Dexamethasone

General Information

Dexamethasone suppression tests are typically conducted as part of the front line investigations for Cushing's syndrome. Despite having a specificity of 80%, dexamethasone suppression tests can be subject to false positive results due to altered dexamethasone absorption, clearance, cytochrome P450 induction or inhibition or non-compliance with the testing protocol which complicate interpretation.

Simultaneous measurement of dexamethasone and cortisol in serum can assist with identification of false positive dexamethasone suppression tests. When dexamethasone is >3.3 nmol/L, a cortisol of ≤50 nmol/L is typically observed in patients without Cushing's syndrome. Dexamethasone levels of ≤3.3 nmol/L indicate that insufficient dexamethasone is present in circulation to suppress cortisol and a corresponding cortisol ≥50 nmol/L may be a false positive result. In these cirucmstances the test may need to be repeated or an alternative investigation for Cushing's syndrome conducted eg 24 hour urine cortisol.