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A Fully Automated Enzyme Immunoassay for the Measurement of Cortisol in Biological Fluids

This paper describes a fully automated assay on the Serono SR1 for the measurement of cortisol in serum, heparinised plasma and urine. The assay incorporates a specific polyclonal antibody to cortisol and cortisol conjugated to alkaline phosphatase as a label. Following immunoincubation, bound and free labelled cortisol are separated by magnetic sedimentation of the antibody complex. Phenolphthalein is liberated by the enzymatic hydrolysis of the substrate (phenolphthalein monophosphate) and the absorbance generated is measured as the assay end-point. All processing steps are performed by the SR1. The assay has good analytical performance with respect to precision (within and between run CVs less than 10 and 11.5% respectively), detection limit (less than 5 nmol/l) and recovery of added cortisol (100 +/- 10%). The assay agrees well with cortisol concentrations determined by ID-MS and by established immunoassays (r > 0.96). Reference ranges of normal urine samples (pretreated by solvent extraction) are in good agreement with accepted values. The study demonstrates that the SR1 Cortisol assay on the SR1 analyzer is suitable for the routine determination of cortisol in serum, heparinised plasma and urine samples.