Comparison of topiramate concentrations in plasma and serum by fluorescence polarization immunoassay

Topiramate has been recently licensed as an antiepileptic drug. A fluorescence polarization immunoassay (FPIA), the Innofluor, has been developed to determine topiramate in heparinized plasma. Since therapeutic drug monitoring laboratories may not have control over collection of the samples submitted to them, it is important for analytical methods to be robust and able to cope with any specimen. The effect of different anticoagulants on the topiramate FPIA assay was investigated by collecting blood from 50 patients with epilepsy being maintained on a range of topiramate doses as part of their therapy. After venesection the blood was divided among four tubes: plain, heparinized, EDTA, and fluoride/oxalate. Erythrocytes were separated by centrifugation and supernatant fluid frozen to await duplicate assay by FPIA. Results were compared by means of Altman and Bland difference plots which indicated that there was no significant difference between values obtained with heparinized plasma and the other fluids. It was concluded that the Innofluor assay is robust and gives similar results when blood samples are collected into any of the specified anticoagulants.