A new generation IRMA for ACTH with improved specificity: validation in various physiological and pathological conditions

Objective: Measurement of plasma ACTH is a key step for the exploration of hypothalamic-pituitary-adrenal disorders. To further improve ACTH recogniti on a new generation of ACTH IRMA was developed using antibodies directed to wards succinylated ACTH (sACTH IRMA). Design: The usefulness of this assay was compared with that of another comm ercially available ACTH IRMA assay using intact ACTH (ELSA-ACTH) in various pathophysiological situations: patients with low ACTH plasma levels, high ACTH plasma levels with normal or tumoural pituitaries, or ectopic ACTH syn drome, and pregnant women with high proopiomelanocortin (POMC) plasma level s, Methods: All plasma samples were assayed simultaneously with the two differ ent IRMAs, Comparisons were assessed by plotting the results along the theo retical line of identical values, and by the graphical method of Bland and Altman. Results: In the ELSA-ACTH IRMA, CLIP (or ACTH(18-39)) showed true cross-rea ctivity, and alpha -melanocyte-stimulating hormone and purified POMC both i nterfered and induced falsely lower ACTH results: in the sACTH IRMA no pept ide showed any cross-reactivity, and only extremely high values of CLIP (50 000 pg/ml) interfered and induced falsely lower ACTH results. in ACTH hype rsecretory syndromes, of tumoural (Cushing's disease, ectopic ACTH secretio n) or non-tumoural (Addison's disease, congenital adrenal hyperplasia) orig ins a good agreement between the two assays was observed except for very hi gh ACTH plasma values (above 1000 pg/ml) and in some tumours where the sACT H IRMA yielded lower results; in some cases, the presence of circulating CL IP, demonstrated by HPLC studies, may contribute to this discrepancy, It is also likely that the calibration of the ELSA-ACTH kit itself generates hig her ACTH values, In normal pregnant women both IRMAs gave highly correlated values, yet lower results were obtained with the sACTH IRMA. Conclusion: These data show that the sACTH IRMA has improved qualities of s pecificity and usefulness for rapid assessment of ACTH plasma levels.