Endocrine Abstracts

Introduction: ACTH is measured to confirm impairments in the hypothalamic–pituitary–adrenal axis. Patients with small cell lung cancer (SCLC) can have ectopic adrenocorticotropic secretion. In December 2009, a SCLC patient showed an enormous discrepancy between the ACTH results obtained with two different ACTH assays. The aim of this study was to compare different ACTH assays in SCLC patients.

Methods: In a pilot study EDTA plasma samples of nine SCLC patients were measured with three different ACTH assays; IRMA, electrochemiluminescent immunometric assay (Elecsys), RIA. Twelve EDTA plasma samples of patients who underwent an insulin tolerance test (ITT) were measured with the IRMA and RIA assays. At our laboratory the specific IRMA assay is used as standard method.

Results: The average ACTH Elecsys/IRMA and RIA/IRMA ratios were respectively 0.84 (range: 0.61–0.94) and 4.66 (range: 1.52–17.0) for the nine SCLC patients. The RIA/IRMA ratio was 1.96 (range: 0.90–5.25) for the ITT group. The average RIA/IRMA ratio in the SCLC group is 2.4 times higher with respect to the ITT group.

Conclusions: Ectopic adrenocorticotropic secretion can lead to discrepancies in the ACTH concentrations measured with different ACTH assays. In the case of ectopic adrenocorticotropic secretion, not only the concentration of the intact ACTH is of interest, but also in the total ACTH concentration including fragments, degradation products, and precursors. In those cases it happened to be meaningful to use a RIA assay. Mostly, non-competitive ACTH are used in standard laboratory practice. When interpreting ACTH results one should be aware of the specifications of the assay. If there is any doubt that biochemical effects of ACTH-like molecules, not measured with the routine assay, could explain clinical status of a patient, a competitive assay should be used additionally. Competitive assays appear to cross react with more ACTH-like analytes than non-competitive assays.