CARBAMAZEPINE INTERFERENCE WITH AN IMMUNE ASSAY FOR TRICYCLIC ANTIDEPRESSANTS IN PLASMA

Background: Drug toxicological screening is commonly used res a diagno stic tool in patients with suspected toxic ingestion. False positive r esults due to cross-reactive compounds in drug assays may lead to misd iagnosis and mismanagement, especially when child abuse is suspected. Case Report: Two of our patients with history of ingestion of carbamaz epine were tested positive on screening with the tricyclic antidepress ant immunoassay. The immunoassay's known cross-reactivity for carbamaz epine is reportedly as low as 0.3%. Plasma samples of our patients wer e initially considered positive for tricyclic antidepressants because the cross-reaction of carbamazepine gave tricyclic antidepressant conc entrations as imipramine equivalent sufficiently above the assay cut-o ff point (20 ng/mL). Later, confirmatory urine testing of both patient s using high-performance liquid chromatography was negative for tricyc lic antidepressants, Conclusion: This interference has significant cli nical implications, and can be avoided on mine testing using a specifi c chromatographic assay such as high-performance liquid chromatography .