Objective: The diagnosis of subclinical Cushings syndrome in patients with incidentalomas is not always straight forward and a number of different criteria have been used. The 1 mg overnight dexamethasone suppression test has been recommended as a screening test, followed up by other tests of the hypothalomopituitaryadrenal axis to confirm the diagnosis. In this study we investigate whether the low-dose dexamethasone suppression test offers additional information to the overnight dexamethasone suppression test in establishing diagnosis.
Design: Retrospective, observational study.
Patients and measurements: Demographic and clinical data were collected on 137 patients diagnosed with incidentalomas. A full endocrinology work up was performed.
Results: There were 76/137 (55%) patients who had either a positive overnight dexamethasone suppression test or/and a low-dose dexamethasone suppression test. 60/103 (58%) patients were found to have a positive overnight dexamethasone test whilst 45/63 (71%) patients had a positive low-dose dexamethasone test. 29 patients had both tests done of which 20/21 patients with a cortisol level >70 nmol/l after the overnight dexamethasone test had a positive low-dose dexamethasone test, whilst the other 8 patients, all of which had a cortisol level <70 nmol/l post-overnight dexamethasone test, had a negative low-dose dexamethasone test. Correlation analysis revealed a significant positive correlation between cortisol levels for both tests (r=0.78; P<0.001). Mean cortisol levels after each test were similar (103 vs 104.8 nmol/l; P=0.9).
Conclusion: We have established that in patients with a cortisol level of >70 nmol/l after an overnight dexamethasone test, the low-dose dexamethasone suppression test is usually positive and will not offer more information than the overnight dexamethasone suppression test in the diagnostic work-up of subclinical Cushings syndrome.