Quetiapine (Seroquel[TM] by AstraZeneca) interference with tricyclic antidepressant immunoassays

Background: Urine immunoassays for tricyclic antidepressants (TCAs) suffer from many cross-reactivities which vary by concentration of the interfering substance and manufacturer. A recent report (Sloan et al., Am J Psych 2000) indicated that the new atypical antipsychotic, quetiapine fumurate, gave a positive TCA result in an immunoassay marketed by Diagnositc Reagents, Inc. This assay uses a calibrator cutoff of 300ng/ml for nortriptyline. TCA immunoassays rely on polyclonal sheep antibodies. We have seen two patients with quetiapine overdoses who had positive EMIT TCA assays; the presence of quetiapine and absence of typical TCAs was confirmed by high performance liquid chromatography (Remedi, BioRad Labs, Hercules, CA). An in vitro study of quetiapine cross-reactivity with three TCA immunoassays was conducted. Methods: A 25 mg tablet of quetiapine was dissolved in 250 ml of water and serial dilutions tested by the EMIT system (Syva Co.; cutoff of 300 ng/ml), and two point-of-care (POC) devices, Triage Drugs of Abuse Panel (Biosite Diagnostics; cutoff 1000ng/ml), and SureStep (Applied Biotech; cutoff 1000ng/ml). Results: The EMIT assay was positive at 4.5[micro]g/ml (cross reactivity 6.6%), comparable to the previous report of 4.3% using the Diagnostics Reagents, Inc. assay. There was no cross-reactivity with either of the two POC assays when tested up to 1001ag/mi (cross reactivity
McKay CA, Wu, AHB. Hartford Hospital/UCONN School of Medicine, Hartford, [...]