Acta Clin Croat, Vol. 51, No. 2, 2012 269 Acta Clin Croat 2012; 51:269-272 Case Report FALSE-POSITIVE METHADONE URINE DRUG SCREEN IN A PATIENT TREATED WITH QUETIAPINE* Davor Lasić 1 , Boran Uglešić 1 , Marija Žuljan-Cvitanović 1 , Daniela Šupe-Domić 2 and Lovro Uglešić 1 1 University Department of Psychiatry, 2 Department of Medical Laboratory Diagnosis, Split University Hospital Center, Split, Croatia SUMMARY – We present a case of T.M. admitted to University Department of Psychiatry, Split University Hospital Center, in Croatia, because of the acute psychotic reaction (F23.9). Te patient’s urine tested positive for methadone without a history of methadone ingestion. Urine drug screen was performed with the COBAS Integra Methadone II test kit (kinetic interaction of micro- particles in solution /KIMS/ methodology) by Roche. Drugs that have been shown to cross-react with methadone feature a tricyclic structure with a sulfur and nitrogen atom in the middle ring, which is common for both quetiapine and methadone. Terefore, it is plausible that this structu- ral similarity between quetiapine and methadone could underlie the cross-reactivity on methadone drug screen. Besides quetiapine, a number of routinely prescribed medications have been associated with triggering false-positive urine drug screen results. Verifcation of the test results with a dife- rent screening test or additional analytical tests should be performed to avoid adverse consequences for the patients. Key words: Quetiapine; Methadone; False-positive urine drug screen Correspondence to: Davor Lasić, MD, University Department of Psychiatry, Split University Hospital Center, Spinčićeva 1, HR- 21000 Split, Croatia E-mail: dlasic@kbsplit.hr Received June 26, 2012, accepted July 15, 2012 Introduction Quetiapine is a dibenzothiazepine atypical antip- sychotic. It has been proposed that this drug’s antip- sychotic activity is mediated through a combination of dopamine type 2 (D 2 ) and serotonin type 2 (5-HT 2 ) antagonism. It is an antagonist at multiple neurotrans- mitter receptors in the brain: serotonin 5-HT 1A and 5-HT 2 , dopamine D 1 and D 2 , histamine H 1 , and adrenergic alpha 1 - and alpha 2 -receptors, but appears to have no appreciable afnity at cholinergic muscar- inic and benzodiazepine receptors. Norquetiapine, an active metabolite, difers from its parent molecule by exhibiting high afnity for muscarinic M1 receptors 1 . Methadone is a synthetic opioid, used medically as an analgesic and a maintenance anti-addictive for use in patients with opioid dependency. It was devel- oped in Germany in 1937. Although chemically un- like morphine or heroin, methadone acts on the same opioid receptors as these drugs, and thus has many of the same efects. Methadone is also used in manag- ing severe chronic pain, owing to its long duration of action, extremely powerful efects, and very low cost. Methadone is a partial µ-opioid agonist. Methadone also binds to the glutamatergic NMDA (N-methyl- D-aspartate) receptor, and thus acts as a receptor an- tagonist against glutamate 1 . Drug screening through urinalysis is a widely ac- cepted method for rapid detection of the potential drug abuse. Te most commonly used tests to screen urine for drugs of abuse are immunoassays, even though false-positive results for drugs of abuse have been reported with a number of these rapid-screening products 2 . Confrmation of presumptive positive urine * First poster prize in psychiatry at 52 nd International Neuropsy- chiatric Pula Congress 2012.