A comparison of immunometric and radioimmunoassay measurement of ACTH for the differential diagnosis of Cushing's syndrome

Objective: Measurement of plasma ACTH levels by radioimmunoassay (RIA) is used to identify adrenal causes (AA) of Cushing's syndrome (CS) and to distinguish ectopic CS (EAS) from Cushing's disease (CD) using CRH stimulation testing and inferior petrosal sinus sampling (IPSS). We wished to determine whether diagnostic criteria developed with RIA would also be applicable for immunoradiometric (IRMA) or immunochemiluminescent (ICMA) assays. Subjects and methods: ACTH was measured by RIA, immunoradiometric and/or immunochemiluminescent assay on samples obtained during three types of diagnostic testing in a tertiary referral setting: a) basally (63 CD, 5 AA, 2 EAS and 37 non-CS patients); b) in 44 CD patients following CRH; c) in 6 ectopic CS and 17 CD patients during IPSS. The primary outcome was comparison of diagnostic utility. Results: a) IRMA results, while lower, correlated highly with RIA (r=0.9, p < 0.0001) and had similar sensitivity (100 vs 80%) and specificity (89 vs 94%) for the diagnosis of AA (p=0.3); b) the sensitivity for CID of CRH testing using IRMA was similar to that of RIA (85 vs 83%, p=11.0); c) during IPSS, IRMA had similar sensitivity (100%) and specificity (100 vs 83%) compared with ICMA or RIA (p=1.0). Conclusions: ACTH immunometric assays correlate closely to RIA and offer similar diagnostic utility.