(22R,S)Budesonide was isolated from human plasma by solid-phase extraction. Switching from reversed-phase conditions during sample application and washing to normal-phase conditions during elution afforded a very clean extract. Budesonide was derivatized with acetic anhydride to form the 21-acetyl derivative before analysis by reversed-phase liquid chromatography combined with thermospray mass spectrometry. Deuterium-labelled budesonide was used as internal standard. Standard samples prepared in human albumin solution were used for the calibration curve. An automated liquid chromatography/mass spectrometry system, allowing unattended overnight operation, was used for routine analysis. The recovery of budesonide from plasma was 88.9 +/- 5.9% (mean +/- SD) and the method was linear over the range 0.30-30 pmol (amount analysed), corresponding to plasma concentrations of 0.10-10 nmol l-1. Budesonide could be measured down to 0.10 nmol l-1 with a within-day variation of 10-18% (CV). The error was less than +/- 15% at 0.10 nmol l-1 and less than +/- 7% at concentrations of 0.20 nmol l-1 or higher. The total imprecision between days was 9% (CV) at a concentration of 0.30 nmol l-1.

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