Impact of old and current immunoassays on the 1 mg overnight dexamethasone suppression test: comparison with LC-MS/MS.

Objective: The influence of immunoassay performance in hypercortisolism and dexamethasone suppression test (DST) settings was scarcely investigated. We evaluated the effectiveness of two immunoassays in detecting hypercortisolism compared to recommended liquid chromatography-tandem mass spectrometry (LC-MS/MS), and compared immunoassay analytical performance in basal and post-DST conditions.

Methods: We measured cortisol in post-DST sera of patients with suspected hypercortisolism or adrenal incidentalomas by Elecsys gen I (n=260), and by Access (n=217). All samples were also measured by a validated LC-MS/MS method. We estimated hypercortisolism rate according to the established 50 nmol/L cut-off, and generated immunoassay-specific cut-offs providing >95% sensitivity and >80% specificity. Finally, we compared cortisol measurements in basal and post-DST samples.

Results: Using the 50 nmol/L cut-off, both immunoassays detected lower rates of hypercortisolism compared with LC-MS/MS, particularly in patients with adrenal adenomas (P<0.050). Elecsys gen I and Access determined 6.9% and 6.4% possible false negatives, respectively. Elecsys gen I also caused 3.8% possible false positives. Optimal cut-off was 41 nmol/L for Elecsys gen I (sensitivity: 97.7%; specificity: 80.8%), and 33 nmol/L for Access (sensitivity: 97.5%; specificity: 78.3%). In basal and post-DST samples, Elecsys gen I overestimated by 32.5% and 6.1%, whereas Access underestimated by -4.7% and -5.9% compared to LC-MS/MS cortisol measurements, respectively. Sex differences in method deviations were noted.

Conclusions: Both immunoassays demonstrated remarkable underdetection of hypercortisolism, suggesting the application of a method-specific cut-off. Immunoassay performance may not be uniform in basal and post-DST conditions and should be purposely examined. Accurate LC-MS/MS methods should be preferred in hypercortisolism settings.