Evaluation of a Sensitive Radioimmunoassay of Plasma Corticotropin Using Commercial Reagents

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Abstract
A radioimmunological [RIA] method for the sensitive estimation of human plasma ACTH immunoreactivity using commercially available reagents is presented. The method involves silica extraction of ACTH from plasma, desorption with acid protein solution, neutralization and subsequent RIA. [125I]ACTH is added as internal standard to the plasma sample. The amounts of ACTH extractable with silica differed considerably between the plasma samples of individual subjects. There were marked differences in the affinity of 5 different ACTH standards to the antiserum used. The detection limit of the method was 1.47 pmol/l. Blanks arising in H2O and in charcoal stripped serum were lower than the detection limit. Precision and accuracy were within the range commonly achieved for RIA-methods. Morning levels of normal subjects ranged from 6.5-10.9-18.5 pmol/l. Hydrocortisone infusion suppressed plasma ACTH from 12.6 .+-. 6.4 (SD) to 4.4 .+-. 3.1 (SD) pmol/l. Metyrapone infusion increased ACTH levels from 7.3 .+-. 4.2 (SD) to 15.3 .+-. 6.0 (SD) pmol/l.