Dexamethasone Supression Test

Indications

Overnight Low Dose Dexamethasone Suppression Test

To screen for Cushing’s Syndrome

Low Dose Dexamethasone Suppression Test

Confirmation of Cushing’s Syndrome

High Dose Dexamethasone Suppression Test

To rule out pituitary dependent Cushing’s Disease

Patient Preparation

1. Estrogen containing medications, including the contraceptive pill and hormone replacement therapy, should be stopped for six weeks prior to measuring serum cortisol. This is because estrogen induces cortisol binding globulin and leads to elevations in measured serum cortisol. Any steroid containing medications should also be documented, and avoided if possible, as they may interfere with the hypothalamo- pituitary-adrenal axis or cross-react with the cortisol assay.

2. Care is needed in patients with suspected or active peptic ulcer disease (referring physician may prescribe proton pump inhibitor prior to test). Care is also needed in patients with diabetes mellitus, as blood sugars may rise during test. Referring clinician will advise patient whether they need to increase dose of insulin, or just test blood glucose more frequently during test.

3. Potent inducers of the cytochrome P450 system accelerate the clearance of dexamethasone and so invalidate this test. Patients taking rifampicin, phenytoin, phenobarbitone and carbamezepine should therefore be discussed with the endocrinologist, before proceeding with the test as interpretation will be difficult.

Sample Labeling

Label plain tube with two identifiers (Name and identity no - NRIC/MRN) and label clearly test requested: Overnight Low Dose Dexamethasone Suppression Test, Low Dose Dexamethasone Suppression Test or High Dose Dexamethasone Suppression Test and sample type- baseline, Day 2 or Day 3. All samples must be sent to the lab immediately after blood sampling accompanied by the completed form.

Sample Collection

Overnight Low Dose Dexamethasone Suppression Test 

Day 1

Blood drawing for cortisol baseline at 8-9 a.m. Dexamethasone 1 mg is administered orally between 11p.m to 12a.m. (optional)

Day 2

Serum cortisol levels are drawn on 8-9 a.m.

Low Dose Dexamethasone Suppression Test 

Day 1

Blood drawing for cortisol baseline at 8-9a.m. Immediately after sampling, Dexamethasone 0.5mg is administered orally every 6 hours for 48 hours.  (optional)

Day 2

Continuously Dexamethasone 0.5mg is administered orally every 6 hours.

Day 3

Serum cortisol levels are drawn on 8-9a.m.

High Dose Dexamethasone Suppression Test 

Day 1

Blood drawing for cortisol baseline at 8-9a.m. Immediately after sampling, Dexamethasone 2mg is administered orally every 6 hours for 48 hours.

Day 2

Continuously Dexamethasone 2mg is administered orally every 6 hours.

Day 3

Serum cortisol levels are drawn on 8-9a.m.