Endocrine Abstracts

Background: The 1 mg overnight dexamethasone suppression test (ONDST) is recommended for the differential diagnosis of Cushing’s syndrome and the investigation of adrenal incidentalomas. However, diagnostic performance of the test relies on accurate methods to quantitate cortisol in serum. Although the variable performance of serum cortisol immunoassays has been well-documented, little has been published on their performance following the ONDST.

Aims: Assess the performance of three common immunoassay platforms (Roche Elecsys II, Abbott Alinity & Siemens Centaur) when compared to a liquid chromatography tandem mass spectrometry (LC–MS/MS) method.

Methods: Samples (n=77) sent to the laboratory as part of an ONDST were retrieved prior to disposal and stored at −80°C until commencement of the study. Samples were pseudononymised, aliquoted and frozen prior to distribution to participating laboratories for immunoassay analysis. Samples with cortisol >250 nmol/l and samples with cortisol lower than the limit of quantitation for each assay were excluded. Immunoassay results were compared to a routine LC–MS/MS serum cortisol method that is metrologically traceable to a candidate reference measurement procedure. Statistical analysis was done, for total results and split into male and female cohorts, through Passing-Bablok regression and Bland-Altman plots.

Results: The Roche gen II compared well, with a mean bias of −2.4 nmol/l and a Passing-Bablok regression fit of y=−1.332+0.9857x. This bias was not affected by sex. The worst comparison was observed with the Abbott immunoassay. The mean bias here was −17.7 nmol/l, with a Passing-Bablok regression fit of y=−5.565+0.8362x. This negative bias was exacerbated in the samples taken from female patients (−20.9 nmol/l) vs male patients (−15.0 nmol/l). The Siemens assay had a mean bias of 3.2 nmol/l and a Passing-Bablok regression fit of y=0.9035+1.044x. This positive bias was worse in male patients (6.7 nmol/l) vs female patients (0.01 nmol/l).

Conclusions: Method-dependent variation exists in the analysis of serum cortisol as a part of ONDSTs. Of the immunoassays, Roche gen II and Siemens more closely aligned with LC–MS/MS. Use of the Abbott immunoassays may result in the underestimation of cortisol and a reduction in ONDST diagnostic sensitivity, particularly in female patients. Clinicians should be aware of the performance of their local assay and on the basis of this data there is evidence to support assay-specific cut-offs for the ONDST.