Liquid chromatographic analysis of phenobarbital, phenytoin, and theophylline

Sera from the routine of therapeutic drug monitoring were assayed for phenobarbital, phenytoin, and theophylline with three different methods: fluorescence polarization immunoassay as the standard procedure, the new CEDIA assays within a multicenter evaluation and HPLC which is known to yield results with a high specificity. CVs for between-day imprecision ranged from 2.6-8.6%, depending on the concentration of the drugs. There was a tendency to lower CVs for the HPLC procedure. Accuracy was verified with commercial control materials and spiked sera and proved to be satisfactory for all three methods and parameters. The linear range was approx. twice as wide for the HPLC compared with the other methods. The method comparisons were quite favorable. Deviations occurred mainly in the subtherapeutic concentration range.