The use of baseline cortisol level in predicting the outcome of 1 μg Synacthen tests in an outpatient endocrinology unit

Introduction: The clinical requirements and the indication of the Synacthen test are increasing. The objective of our
study is to determine a baseline cortisol level that reliably predicts the response to Synacthen test in a low-risk
group of patients.
Materials and methods: We performed a cross-sectional analysis of all Synacthen tests conducted between January
2017 and June 2018. The diagnostic accuracy of basal cortisol levels as a predictor of an adequate response to
Synacthen test was evaluated by ROC curve analysis.
Results: One hundred and fifty-three patients were included. A baseline cortisol level <40 ng/mL had a sensitivity
of 100% but a specificity of 5.8% for the failure of the Synacthen test, while a baseline cortisol level> 147.5 ng/
mL showed a specificity of 100% but a sensitivity of 1.2% for an adequate response to the Synacthen test. According to the ROC curve, the optimal baseline cortisol level for predicting an adequate response to the Synacthen
test was 85 ng/mL with an AUC of 0.808 (95% CI [0.738–0.877]).
Conclusion: We propose a basal cortisol level assay as a first step in the evaluation of patients with suspected
adrenal insufficiency