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Evaluation of a Luminescent Enhanced fmmunoenzymometric Assay for Plasma Corticotropin (ACTH) Using the IMMULITE® System by Michael Vogeser, Werner Kuehnel, Hans-Georg Lambrecht, Jutta Sitte, Hilmar Stracke

The assay for determination of the adrenocorticotropic hormone (ACTH) implemented on the IMMULITE® analyzer of DPC (Diagnostic Products Corporation) evaluated in this study is a solid-phase sandwich assay with a monoclonal ACTH capture antibody (mouse) coupled to the solid phase (bead) and a polyclonal liquid-phase antibody (rabbit) coupled with alkaline phosphatase. A precision profïle of the assay was determined which allowed the functional sensitivity (CV <20%) to be calculated as 6.8 pg/ml. For a non-automated immunoradiometric assay (ACTH IRMA, Nichols) used for comparison, a similar functional sensitivity of 7.2 pg/ml was found. With an average recoyery of 97%, the dilution linearity of the IMMULITE ACTH assay was satisfactory. The addition of a high ACTH concentration to a sample with an ACTH level < 5 pg/ml led to a 106% recovery in comparison to the Nichols IRMA. Lipemia, bilirubinemia, uremia and hemolysis led to a 5-15% reduction in the ACTH levels found. A high-dose hook effect could not be shown up to a concentration of 4.5 x 107 pg/ml and an ACTH concentration of 45.000 pg/ml did not cause carryover to a subsequent sample. In healthy volunteers (n=59), a reference range of 10.3 - 48.3 pg/ml (2.5 - 97.5 percentile) was calculated for the IMMULITE® ACTH assay. To investigate the clinical suitability of the IMMULITE® ACTH assay, ACTH was determined in a total of 30 functional tests (CRH test, global pituitary stimulation test) carried out for various clinical areas of inquiry in parallel to the routine assays used in the respective laboratories (Nichols IRMA, and B.R.A.H.M.S. LUMItest). In all cases, the response of the individual tests and the regression analysis of all measured values were consistent with those from the assays used for comparison. The regression analyses resulted in the following data: ACTH IMMULITE vs. ACTH IRMA Nichols; ACTH IMMULITE = 1.00 x ACTH IRMA -2.21pg/ml (r=0.97; n=111). ACTH IMMULITE vs. ACTH LUMItest B.R.A.H.M.S.; ACTH IMMULITE = 1.10 x ACTH LUMItest -0.02 pg/ml (r=0.97; n=41). In summary we conclude from our data that the IMMULITE® ACTH assay permits the rapid and clinically reliable determination of ACTH and yields results that correspond well to those obtained with established non-automated immunoassays.

DOI: Clin. Lab. 1999; 45:37-45