Carbamazepine interference with an immune assay for tricyclic antidepressants in plasma.
BACKGROUND
Drug toxicological screening is commonly used as a diagnostic tool in patients with suspected toxic ingestion. False positive results due to cross-reactive compounds in drug assays may lead to misdiagnosis and mismanagement, especially when child abuse is suspected.


METHODS
Two of our patients with history of ingestion of carbamazepine were tested positive on screening with the tricyclic antidepressant immunoassay. The immunoassay's known cross-reactivity for carbamazepine is reportedly as low as 0.3%. Plasma samples of our patients were initially considered positive for tricyclic antidepressants because the cross-reaction of carbamazepine gave tricyclic antidepressant concentrations as imipramine equivalent sufficiently above the assay cut-off point (20 ng/mL). Later, confirmatory urine testing of both patients using high-performance liquid chromatography was negative for tricyclic antidepressants.


CONCLUSIONS
This interference has significant clinical implications, and can be avoided on urine testing using a specific chromatographic assay such as high-performance liquid chromatography.
Chattergoon
D S
DS
Hospital for Sick Children, University of Toronto, Ontario, Canada.
Verjee
Z
Z
Anderson
M
M
Johnson
D
D
McGuigan
M A
MA
Koren
G
G
Ito
S
S
D000293:Adolescent; D000927:Anticonvulsants / Q000097:blood* / Q000506:poisoning / Q000627:therapeutic use; D000929:Antidepressive Agents, Tricyclic / Q000097:blood* / Q000652:urine; D002220:Carbamazepine / Q000097:blood* / Q000506:poisoning / Q000627:therapeutic use; D002675:Child, Preschool; D002851:Chromatography, High Pressure Liquid; D003429:Cross Reactions; D004347:Drug Interactions; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007118:Immunoassay; D008297:Male; D012640:Seizures / Q000188:drug therapy; D013406:Suicide, Attempted
D000927:Anticonvulsants; D000929:Antidepressive Agents, Tricyclic; D002220:Carbamazepine