Endocrine Abstracts

Objective: The 250 µg short Synacthen stimulation test (SST) is the most commonly used dynamic assessment to diagnose adrenal insufficiency (AI). There are challenges to the use of the SST in routine clinical practice, including staff and resource limitation in the current COVID-19 pandemic and Synacthen cost. We aimed to investigate the value of basal cortisol level for predicting AI in our selected cohort of patients at risk of secondary adrenal insufficiency from pituitary disease, pituitary surgery or pituitary irradiation.

Methods: A ten-year retrospective review was performed in our tertiary unit. SSTs were performed before midday. AI was defined as peak serum cortisol level of <550 nmol/l on Roche Cortisol I assay or <420 nmol/l on Roche Cortisol II assay. Conversion of baseline cortisol level from Roche I to Roche II equivalent measurement was done using validated regression equation. Diagnostic performance was evaluated by Receiver Operating Curve (ROC) analysis.

Results: 595 SSTs performed from 2010 to 2020 were included. 51 (8.6%) were positive for AI. The ROC analysis showed an overall area under the curve (AUC) for basal cortisol of 0.975 (95% CI 0.959 to 0.986). If a basal cortisol level cut-off of ≤237 nmol/l was implemented to predict AI, no failed SST would be missed, hence the negative predictive value to rule-out AI was 100% (95% CI 93 to 100%). By using a cut-off value of 237 nmol/l, 399 out of 544 (73%) SSTs could be eliminated. With a lower basal cortisol cut-off of ≤165 nmol/l, 5 out of 51 failed SSTs would be missed but 503 out of 544 (92%) normal SSTs would be avoided.

Conclusion: In our cohort of patients at risk of secondary adrenal insufficiency, basal morning serum cortisol concentration can be utilised as a convenient screening test, with high diagnostic performance, to identify patients requiring confirmatory dynamic testing using SST. Further prospective studies are required to validate the cut-off values proposed.