Dexamethasone suppression test

Investigation Protocol

Indications

Diagnosis or exclusion of Cushing’s syndrome. Usually performed following 2x24 hour urinary free cortisol collections and prior to admission for formal diagnosis. 1mg test may occasionally be sufficient in low probability patients, however a full low dose dexamethasone suppression test is usually required. The high dose test is performed in patients with confirned Cushing's to help distinguish between adrenal and other causes.

This test is also performed to aid the differential diagnosis of polycystic ovarian syndrome from autonomous androgen secreting tumours. 

Preparation and precautions

Patients can eat and drink normally and take all their usual medications prior to the test. 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.

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.

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. 

Procedure: Low dose dexamethasone suppression test

Day 1, 9am, base line blood test taken into a plain clotted tube for cortisol. In patients with virilisation, the blood will also be tested for other adrenal steroids: testosterone, dehydroepiandrosterone, 17hydroxyprogesterone and androstenedione as specified by the requesting clinician. The bottle and request form should be labelled 2+0 (2mg total dose over 24h, time=0).

The patient then takes their first dexamethasone tablet at 9am. The patient is given 9 tablets of 0.5mg dexamethasone altogether which includes this first supervised dose, and one spare tablet in case of mishap. The patient then takes the following tablets strictly eight hourly for the next 48 hours: at 3pm, 9pm, 3am and 9am. 

Day 3, 9am, patient returns for the second blood test labelled 2+48, having taken their last dose of dexamethasone at 3am that morning. In patients admitted to hospital, an extra blood test is taken at 24 hours, at 9am on day 2, prior to their fifth dose of dexamethasone and labelled 2+24. 

Procedure: High dose dexamethasone suppression test

The procedure for the high dose dexamethasone suppression test is identical to that for the low dose dexamethasone suppression test, except that each dose is 2mg rather than 0.5mg dexamethasone. Samples are labelled 8+0, 8+24, and 8+48 for 8mg total daily dose, at times 0, 24 and 48 hours. Since this test requires the patient to take high doses of steroids which may cause more severe side effects, this is usually supervised during a hospital admission.

Procedure: Overnight 1mg test

1mg dexamethasone is administered to the patient to be taken at home at 10pm. The patient is then advised to return for a blood test (plain clotted tube only for cortisol) at 9am the following morning. The tube must be clearly labelled as post 1mg dexamethasone.

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Patient information sheet

Dexamethasone suppression test

Your doctor has referred you for a low dose dexamethasone suppression test. This is to see if your body is making too much of its natural steroid hormones. The test starts with a blood test taken at 9am that morning. You can take all your normal medications and eat breakfast normally first, but it is very important that the test is done at 9am.

I will then give you some tablets called dexamethasone, which you take strictly every six hours for 48 hours: that is at 9am, 3pm, 9pm and 3am for two days in a row. This will involve setting your alarm clock for 3am both nights. The dexamethasone is a very small dose of steroid (0.5mg) and should switch off your body’s natural steroid hormones but will not affect your normal daily activities.

You then return for a blood test at the end of the 48 hours, again at 9am. The timings of this test are crucial, and if for any reason any of the tablets are taken at the wrong time, or the blood tests are not taken at 9am, the whole test will need to be repeated. We will usually arrange to admit you to hospital for this the second time round, or to repeat the test if the results are not completely normal.

You may be able to get the 9am blood tests done through your GP surgery. However since the timings are so critical, and this is a test requested by the hospital, it is usually best done in the unit. If you are very keen to have the blood tests performed at your GP surgery discuss this with your practice before the test is needed, to see if they may be able to help. 

If you have diabetes, this test might cause a temporary rise in your blood glucose levels. We will discuss this with you when you attend your appointment.

You may take your other medication as normal on the day of the test. However, if you take the contraceptive pill, hormone replacement therapy, or steroid tablets, nasal drops, inhalers, skin cream, or have recently had a joint injection, it is important that you inform us before the test date.

You can contact us either by writing to the address at the top, or by phoning the number given at the end of this sheet.

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