Development and evaluation of a highly sensitive and specific radioimmunoassay for measurement of adrenocorticotropin levels in plasma

The authors developed and evaluated a radioimmunoassay for adrenocorticotropin (ACTH) using Sep-Pak C18 cartridges for the extraction of the hormone prior to assay. The assay has a sensitivity of 5 pg/mL. Inter- and intra-assay coefficients of variation were 9.1% and 4.8% at the level of 30 pg/mL (n=22). The mean ACTH level between 8 and 9 AM in 48 normal individuals was 20.3 ± 9.0 (S.D.) pg/mL with a range of 5 - 48 pg/mL. Mean (± S.D.) ACTH levels (8 - 9 AM) in nine normal subjects before and after 1 mg dexamethasone at midnight were 21 ± 8.8 and 9 ± 4.8 pg/mL, respectively, showing a mean suppression of 57 % with a range of 38-76%. ACTH levels increased two- to fivefold in five normal subjects after a single oral dose of metyrapone. In 17 patients with hypercorticolism, ACTH levels indicated the status of the pituitary-adrenal axis.