Quetiapine Cross Reactivity With Urine Methadone Immunoassays

Using the Cobas Integra Methadone II testkit by Roche (1) , we found methadone-positive results that were unexpected, given the patients’ medical histories. The Cobas Integra Methadone II testkit contains an in vitro diagnosticum for semiquantitative and qualitative methadone detection in human urine, with a threshold of 300 ng/ml for methadone positivity. In a second step, we tested blood samples taken on the day after the last quetiapine intake to cross check with mass spectrometry (LC/MS/MS). Chromatographic peaks were confirmed and quantified by tandem mass spectrometry on a Micromass Quattro Ultima, using the mass transitions 384 >253 amu for quetiapine and 310 >265 amu for methadone as described previously (2) . Samples (190 microliter) were spiked with 10 microliter d3-methadone internal standard (1 microgram/ml in methanol, mass transition 268 >265) and extracted with 500 microliter –20°C cold acetonitrile. Hundred microliter supernatant were directly injected into the HPLC/MS/MS instrumentation. Chromatographic separation of the analytes was performed on a reverse-phase C18 column (Waters Acquity, 1.7 micrometer, 2.1×50 mm [www.waters.com]), with a mobile phase gradient from 50% acetonitrile and 50% 5 mM formic acid in water (buffer A) to 100% acetontrile (buffer B) over 2.2 over 1.8 minutes at a flow rate of 0.25 ml/minute. The level of quantification was 1 ng/ml for quetiapine and 2 ng/ml for methadone. Quetiapine metabolites were not quantified (determined). In contrast to the rechecked urinalyses (again methadone-positive), the quantitative assay, which was performed at the same time, did not reveal methadone positivity in the patients’ plasma.