Evaluation of immunoradiometric succinylated. ACTH assay

A new immunoradiometric assay for succinylated ACTH using 3 monoclonal anti bodies has been developed by Immunotech (I-IRMA). It was compared to a comm ercial immunoradiometric assay of ACTH (Nichols Institute, N-IRMA). The fun ctional sensitivity of I-IRMA assay was estimated at 1.5 ng/L. The comparis on of both methods on plasma samples withdrawn at 8 h from 47 normal subjec ts showed a good correlation coefficient (r = 0.83: P < 0.001). The 24-hour s secretion profiles obtained by both methods were similar in 14 normal sub jects. Nevertheless, the I-IRMA mean values were about 30 % lower than the corresponding N-IRMA values. This difference increased to 50 % when the ACT H concentrations were low, as it the case at 24 h or during the dexamethaso ne suppression rest. During insulin hypoglycemia stimulation test, the two procedures gave similar values. Both methods applied to a pathologic popula tion gave similar result to those obtained on normals. In 10 patients beari ng 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 vales during the base state or after the 8 mg-dexamethasone suppressio n test. This difference was also observed in 6 patients with corticotroph i nsufficiency. In conclusion, the comparison of N-IRMA and I-IRMA methods sh owed the validity of the new succinylated-ACTH assay which is more efficien t in the lower range of ACTH concentration. This significant decrease in th e sensitivity threshold may be useful in the establishment of the cure crit eria in Cushing disease.