Assessment of a simple competitive protein-binding technique for plasma cortisol assay

The Cortipac kit for cortisol assay by a competitive protein-binding technique (CPB) which utilizes 75Se cortisol has been evaluated. Results obtained by it agree well with those by the Mattingly fluorimetric method. Assays can be carried out on either plasma or serum and haemolysis does not interfere. The specificity of the assay was checked in blood samples from patients receiving synthetic steroids. Prednisone and prednisolone therapy caused significant interference with the assay; fluorocortisol and dexamethasone therapy did not. The increased progesterone in late pregnancy blood samples had only a small effect on the assay. Plasma samples for cortisol assay could be stored for at least 4 weeks at 4 degrees C and for at least 12 weeks at -20 degrees C.