin Chem ; 48 : —7. Venlafaxine usage resulted in a false positive immunoassay for phencyclidine. Massive venlafaxine overdose resulted in a false positive Abott AxSYM urine immunoassay for phencyclidine.
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False positive phencyclidine result on urine drug testing: a little known cause