Chemical Pathology


The overnight dexamethasone suppression test can be used as a screening procedure for Cushing's syndrome, but may not be appropriate in all patients.

Failure to suppress cortisol output will NOT diagnose Cushing's but cortisol supression will exclude the majority of patients thought to have the disease.

Patient preparation

  • No patient preparation is required for this out-patient test.
  • If the patient is also required to collect a 24h urine for cortisol, this should be completed before this test is done.
  • No contraindications have been reported but caution is warranted in patients with diabetes mellitus and active peptic ulceration.
  • Patients on enzyme inducing drugs e.g. anti-convulsants and rifampicin, may rapidly metabolise dexamethasone and give a false positive result.
  • Women on oestrogen therapy may fail to suppress adequately due to increased cortisol binding globulin. 24h urine cortisol is a better test in this situation, alternatively progress to a low dose dexamethasone suppression test.
  • Patients with depression, chronic alcohol abuse, those who have serious illness or who are on dialysis are likely to give false positive results.
  • Recently hospitalised patients and those with concurrent severe illness may give false positive results.

Materials required

1mg tablet (or 2 x 0.5 mg e.g. Decadron MSD) of dexamethasone to be taken orally.


  1. The patient is instructed to take 1mg dexamethasone orally at 23:00h (dose of dexamethasone for children is 0.3 mg/m2surface area)
  2. Take at least 5mL blood into a narrow gold top tube (or rust top for the Acute Unit) at exactly 09:00h the following morning.
5ml gold tube
  1. Send the blood sample at ambient temperature to the laboratory on the day of sample collection, for serum CORTISOL analysis.

Required information

Relevant clinical details including whether patient is hypertensive, any history of steroid treatment and particularly note on the request form that dexamethasone has been taken (state time and dose).

Turnaround times

The assays are run throughout the day and night.

The in-lab turnaround time is less than 24 hours.

Result Interpretation

A normal response is shown by suppression of serum cortisol to less than 50 nmol/L.

  • Poor absorption or increased metabolism of dexamethasone may also result in failure of suppression of cortisol levels.
  • Delay in collection of the blood sample may result in a false positive result as the pituitary-adrenal axis escapes from suppression.
  • Suppression in patients with Cushing's syndrome is rare with this test (2%). In the reported cases the patients have been shown to metabolise dexamethasone slowly and so achieve higher circulating levels than expected. If there is strong clinical or biochemical evidence for Cushing's syndrome then an endocrine referral is advised.

Further information

To learn more about Cushing's syndrome visit Endobible or Lab Tests Online

Also see "The diagnosis of Cushing's syndrome: An Endocrine Society Clinical Practice Guideline".

The diagnosis of cushing's syndrome: An Endocrine Society Clinical Practice Guideline

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