An enzyme-linked immunosorbent assay for corticosteroid-binding globulin using monoclonal and polyclonal antibodies: decline in CBG following synthetic ACTH.

BACKGROUND Adrenal function is commonly assessed by measuring plasma cortisol following synthetic ACTH (synacthen) challenge. Generally little regard is given to plasma levels of corticosteroid-binding globulin (CBG). We have developed and validated an enzyme-linked immunosorbent assay (ELISA) for CBG and together with plasma cortisol calculated the "free cortisol index" as an additional parameter for assessing adrenal function. METHODS A monoclonal antibody was raised against CBG. The antibody was characterized by Western blotting and used with a polyclonal antibody to develop a direct ELISA for CBG. Together with total plasma cortisol, the free cortisol index was derived and correlated with an "in-house" ligand binding method for assessing free cortisol. The free cortisol index was also used as an adduct to total plasma cortisol in assessing adrenal function. RESULTS The ELISA has acceptable performance and the free cortisol index correlates well with free cortisol determined by ligand binding. In addition, we show that CBG levels following synthetic ACTH (synacthen) show a modest but significant decline. CONCLUSION We conclude that the measurement of both plasma CBG and total cortisol to derive the free cortisol index may provide an additional parameter in the interpretation of the short synacthen test and that there is a decline in plasma CBG over this test.