False positive phencyclidine result on urine drug testing: a little known cause

Abstract

Presents the case reports of two patients of false positive results for Phencyclidine (PCP) on urine drug screening at a community mental health rehabilitation centre. Patient A was a 25-year-old male with paranoid schizophrenia, admitted to an acute psychiatric ward under Section 2 of the Mental Health Act 1983 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. Patient B was a 38-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 transferred to the rehabilitation centre under Section 3 of the Act 8 months later with ongoing psychotic symptoms including ‘electric shock sensations’ which he attributed to possible chemical warfare. Based on this information, the urine assay results showing PCP for patients A and B were determined to be false positives due to cross-reactivity with venlafaxine. Patient A’s leave was reinstated as it had been cancelled until drug testing was negative. 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. (PsycINFO Database Record (c) 2018 APA, all rights reserved)