Comparative value of plasma ACTH and beta-endorphin measurement with three different commercial kits for the etiological diagnosis of ACTH-dependent Cushing's syndrome
Recent reports suggest that, contrary to radioimmunoassays (RIA), immunoradiometric assays (IRMA) artifactually decrease plasma ACTH levels in patients with the ectopic ACTH syndrome. Discrepancies between RIA and IRMA results may provide a means of discriminating this entity from Cushing's disease. We have compared the results of these two techniques, together with those of a β-endorphin assay, in 1 7 patients with Cushing's disease, 9 with the ectopic ACTH syndrome and 30 controls. ACTH-RIA and ACTH-IRMA levels in patients with Cushing's disease were similar (17.5±2.5 vs 15.1±2.8 pmol/l) and were correlated (rs=0.59, p<0.01). ACTH-RIA levels in patients with the ectopic ACTH syndrome were higher than ACTH-IRMA levels (27.3±2.9 vs 14.5±2.5, p<0.01) and these did not correlate. The ACTH-RIA and ACTH-RIA/ACTH-IRMA ratio levels in patients with the ectopic ACTH syndrome were higher than those of patients with Cushing's disease (p<0.01), but they overlapped with these in 27 and 31% of cases respectively. Plasma β-endorphin level was higher in patients with the ectopic ACTH syndrome than in patients with Cushing's disease (81.9±19.4 vs 26.4±5.6 pmol/l, p <0.01) and was correlated with ACTH only in patients with Cushing's disease. The overlap in β-endorphin and β-endorphin/ACTH-IRMA molar ratio levels between the two groups were 19 and 27% respectively. Although no parameter could be used to make clearcut distinction between Cushing's disease and the ectopic ACTH syndrome, the discriminative power of β-endorphin level was clearly better than that of the comparison between ACTH-RIA and ACTH-IRMA levels.