Seradyn quantitative microsphere system lamotrigine immunoassay on a Hitachi 911 analyzer compared with HPLC-UV

Lamotrigine (LTG) is used currently as monotherapy or, more frequently, as add-on therapy with other antiepileptic drugs. It demonstrates efficacy against partial seizures, primary and secondary tonic clonic seizures, absence seizures, and drop attacks. LTG pharmacokinetics is complicated by coadministration with other antiepileptic drugs such as valproic acid, phenytoin, or carbamazepine. The wide interpatient variability in LTG dosage required to attain therapeutic plasma LTG concentrations for seizure control suggests that LTG is a good candidate for therapeutic drug monitoring (TDM). In this study, we compared the quantitative microsphere system (QMS) LTG immunoassay with the LTG high-performance liquid chromatography-ultra violet (HPLC-UV) assay routinely employed for TDM in our laboratory. Samples tested by these methods were patient samples presented for TDM and from a quality assurance program. Quality control material demonstrated within- and between-run (n = 6) coefficient of variation and biases of less than 10%. Patient samples demonstrated a Deming regression of QMS = 1.09 HPLC-UV - 0.17 and quality assurance program samples had a Deming regression of QMS = 1.03 HPLC-UV - 0.11. Patient samples demonstrated a mean bias of 6.1% and quality assurance program samples had a mean bias of 0.2%. The QMS LTG assay had a clinically small but significant overestimation of plasma LTG concentrations. It may be useful as a convenient alternative method that would provide TDM guidance if a chromatographic assay was not available.