Interference by Carbamazepine and Oxcarbazepine With Serum- and Urine-Screening Assays for Tricyclic Antidepressants

Abstract

OBJECTIVE. The purpose of this work was to evaluate the potential cross-reactivity of 2 antiepileptic medications containing 3-ringed structures, namely, carbamazepine and oxcarbazepine, with screening assays for tricyclic antidepressants. METHODS. A cross-sectional study of 52 patients between 3 and 19 years of age who had been prescribed either carbamazepine or oxcarbazepine was conducted. A serum fluorescence-polarized immunoassay and a urine enzyme-linked immunoassay were used. The serum carbamazepine or oxcarbazepine level was measured. Gas chromatography/mass spectrometry, a confirmatory test for tricyclic antidepressant detection, was subsequently performed on the serum specimen. RESULTS. A linear dependency on medication level was observed with the serum fluorescence-polarized immunoassay assay. This relationship was stronger for carbamazepine (4.2 μg/L tricyclic antidepressant detected per microgram/liter of carbamazepine) than for oxcarbazepine (0.7 μg/L tricyclic antidepressant detected per milligram/liter). At higher carbamazepine levels (8.0–11.6 mg/L), 12 of 13 patients had a positive serum fluorescence-polarized immunoassay result; at lower levels (0.1–7.9 mg/L), only 1 of 20 had a positive result. None of the patients who were receiving oxcarbazepine showed significant tricyclic antidepressant activity on either assay. CONCLUSIONS. Carbamazepine interferes at a statistically significant level with serum fluorescence-polarized immunoassay assay and in a dose-dependent fashion. Neither carbamazepine nor oxcarbazepine exhibit significant tricyclic antidepressant activity on urine enzyme-linked immunoassay assay.