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).

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.

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.