Scintillation proximity assay: determination of steroid hormones without separation of antibody-bound and free ligand.

Scintillation proximity assay (SPA), which does not require the physical separation of bound and free hormone, was applied to the determination of cortisol in plasma. Results showed excellent agreement with a conventional radioimmunoassay using dextran-coated charcoal to separate bound and free cortisol, with similar precision and sensitivity. When the SPA was applied to the measurement of plasma aldosterone, it was evident from the SPA and radioimmunoassay results that the blocking of aldosterone binding to corticosteroid-binding globulin by 8-amino-1-naphthalene-sulphonic acid was not completely effective, and that a different sample preparation technique should be used. SPA may become the method of choice for the determination of hormones in blood.