Pathology tests database

Sample and container requirements

Serum

Minimum volume required

1 mL serum

Special requirements

None

Patient preparation

For the 1 mg dexamethasone suppression test, 1 mg dexamethasone is given between 23:00 and 24:00. Samples are collected between 08:00 and 09:00.

Sample viability

Turnaround time

21 days

Reference range

>3.3 nmol/L is sufficient to suppress cortisol to ≤50 nmol/L (Source: Hawley et al., 2018, doi: 10.1177/0004563218766566)

Clinical guidance

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 e.g. 24 hour urine cortisol.

Notes