A Simple Direct Solid-Phase Enzymeimmunoassay for Cortisol in Plasma

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Abstract
A simple, direct solid-phase enzyme-labelled immunoassay for plasma cortisol was established using horseradish peroxidase/cortisol 21-hemisuccinate conjugate as ‘enzyme label’. The antiserum, raised against a cortisol 21-hemisuccinate/bovine serum albumin conjugate, was coupled to cellulose to facilitate separation of free and bound steroid. Solvent extraction was avoided by the use of heat denaturation of the cortisol-binding globulin. This assay had a lower limit of sensitivity of 16·6 nmol/l and satisfied the standard criteria of accuracy and precision. Cortisol concentrations determined by enzymeimmunoassay were in excellent agreement with a gas liquid chromatography/mass spectrometry procedure (r = 0·98, n = 19) and also with the radioimmunoassay in current use (r = 0·95, n = 20). Cortisol levels after ACTH stimulation and dexamethasone suppression in various subjects are presented. This enzymeimmunoassay is particularly applicable to the routine determination of plasma cortisol in small clinical laboratories or in those with a fluctuating workload.