Endocrine Abstracts

Introduction: The plasma cortisol response to the ACTH test is known to vary significantly by assays. An automated cortisol immunoassays with increased specificity due to the shift from polyclonal to MABs, and standardized against mass spectrometry was recently introduced, with an expected decrease in cortisol concentrations by 20%. Cut-offs used in clinical practice for patient evaluation will thus have to be adjusted. We aimed to assess the normal cortisol response to ACTH stimulation measured by Elecsys Cortisol II.

Methods: An ACTH test (250 μg i.v. ACTH_1–24) was undertaken in 100 healthy volunteers (age, 18–70 years; 50 women) and 13 women on oral contraceptives. Serum cortisol was measured by two commercially available immunoassays: Elecsys Cortisol (polyclonal Ab)(Roche) and Elecsys Cortisol II (MAB). The estimated lower reference limit for the 30 min cortisol response to ACTH was derived from the 2.5th percentile of log-transformed concentrations.

Results: Cortisol concentrations measured by Elecsys Cortisol II were approximately 20% lower relative to Elecsys Cortisol. The 30 min cortisol response was normally distributed in males but not in females, with no significant gender difference at baseline nor post-ACTH (P=0.1). The lower reference limit for the 30 min cortisol response was method-specific; Elecsys Cortisol (range: 534–1013; lower reference limit: 572 nmol/l) and Elecsys Cortisol II (range: 399–817; lower reference limit: 432 nmol/l), and remained significantly elevated by both methods in women on oral contraceptives.

Conclusion: The cutoff defining a normal 30 min. cortisol response to the ACTH test is influenced significantly by assay employed as well as oestrogen treatment. New cutoffs should be introduced with the introduction of new generation immunoassays with higher specificity.