False-Positive Carbamazepine Results by Gas Chromatography-Mass Spectrometry and VITROS 5600 Following a Massive Oxcarbazepine Ingestion.

Abstract

A 23-month-old patient with a seizure disorder presented to the emergency department after a suspected unintentional ingestion of his seizure medication. He had significant central nervous system depression and seizure-like movements. Although at the time of presentation it was known that the patient ingested his antiseizure medication, the name of the medication was not known. Also, it was not clear if the patient coingested any other drug. Broad-spectrum urine drug screening by immunoassay and GC-MS that includes presumptive screening of >200 drugs and toxins was ordered. Additionally, volatile screening for ethanol, methanol, acetone, and isopropanol was ordered. Carbamazepine testing that is available on a stat basis on the Vitros 5600 Chemistry Analyzer was also requested. Urine drug screening by GC-MS (GC-7890, MS-5975; Agilent) showed the presence of carbamazepine and oxcarbazepine. Carbamazepine concentration measured by immunoassay on the Vitros 5600 Chemistry Analyzer (Ortho-Clinical Diagnostics) was 7.9 μg/mL (therapeutic range, 4–12 μg/mL). Unless there was a coingestion of another drug, the patient's symptoms of central nervous system depression and seizure-like movements were not consistent with the reported carbamazepine level. Because the presence of carbamazepine was reported on both GC-MS and immunoassay methods, it was thought that the patient ingested carbamazepine. Owing to the presence of oxcarbazepine on the GC-MS, it was thought that the …