Case report of a phencyclidine false positive due to lamotrigine use with confirmatory testing.

BACKGROUND

Urine toxicology screens are useful in diagnosing patients who present with acute psychosis with a history of substance abuse. Being aware of potential false positive reactants is paramount in diagnostic accuracy. Currently, lamotrigine is not listed among common cross-reactants with phencyclidine (PCP).

CASE REPORT

A 49 year old male (98 kg) was brought to the ED by a family member for worsening confusion and agitation. He had a history of Bipolar I, PTSD, schizoaffective disorder, hypertension, and cannabis/opioid abuse. His home medications included paliperidone, duloxetine, lamotrigine, tizanidine, hydroxyzine, and lisinopril. Upon examination, he denied intentional overdose or illicit substances, but largely mumbled incoherently. Blood pressure was 140/90 mmHg, pulse 113. A urine toxicology screen was positive for PCP and cannabinoids. Other labs were unremarkable, co-ingestants negative. By day three, his mental status vacillated but he largely gave unintelligible responses. Given the short half-life of PCP, false positives were investigated. A confirmatory blood test (collected upon admission) for PCP was found to be negative, and a serum lamotrigine level was confirmed to be positive (1.5μg/ml). Once more lucid, the patient admitted to taking large quantities of mirtazapine and tizanidine, making serotonin syndrome the more likely diagnosis.

DISCUSSION

There is little in the medical literature describing cross-reactivity of lamotrigine and PCP on urine drug screens. This can be especially difficult to deduce in a known drug abuser who presents psychotic and non-contributory in their work up.