(174) Methamphetamine Use: The Unlikely Culprit

Background/Significance: Methamphetamine use is prevalent and psychiatric complications, particularly psychoses are common presentations to emergency settings. Urine drug screens are frequently used as diagnostic tools for detecting methamphetamine-induced psychosis, however results can be misleading. The following case illustrates that point.

Methods: A 62 year old man with a history of depression, treated with selegiline, presented to the emergency room with new-onset psychosis. He endorsed auditory hallucinations and paranoia for the previous 10 days. Urine drug screen was positive for amphetamine, however the patient denied any illicit drug use. No other cause for the psychosis was identified at that visit and it was thought that the drug screen result was likely a false positive. The patient was subsequently discharged but returned to the emergency department four times over the next month complaining of psychotic symptoms. Urine drug screen continued to be positive for amphetamine on all of these visits. A complete evaluation including CBC, chemistries, RPR, HIV, TSH, ceruloplasmin, ANA and brain imaging were all unremarkable. Confirmatory test by GC-MS showed a mixture of amphetamine and methamphetamine leading to the patient receiving a diagnosis of methamphetamine-induced psychosis although he continued to deny any drug use. Ultimately, a literature search and toxicology consultation clarified the cause of the patient's psychosis.

Results: Literature search and toxicology consultation revealed that selegiline may cause a false positive result on urine drug screens using immunoassay technology. Selegiline is metabolized into N-desmethyl selegiline, R-amphetamine, R-methamphetamine and their conjugated p-hydroxy derivatives, thus causing a positive result on both the screening test and the confirmatory GC-MS test. Isomer levels help differentiate between a positive test from selegiline use and a positive test from use of methamphetamine. The R-isomer is found in prescription medications while the pure S-isomer is most frequently detected in samples from individuals with methamphetamine use disorders. Our patient's psychosis was most likely caused by selegiline rather than use of methamphetamine.

Discussion/Conclusions/Implications: This case illustrates the importance of considering false positives and false negatives when evaluating urine drug screening results. Confirmatory tests using GC-MS should be obtained in cases where a false positive is suspected. Many commonly prescribed medications including amantadine, bupropion, metformin, promethazine, pseudoephedrine, ranitidine and trazodone may lead to false positives on screening tests. In addition to selegiline, Vick's vapor rub may lead to a false positive on both screening and confirmatory tests.The diagnosis of methamphetamine-induced psychosis should not be made based on the results of the drug screen alone but in the context of the entire patient presentation. Proper care of this patient was delayed because of the suspicion of methamphetamine use.

References:
Maurer H, Kraemer T. Toxicological detection of selegiline and its metabolites in urine using Fluorescence Polarization Immunoassy and Gas Chromatography-Mass Spectrometry and differentiation by entantioselective GC-MS of intake of selegiline from abuse of methamphetamine or amphetamine. Archives of Toxicology; 66:675-78, 1992.

Moeller K, Kissack J, Atayee R, Lee K. Clinical interpretation of urine drug tests: what clinicians need to know about urine drug screens. Mayo Clinic Proc; 92; 774-96, 2017