A Curious Cause of Positive PCP Test Results: Dextromethorphan Intoxication

Tulisa LaRocca, MD
Volume 56 - Issue 6 - June 2016

A 24-year-old man presented to the emergency department with altered sensorium. He was awake, alert, and oriented to person and place only, with erratic, dissociative behavior, including hallucinations. He had been given naloxone in the field, which had resulted in mild improvement.

On physical examination, the patient was in hypertensive urgency, with a blood pressure of 209/118 mm Hg. His mood was euphoric and hyperalert, with inappropriate laughing. He was hallucinating. He was agitated and diaphoretic. His pupils were dilated and equally reactive, with vertical and horizontal nystagmus. Mucous membranes were dry. His Glasgow Coma Scale score was 14 of 15. No focal neurologic deficits were appreciated. Cardiac examination revealed only sinus tachycardia, with the beats per minute in the 120s.
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Results of laboratory studies showed hypokalemia (2.5 mEq/L potassium), acute renal insufficiency (creatinine clearance, 1.8 mg/dL; baseline, 1.0 mg/dL), and elevated creatine kinase (2400 U/L).

Computed tomography scan of the head revealed no acute findings. Electrocardiography revealed sinus tachycardia. Chest radiography revealed no acute pulmonary findings. Results of a urine toxicology screen were positive for phencyclidine (PCP) and marijuana.

He received a presumptive diagnosis of PCP intoxication and was managed with intravenous fluids, benzodiazepines, electrolyte replacement, antihypertensives, and soft restraints as needed.

Overnight, his condition improved. His electrolyte levels normalized, and his renal function returned to baseline. His mentation cleared, and when he was interviewed the next morning, he denied the use of PCP but admitting to taking approximately 10 pills of a Coricidin HBP product (containing dextromethorphan) the night before to “get high.”

isomer of codeine and shares a similar chemical structure to PCP

Discussion

Dextromethorphan is an over-the-counter (OTC) cough suppressant that is commonly used as a recreational drug among young adults. It is an isomer of codeine and shares a similar chemical structure to PCP (Figure).1 Its active metabolite is dextrorphan, an N-methyl-d-aspartate (NMDA) receptor antagonist, which in cases of intoxication leads to a unique combination of hallucinations, out-of-body sensations, dissociation, and inhibition of adrenergic reuptake. This in turn leads to tachycardia, hypertension, and diaphoresis.1

In addition to dextromethorphan, OTC cough formulations frequently contain other agents such as the antihistamine chlorpheniramine.  Coingestion of dextromethorphan and antihistamines leads to features of anticholinergic poisoning, such as tachycardia, hypertension, hyperthermia, flushing, warm and dry skin, and agitated delirium.2 Hyperthermia, agitation, seizures, and muscle rigidity may lead to rhabdomyolysis and renal failure.2 Serotonin syndrome can complicate the presentation.2

Approximately 1 million US young adults misuse OTC cough and cold medicines containing dextromethorphan each year.3 The brands Coricidin, Nyquil, and Robitussin account for almost 66% of such cases of misuse.3 Furthermore, young adults often simultaneously use marijuana with dextromethorphan-containing products.4

Testing for dextromethorphan ingestion is difficult, because rapid urine drug screens do not detect it; thus, the diagnosis usually is based on the patient’s history and clinical findings.5 Moreover, dextromethorphan can cause a false positive result for PCP on rapid urine drug screens,1,6 because dextromethorphan and PCP are both NMDA receptor antagonists, are dopamine agonists, and inhibit serotonin reuptake, and because the screening test for PCP uses an antibody with a structure similar to dextromethorphan.1,7

Tulisa LaRocca, MD, is an affiliated assistant professor of medicine at the University of Miami Miller School of Medicine, Palm Beach Regional Campus, in Atlantis, Florida.

References:

  1. Brahm NC, Yeager LL, Fox MD, Farmer KC, Palmer TA. Commonly prescribed medications and potential false-positive urine drug screens. Am J Health Syst Pharm. 2010;67(16):1344-1350.
  2. Dextromethorphan (DXM). Center for Substance Abuse Research (CESAR), at the University of Maryland at College Park. cesar.umd.edu/cesar/drugs/dxm.asp. Accessed May 3, 2016.
  3. Substance Abuse and Mental Health Services Administration, Office of Applied Studies. The NSDUH report: misuse of over-the-counter cough and cold medications among persons aged 12 to 25. http://www.medpagetoday.com/upload/2008/1/10/coughmedssamhsa-cough.pdf. January 10, 2008. Accessed May 3, 2016.
  4. Schier J, Díaz JE. Avoid unfavorable consequences: dextromethorphan can bring about a false-positive phencyclidine urine drug screen. J Emerg Med. 2000;18(3):379-381.
  5. Poklis JL, Guckert B, Wolf CE, Poklis A. Evaluation of a new phencyclidine enzyme immunoassay for the detection of phencyclidine in urine with confirmation by high-performance liquid chromatography-tandem mass spectrometry. J Anal Toxicol. 2011;35(7):481-486.
  6. Rengarajan A, Mullins ME. How often do false-positive phencyclidine urine screens occur with use of common medications? Clin Toxicol (Phila). 2013;51(6):493-496.
  7. Krasowski MD, Pizon AF, Siam MG, Giannoutsos S, Iyer M, Ekins S. Using molecular similarity to highlight the challenges of routine immunoassay-based drug of abuse/toxicology screening in emergency medicine. BMC Emerg Med. 2009;9:5.