Objective: Adrenal insufficiency (AI) is diagnosed with morning cortisol but ACTH stimulation test is usually needed to confirm the diagnosis. In this study we investigated different morning cortisol thresholds that can safely rule out AI without requiring a confirmatory ACTH stimulation test. Design: Retrospective cohort study. Methods: We included patients aged 18 and above who underwent the 250 mcg ACTH stimulation test from June 2018 to June 2022. Basal and post-ACTH serum cortisol values at 30 and 60 min were documented. Sensitivity, specificity and logistic regression analysis were employed to assess morning cortisol level's ability to predict AI as confirmed by ACTH stimulation test. Results: 237 patients were included, 66 diagnosed with AI and 171 had normal ACTH results. Hypertension and type 2 Diabetes correlated with lower AI incidence. Median morning cortisol was 138.0 nmol/L for AI group and 286.0 nmol/L for non-AI patients. A morning cortisol of 285 nmol/L had 90.6 % sensitivity, 50.3 % specificity, and a negative predictive value of 93.3 % for ruling out AI. A threshold of 306 nmol/L increased sensitivity to 95.3 % with 40 % specificity. Conclusion: Morning cortisol is an effective diagnostic tool for ruling out AI. Using multiple thresholds based on clinical suspicion and the integration of predictive pre-test probability can reduce the need for excessive ACTH stimulation tests. This study contributes to the growing evidence of utilizing morning serum cortisol in the diagnosis of adrenal insufficiency. Clinical relevance statement: Diagnosing Adrenal Insufficiency (AI) can be challenging due to debate regarding the cortisol cut-off value that can exclude AI without additional tests. The confirmatory short synacthen test has certain limitations including financial implications and time restrictions. We investigated the performance of various morning cortisol levels that can diagnose AI without additional testing.
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