Determination of Phenytoin in Plasma: A Comparison of Procedures Using a New Radioimmunoassay, Gas Chromatography, and Enzyme Immunoassay

Abstract
A new commercially available radioimmunoassay (RIA) procedure for the anticonvulsant drug phenytoin was assessed and compared with a GLC and an enzyme immunoassay (EMIT) procedure. Human plasma samples (134) containing phenytoin were analyzed by the 3 methods and the results were found to be essentially independent of the procedure used and so would indicate similar clinical interpretation. The correlation coefficients were: RIA/GLC = 0.988; EMIT/RIA = 0.978; EMIT/GLC = 0.981. The cross reactivity of 33 drugs and related compounds was tested in the RIA and EMIT procedures and only a few very closely related substances showed significant cross reactivity. The 3 methods were compared with respect to the technical difficulties involved as well as time and cost for the assay of single and multiple samples.