Determination of ethambutol in plasma by high-performance liquid chromatography after pre-column derivatization

A new HPLC assay using UV detection (200 nm) was developed to determine ethambutol (EMB) concentrations in plasma. Following extraction (0.1 ml plasma) with chloroform, EMB and octylamine (used as internal standard) were derivatized with phenylethylisocyanate. Quantitation in plasma was achieved at 200 nm. There were no interferences from endogenous compounds. Intra- and inter-day variabilities were lower than 5.2 and 7.6%, respectively. The limit of quantitation of the method was 0.2 μg/ml. In plasma, ethambutol was found to be stable for at least one month when samples were stored at −20°C. This assay was applied to the therapeutic monitoring of EMB concentrations in 19 patients suffering from tuberculosis.