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Immunoassay (IA) based serum toxicology test results have the advantage of being quickly obtained, easily automated, inexpensive, and largely independent of operator skill. This type testing has the disadvantages of [...]

Background: Interpretation of TCA urine immunoassays is confounded by frequent false positive results, often leading to problematic interpretation. Although clinical evaluation of TCA toxicity is most important, increased specificity of these frequently ordered tests would be helpful. We compare two TCA immunoassays with cutoffs of 1000ng/mL (SureStep by Applied Biotech, Inc., and Triage Drags of Abuse Panel by Biosite Diagnostics) with our existing immunoassay (EMIT by Syva, cutoff 300 ng/mL nortriptyline) to determine if the increased threshold is sufficient to decrease false positive results. We also investigate the ability of these assays to identify patients with clinical TCA toxicity. Methods: Adult patients with positive TCA urine assays by EMIT who had stored urine were included in the study. A retrospective chart review was performed, abstracting data for toxicants and clinical evidence of TCA toxicity (hypotension, tachycardia, altered mental status, anticholinergic toxidrome). Available EKGs were reviewed for evidence of abnormal terminal R wave in lead a VR and QRS duration. Results: 66 patients met inclusion criteria. 3 patients demonstrated clinical evidence of TCA toxicity. Both higher cutoff assays detected these 3. Positive predictive values were 4.5%, 12% and 13% for the EMIT, Biosite and Applied Biotech assays respectively. Specificity (95% confidence intervals) was 65% (52-78%) and 68% (56-80%) for the SureStep and Triage assays, respectively. Conclusions: Utilizing a TCA immunoassay with a cutoff of 1000ng/mL maintains sensitivity for TCA overdose, while eliminating more than 1/2 of the false positive results in this patient population. Further study may support the use of low and high threshold testing combined with clinical presentation to suggest the presence of TCA immunoassay cross-reactants., Gee A, McKay CA, Wu, AHB. Hartford Hospital/ UCONN School of Medicine, Hartford, [...]

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, [...]