Fast, simple and cost-effective determination of thiopental in human plasma by a new HPLC technique

Background: Thiopental is an anaesthetic drug that is largely used in both short-term and long-term infusion. After long-term infusion of thiopental, non-linear and inter-individual-dependent pharmacokinetics occur because of the saturation and/or induction of the metabolism. Clinical monitoring is important so that therapeutic adjustments can be made in many of the different pharmacological treatments, especially when long-term infusion is required. We describe a new, rapid HPLC method for the determination of plasma thiopental. Methods: Sample preparation involved precipitation of plasma proteins using a mixture of methanol, zinc sulfate and ethylene glycol, and containing the internal standard 5-ethyl-5-p-tolyl-barbituric acid. After adding trichloroacetic acid, the sample was centrifuged and the supernatant was injected into a C₁₈ reversed-phase column. The mobile phase used was water-methanol-acetonitrile (50:40:10, v/v). The eluent was monitored at 290 nm. Results: The calibration curve was linear from 0.2 to 100 μg/mL. Precision, calculated as the coefficient of variation (%), was in the range of 3.62-0.70% for the within-day assay and 5.77-1.51% for the between-day assay. The absolute recoveries obtained from supplemented samples were never less than 100%. Conclusions: This technique shows good reliability and seems to be suitable for a very fast and simple therapeutic monitoring of plasma thiopental.