[Evaluation of an immunoradiometric assay for succinylated ACTH]

A new immunoradiometric assay for succinylated ACTH using 3 monoclonal antibodies has been developed by Immunotech (I-IRMA). It was compared to a commercial immunoradiometric assay of ACTH (Nichols Institute, N-IRMA). The functional sensitivity of I-IRMA assay was estimated at 1.5 ng/L. The comparison of both methods on plasma samples withdrawn at 8 h from 47 normal subjects showed a good correlation coefficient (r = 0.83; P < 0.001). The 24-hours secretion profiles obtained by both methods were similar in 14 normal subjects. Nevertheless, the I-IRMA mean values were about 30% lower than the corresponding N-IRMA values. This difference increased to 50% when the ACTH concentrations were low, as it the case at 24 h or during the dexamethasone suppression test. During insulin hypoglycemia stimulation test, the two procedures gave similar values. Both methods applied to a pathologic population gave similar result to those obtained on normals. In 10 patients bearing corticotroph adenomas, the profiles of ACTH secretions during 24 h were similar using both methods. The I-IRMA values were lower about 30% than N-IRMA values during the base state or after the 8 mg-dexamethasone suppression test. This difference was also observed in 6 patients with corticotroph insufficiency. In conclusion, the comparison of N-IRMA and I-IRMA methods showed the validity of the new succinylated-ACTH assay which is more efficient in the lower range of ACTH concentration. This significant decrease in the sensitivity threshold may be useful in the establishment of the cure criteria in Cushing disease.