Lab Test

Dexamethasone Suppression of Cortisol Test, Overnight

Dexamethasone Suppression Test - Overnight, Serum Cortisol

Test Codes





Specimen should be drawn between 7:00 am and 8:00 am following oral intake of 1 mg dexamethasone between 11:00 pm and midnight the night before.

NOTE: The physician ordering the test MUST prescribe the 1 mg dexamethasone tablet.  This is not supplied by the Lab. 

Specimen Collection Criteria

Collect: One Gold-top SST tube. (Minimum Whole Blood: 4.0 mL)

Contact the Laboratory for acceptability of other tube types.

Physician Office/Draw Specimen Preparation

Let SST specimen clot 30-60 minutes then immediately centrifuge to separate serum from cells. Refrigerate (2-8°C or 36-46°F) the centrifuged SST tube within two hours of collection.

Preparation for Courier Transport

Transport: Centrifuged SST tube, refrigerated (2-8°C or 36-46°F). (Minimum: 1.0 mL)

Rejection Criteria

Specimens not collected and processed as indicated. Consult a pathologist prior to specimen rejection or test cancellation for any timed tests.

In-Lab Processing

Let SST specimens clot 30-60 minutes. Centrifuge SST tubes or Microtainers® to separate serum from cells. Deliver immediately to the appropriate testing station.


Specimen Stability for Testing:

Centrifuged SST Tubes and Microtainers® with Separator Gel
Room Temperature (20-26°C or 68-78.8°F): 8 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): Unacceptable

Red-top Tubes and Microtainers® without Separator Gel
Room Temperature (20-26°C or 68-78.8°F): 2-4 hours
Refrigerated (2-8°C or 36-46°F): Unacceptable
Frozen (-20°C/-4°F or below): Unacceptable

Serum Specimens (Pour-Overs)
Room Temperature (20-26°C or 68-78.8°F): 8 hours
Refrigerated (2-8°C or 36-46°F): 7 days
Frozen (-20°C/-4°F or below): 7 days

Specimen Storage in Department Prior to Disposal:

Refrigerated (2-8°C or 36-46°F): 7 days


Dearborn Chemistry Laboratory

Royal Oak Automated Chemistry Laboratory


Monday – Saturday.
Results and interpretation within 48 hours of receipt in the Laboratory.

Reference Range

Normal Dexamethasone Suppression Test: Serum Cortisol is suppressed to a value of less than 1.8 mcg/dL.

Test Methodology

Chemiluminescent Immunoassay for Cortisol.

The patient ingests 1 mg dexamethasone at between 11:00 pm and midnight. A blood sample is collected the following morning between 7:00 am and 8:00 am. 


If the cortisol level does not suppress, the patient should be investigated further. If the AM serum cortisol is less than 1.8 mcg/dL the patient most likely has a normally suppressible pituitary-adrenal axis. However, this test should not be used as the sole criterion to rule out Cushing syndrome. Additional investigations include 24 Hour Urine Free Cortisol or Salivary Cortisol (collected at midnight).

It should be noted that one percent of normal individuals, 13% of obese, about 25% of hospitalized or chronically ill patients, and women with increased cortisol binding proteins due to increased estrogens (e.g. pregnancy) give a false positive result. Less than 2% of false negative values are expected. Additional testing would include 24 Hour Urine Free Cortisol, Salivary Cortisol, and/or a 2 Day, Low Dose Dexamethasone Suppression Test.

Interference: 11-deoxycortisol (arising from metyrapone treatment) and cortisone have cross reactivities of 7.3 and 7.4 respectively. Prednisolone, 6-methyl-presnisolone and prednisone have cross reactivities of 27.0, 20.9, and 6.6% respectively.

Clinical Utility

This assay should be used as a screen for Cushing's syndrome. If positive, follow-up investigations are necessary to confirm.

CPT Codes



Last Updated


Microtainer® and Vacutainer® are registered trademarks of Becton, Dickinson and Company.
UroVysion® is a registered trademark of Abbott Laboratories. ThinPrep® is a registered trademark of Hologic, Incorporated.