This paper describes a fully automated assay on the Serono SRI™ 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

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... e absorbance generated is measured as the assay end-point. All processing steps are performed by the SRI. 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/1) 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 SRI Cortisol assay on the SRI analyzer is suitable for the routine determination of cortisol in serum, heparinised plasma and urine samples.