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.