Direct radioimmunoassay procedure for plasma dexamethasone with a sensitivity at the picogram level

A simple radioimmunoassay (RIA) for the direct quantitation of plasma dexamethasone (DEX) at the picogram level has been developed. An antiserum with high specificity and avidity was obtained by the immunization of a carefully synthesized dexamethasone-21-succinyl-thyroglobulin with a high incorporation ratio. As little as 1 pg of DEX in 50 microL of plasma sample can be detected directly by this RIA without extraction and other purification procedures. Intra- and interassay coefficients of variation were 2.1 and 3.3% for plasma levels at 2.93 ng/mL or 2.3 and 7.2% for plasma levels at 0.88 ng/mL. Blank values for plasma of normal or pre-DEX patients were always under the detection limit (20 pg/mL). Excellent linearity (r = 0.9991-0.9999) was demonstrated between the serial dilutions of six plasma samples and their corresponding DEX concentrations. In single-dose DEX (0.25-1 mg) pharmacokinetic studies, plasma DEX was consistently detectable up to 24 h post dose. Compared with existing methods, this direct RIA demonstrates superior performance with regard to simplicity, sensitivity, specificity, and reproducibility. It also enables high sample throughput and has proven robust in our hands. This assay should be readily transferable to other laboratories for clinical or research purposes.