Clinical Biochemistry & Immunology Department Dynamic Function Tests
Title: Low Dose Dexamethasone Suppression Test
Master copy of this instruction resides on Q-pulse. Uncontrolled if in hard copy. Printed documents may become out of date – always check you are using the current revision.
Principle Dexamethasone is a potent cortisol analogue. In normal patients, administration of dexamethasone suppresses ACTH and therefore serum cortisol is suppressed. In Cushing’s there is incomplete suppression, therefore serum cortisol does not suppress normally. Dexamethasone does not interfere with the serum cortisol analysis in this test.
The dexamethasone suppression test is the test of choice as a screening test for patients suspected of Cushing’s syndrome. However, a positive test (i.e. failure to suppress cortisol secretion with dexamethasone) is not able to reliably differentiate Cushing’s syndrome and pseudo-Cushing’s syndrome.
Indications Assessment of Cushing’s syndrome and cortisol hypersecretion. Contraindications for this test This test may give unreliable results in patients on rifampicin, anticonvulsants or other enzyme inducing therapeutics.
Women on oestrogen therapy may give discrepant results due to increased cortisol binding globulins, although this may be corrected by higher dose dexamethasone.
Patient Preparation There should have been no therapy with corticosteroids, including topical preparations, for several weeks prior to the test. Required materials
Blood collection tube – clotted blood tube for serum
Sample Protocol
Patient takes dexamethasone 1mg tablet at home
Special Considerations Concurrent investigations e.g urinary free cortisol MUST be completed BEFORE the dexamethasone suppression test is started. Interpretation A normal response is the serum cortisol suppressing to <50nmol/L Clinical Biochemistry & Immunology Department Dynamic Function Tests
Title: Low Dose Dexamethasone Suppression Test
Master copy of this instruction resides on Q-pulse. Uncontrolled if in hard copy. Printed documents may become out of date – always check you are using the current revision.
Test Sensitivity & Specificity
Serum cortisol drops after 0900h and false positive suppression may occur if blood sampling is delayed.
Suppression in patients with Cushing’s is rare - 2% of Cushing’s patients will suppress.
Normal subjects rarely fail to suppress (2%).
False positives may occur due to the depression caused by reversible glucocorticoid resistance, and in patients with severe systemic diseases.
Patients undergoing renal dialysis may have a false positive due to increased dexamethasone clearance. Alcohol abuse may lead to rare instances of false positive results.
Simple obesity does not lead to an increase in false positive results.
References 1. Barth JH, Butler GE, Hammond P. Investigations in Laboratory Medicine, ACB Venture Publications,
2. Crapo L. Metabolism 1979; 28: 955-979 3. Wood P et al. Ann Clin Biochem 1997; 34: 222 - 229
Feeling better – Lifestyle management for chronic mental disorders In this module we have learned about three risk factors associated with poor physical health: overweight, lack of physical activity and smoking. All three factors are more common in patients with chronic mental disorders than in the general population and may be associated with a tangible reduction of life expectancy.
Clinical Biochemistry & Immunology Department 
Clinical Biochemistry & Immunology Department