Quetiapine cross-reactivity with plasma tricyclic antidepressant immunoassays.
BACKGROUND
Toxicology screens obtained on patients who have overdosed on drugs frequently include tricyclic antidepressants (TCAs) as part of the evaluation. Quetiapine is an antipsychotic agent with structural similarity to the TCAs.


OBJECTIVE
To determine whether quetiapine may cross-react with plasma TCA immunoassays in vitro using commonly available autoanalyzers.


METHODS
Quetiapine stock solution was added to 9 separate samples of pooled drug-free human plasma to produce concentrations ranging from 1 to 640 ng/mL that were verified by gas chromatography. No quetiapine metabolites were present. Each spiked plasma sample was tested in a blinded fashion using the Abbott Tricyclic Antidepressant TDx Assay on the TDxFLx autoanalyzer in 2 separate laboratories, the Syva Emit tox Serum Tricyclic Antidepressant Assay on the AU400 autoanalyzer and the S TAD Serum Tricyclic Antidepressant Screen on the ACA-Star 300 autoanalyzer. The TDx assay is quantitative, while Emit and S TAD are qualitative screening assays with a threshold of 300 ng/mL for TCA positivity. The outcome of interest was a positive TCA result.


RESULTS
The quantitative assay showed concentration-related TCA cross-reactivity beginning at quetiapine concentrations of 5 ng/mL. The 640-ng/mL spiked sample produced TCA results of 379 and 385 ng/mL in labs 1 and 2, respectively. The qualitative assays were screened as TCA positive at quetiapine concentrations of 160 and 320 ng/mL for the S TAD and Emit assays, respectively.


CONCLUSIONS
Quetiapine cross-reacts with quantitative and qualitative plasma TCA immunoassays in a concentration-dependent fashion. Therapeutic use or overdose of quetiapine may result in a false-positive TCA immunoassay result.
Caravati
E Martin
EM
Utah Poison Control Center, Salt Lake City, UT 84108-1208, USA. martin.caravati@hsc.utah.edu
Juenke
Joetta M
JM
Crouch
Barbara I
BI
Anderson
Kathleen T
KT
D000929:Antidepressive Agents, Tricyclic / Q000097:blood / Q000276:immunology*; D014150:Antipsychotic Agents / Q000097:blood / Q000276:immunology*; D003429:Cross Reactions; D003987:Dibenzothiazepines / Q000097:blood / Q000276:immunology*; D017248:Enzyme Multiplied Immunoassay Technique; D005189:False Positive Reactions; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D007118:Immunoassay; D007202:Indicators and Reagents; D000069348:Quetiapine Fumarate
D000929:Antidepressive Agents, Tricyclic; D014150:Antipsychotic Agents; D003987:Dibenzothiazepines; D007202:Indicators and Reagents; D000069348:Quetiapine Fumarate