Endocrine Abstracts

Introduction: The 1-mg overnight low-dose dexamethasone suppression test is used as a screening tool when Cushing’s syndrome is suspected. However, the biological confirmation of this syndrome is based upon the measurement of 24-hour urinary free cortisol and low-dose dexamethasone suppression test (Liddle test). The aim of this study was to assess the performance of the 1-mg overnight low-dose dexamethasone suppression test in the diagnosis of Cushing’s syndrome.

Methods: This was a retrospective study including patients admitted to our department for suspicion of Cushing syndrome between 2016 and 2021. Clinical and paraclinical data and results of 1-mg overnight low-dose dexamethasone suppression test and Liddle test were collected from medical records.

Results: Fifty-one patients (39 women and 12 men) were enrolled in this study with a mean age of 54.3 ± 15.4 years. The diagnosis Cushing’s syndrome was established in 28 patients and excluded in 23 patients. The median serum cortisol level after the 1-mg test was 7.65 μg/dL in patients with Cushing’s syndrome and 2.1 μg/dL in those without Cushing’s syndrome (P=0.001). It was positively correlated with serum cortisol level after the Liddle test (r=0.852, P<10^-3) and ACTH level (r=0.621, P=0.001). The area under the ROC curve of serum cortisol level after the 1-mg test was 0.773. A cutoff value of 1.8 μg/dL had a sensitivity of 100% and a specificity of 26%. A cutoff value of 5 μg/dL was associated with the diagnosis of Cushing syndrome (Odds Ratio= 4.11, P=0.016) with a sensitivity of 64% and a specificity of 70%. A cutoff value of 9.9 μg/dL confirmed the diagnosis of Cushing’s syndrome in 100% of cases.

Conclusion: With a cutoff value of 1.8 μg/dL, the 1-mg overnight low-dose dexamethasone suppression test is a reliable screening tool for Cushing’s syndrome. However, a higher cutoff value for serum cortisol (9.9 μg/dL) can be used alone to confirm the diagnosis. Further studies involving larger sample sizes would be useful to confirm our findings.