Principle

Dexamethasone, a potent glucocorticoid, suppresses pituitary production of ACTH by the negative feedback effect of corticosteroids on the hypothalamus and pituitary. As dexamethasone does not interfere with the routine methods used for measuring plasma cortisol, a reduction of ACTH output can be assessed by the fall in cortisol levels

Indications

1. Screening test for cortisol excess/Cushing's syndrome
2. Investigation of adrenal incidentalomas

Preparation

Ask the patient's GP to prescribe 1mg of dexamethasone and request blood sampling via the investigation and treatment room, if required, requesting a 09:00 appointment.
Enter “Dexamethasone suppression test” into 'brief clinical details' box.

Protocol and sampling

Advise patient to take 1mg dexamethasone orally at 23:00 hours prior to the blood sample at 09:00 hours the following morning for cortisol (brown gel tube) measurement.

Ensure the blood tube and form are correctly labelled and clearly state “Dexamethasone suppression test” on the request form to avoid confusion for the clinical biochemistry department.

Interpretation

• Serum cortisol should suppress to <50 nanomol/L to exclude Cushing’s syndrome.
• Lack of suppression suggests cortisol excess or a stressed patient. Further assessment may be indicated.
• False positive results can occur in more than 10% of patients.
  Common reasons include: intercurrent illness, oestrogen HRT/COCP (increases CBG and plasma cortisol) and patients on enzyme inducing drugs (eg, carbamazepine, rifampicin) which speed up the metabolism of dexamethasone.