Validation of a radioimmunoassay for the determination of human plasma concentrations of lamotrigine.

A precise and sensitive radioimmunoassay to determine human plasma lamotrigine (430C78), Lamictal) is described. The method is a direct double antibody procedure employing a rabbit polyclonal first antibody raised to a BSA conjugate of lamotrigine, and an iodinated tyrosine methyl ester of lamotrigine as the tracer. Both reagents are added simultaneously to samples containing lamotrigine prior to an overnight incubation at 4 degrees C. The method has a sensitivity of 20 ng ml-1, when plasma samples are initially diluted 1:20 with phosphate buffer and sample volumes of 20 microliters are used. The intra-assay precision at 40, 80 and 160 ng ml-1 was 6.2, 2.1 and 4.8%, respectively, and the inter-assay precision at 500, 2000, 5000 and 10,000 ng ml-1 was 4.6, 5.7, 4.6 and 5.9%, respectively. The method was specific and showed reasonable correlation with an HPLC method [1].