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Clin Chem. 2019 Sep 30. pii: clinchem.2019.306365. doi: 10.1373/clinchem.2019.306365. [Epub ahead of print]

An Intact ACTH LC-MS/MS Assay as an Arbiter of Clinically Discordant Immunoassay Results.

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Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, Canada.
Department of Pathology, University of Utah, Salt Lake City, UT.
Department of Pathology and Laboratory Medicine, Dartmouth-Hitchcock Medical Center, Lebanon NH and The Geisel School of Medicine at Dartmouth, Hanover, NH.
Department of Pathology, Microbiology and Immunology, Vanderbilt University School of Medicine and Vanderbilt University Medical Center, Nashville, TN.
Department of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, Canada;
Department of Pathology and Laboratory Medicine, St. Paul's Hospital, Providence Health Care, Vancouver, Canada.



Measurement of plasma adrenocorticotropic hormone (ACTH) is key in the differential diagnosis of hypothalamic-pituitary-adrenal disorders. Two-site sandwich immunoassays dominate clinical testing of ACTH in North America; however, discordant results between manufacturers have been repeatedly reported. To resolve the discrepancy, we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for the intended measurand, biologically active intact ACTH (iACTH).


The multiple reaction monitoring LC-MS/MS assay was designed to selectively measure full-length iACTH, as well as ACTH analogs and fragments (i.e., ACTH1-24 and ACTH18-39). Epitope assignment of the Roche Elecsys antibodies was performed by MALDI-TOF mass spectrometry. A method comparison between Roche Elecsys and Siemens Immulite ACTH immunoassays was performed and clinically concordant/discordant results identified. In a subset of these samples, the iACTH concentration was determined using the LC-MS/MS method.


The lower limit of the measuring interval of the iACTH LC-MS/MS assay was 9 pg/mL (2 pmol/L). The assay was linear from 9 to 1938 pg/mL (2 to 427 pmol/L). Epitope mapping revealed that the Roche capture and detection antibodies bound residues 9-12 and 36-39 of ACTH, respectively. The iACTH LC-MS/MS analysis demonstrated that for discordant results between 2 immunoassays studied, only the Roche results were highly positively correlated with the iACTH concentration.


Immunoprecipitation of biologically active ACTH molecules followed by LC-MS/MS analysis enabled selective detection of iACTH and relevant biologically active fragments in plasma. Applied to the investigation of clinically discrepant results, this method can act as an arbiter of the concentration of iACTH present.

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