Diagnostic Use
Suppression of post-dexamethasone morning cortisol concentration to below 50nmol/L is considered a normal response, practically excludes Cushing’s syndrome.
The test should be performed in an outpatient setting when the patient is in a more settled state – as up to 35% of stressed (hospitalised) patients may fail to suppress. The test is reliable in obesity, but up to 25% of patients who have recently undergone substantial weight reduction may fail to suppress. Alcoholism and depression are confounding conditions : 30-50% of patients suffering depression fail to suppress; chronic alcoholism may present as pseudo-cushings and distinguishing this from true Cushing’s syndrome may require further testing.
Medications such as rifampicin, phenytion, carbamazepine, phenobarbitol can induce hepatic clearance of dexamethasone, resulting in lower circulating level for it to suppress cortisol production, creating a false positive picture. Occasionally clinicians might choose to use 2mg dexamethasone (instead of 1mg) as the dose challenge for this reason or for patients with significant obesity. With such potential drug-drug interaction or possibility of inadequate dexamethasone oral absorption due to vomiting or being accidentally left out in the evening of the test, the plasma from which the morning cortisol was measured will be automatically sent to LabPLUS for “Dexamethasone” test once the cortisol level is >=50nmol/L (see LabPLUS testguide under “Dexamethaone – plasma/serum” entry).
Patients on oral contraceptive pills will have a raised basal cortisol level due to induction of cortisol binding globulin production. Whether it will create more false positives from the dexamethasone suppression test is debatable.
Reference Intervals
If the 9am cortisol from the morning after 1mg overnight dexamethasone is:
<50nmol/L: normal suppression and excludes Cushing’s syndrome.
>=50 – <=100nmol/L: may be normal but is also consistent with early Cushing’s syndrome
>100nmol/L: failure of normal suppression: causes include Cushing’s syndrome, alcohol, depression, and stress.
Test Method
Refer to Cortisol (Blood)