Introduction: The diagnosis of Cushings syndrome (CS) requires a screening test of high sensitivity, followed by biochemical evaluation aiming to identify the aetiology of hypercortisolism. The high-dose dexamethasone suppression test (HDDST) with the corticotropin-releasing hormone (CRH) test is commonly used for the differential diagnosis of ACTH-depended Cushings syndrome. However, the diagnostic utility of the HDDST has been questioned.
Aim: To compare the diagnostic profile of various combinations of the above tests for the differential diagnosis of ACTH-dependent CS.
Patients and methods: We retrospectively analysed the results of 78 patients with Cushings disease (CD, n=72) or ectopic Cushings syndrome (ECS, n=6) ((mean age=42, F=59(75.6%)) who underwent LDDST (n=59), HDDST (n=77) and CRH test (n=65).The diagnostic profile of various combinations of these tests were assessed by constructing Receiver operating characteristics (ROC) curves.
Results: For the differential diagnosis of CD versus ECS the tests showed:
-Suppression of serum cortisol more than 27.8% at 48 h on the LDDST had a sensitivity of 79% and specificity of 67% (n=59)
-Suppression of serum cortisol more than 53.5% at 48 h on the HDDST had a sensitivity of 79% and specificity of 58% (n=77)
-Increase of serum cortisol more than 27.6% on CRH test had a sensitivity of 83% and specificity of 84% (n=58)
-Increase of serum ACTH more than 58.6% on CRH test had a sensitivity of 78.5% and specificity of 100% (n=58)
-Suppression of serum cortisol more than 27.8% at 48 h on the LDDST and/or increase of serum cortisol more than 27.5% on the CRH test (n=47) had a sensitivity of 100% and specificity of 100%.
Conclusion: LDDST combined with the CRH test provided excellent diagnostic utility in all cases. With the criteria used the HDDST does not increase diagnostic sensitivity compared to LDDST.