Investigation of peptide cross reactivity in equine plasma using two adrenocorticotropic hormone immunoassays

This study aimed to further define and quantify possible cross-reactive peptides when measuring plasma adrenocorticotropic hormone (ACTH) concentration in equids. Equine plasma samples were spiked with known concentrations of exogenous manufactured peptides comprising human ACTH_1-39, ACTH_18-39 (corticotropin-like intermediate lobe peptide, CLIP) and ACTH_7-38 (corticotropin inhibiting peptide, CIP). All samples were assayed in duplicate using Siemens Immulite 2000xpi chemiluminescent assay (CLA) and Tosoh AIA-900 immunoflurorescent assay (IFA). As expected, ACTH_1-39 was measured by both assays although higher values were reported using IFA (mean 132 % of actual concentrations) than CLA (mean 104 % of actual concentrations). ACTH_18-39 was detected by the CLA but not the IFA (mean 29 % actual concentration) whereas ACTH_7-38 was detected by the IFA, but not the CLA (mean 65 % actual concentration). The study further clarifies that these ACTH immunoassays are likely to report higher measured ACTH_1-39 concentrations than are actually present in the sample although additional work is needed to elucidate the diagnostic and pathophysiologic implications of these findings.