Endocrine Abstracts

Introduction: In 2017 the Roche cortisol assay underwent a generation update. The increased specificity of this assay was expected to lower cortisol results by approximately 30%. Based on a local consensus, the 30 min short Synacthen Test (SST) cortisol was lowered from 550 to 420 nmol/l. With the previous assay, a morning cortisol of ≥375 nmol/L gave a 99% PPV of an SST ‘pass’.

Aims: • To compare the numbers of SSTs and the SST pass rates before and after the introduction of the new assay.

• To establish the morning cortisol threshold likely to predict an SST ‘pass’.

Methods: 4 years of SST data was analysed from 2015 to 2019, encompassing 2 years before and 2 years after the new assay

Results: Total numbers of SSTs remained stable (634 vs. 663/2 years). A 30 min cortisol of 420 nmol/l resulted in similar ‘pass’ rates with the new assay to 30 min cortisol of 550 nmol/l with the old assay (71%). A peak response of 380 nmol/l with the new assay gave similar ‘pass’ rates to a peak of 500 nmol/l with the old assay (76%) and a PPV of 98.5%. To achieve a 99% PPV of passing the SST (based on a peak of ≥420 nmol/l with the new assay) morning cortisol was 390 nmol/l.

Conclusion: The more specific cortisol assay has resulted in uncertainty about thresholds for recommending and assessing SSTs. Kline et al. (2017) suggested a cut-off of 350 nmol/l for the peak SST response (reflecting the 30% lower cortisol results with the more specific assay). Most centres are using higher thresholds. We need to consider the potential of missing a diagnosis of adrenal insufficiency, best use of resources, and the adverse consequences of long-term steroid therapy for patients with normal pituitary–adrenal axes.