Abstract
Measurement of plasma ACTH levels by RIA was originally used to identify adrenal causes (AA) of Cushing's syndrome (CS) and for the differential diagnosis of ACTH-dependent CS with ovine corticotropin-releasing hormone (CRH) stimulation testing and inferior petrosal sinus sampling (IPSS). It is unclear whether diagnostic criteria developed with RIA also work for IRMA. Methods: ACTH was measured by RIA and IRMA (Covance Laboratories, Vienna VA) in paired samples i.) basally (63 Cushing's disease (CD), 5 AA, 2 ectopic CS (EAS) and 37 non-CS patients) ii.) in 44 CD patients following CRH (1 μ g/kg i.v.) iii) in 6 EAS and 17 CD patients during IPSS. Results were analyzed using Spearman's rank correlations, the Wilcoxon signed-rank test, linear regression and McNemar's test. Results: i)Basal RIA and IRMA results were highly correlated (r = 0.9, p 2.0 before or ≥3.0 after CRH, IRMA had the same sensitivity (100%) but higher specificity (100% vs 83%, p = 1.0) vs. RIA. Conclusion: ACTH measured by IRMA correlates closely to RIA values and offers similar diagnostic accuracy during the CRH and IPSS testing. However in ACTH samples < 15 pg/ml IRMA probably offers better diagnostic accuracy.
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