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FormalPara Indication:

To assess suppressibility of cortisol in patients with an equivocal screening test such as overnight 1 mg examethasone (dex) suppression test, 24 h urinary free cortisol, and/or late night salivary cortisol.

FormalPara Preparation:

None.

FormalPara Materials Needed:

Eight dexamethasone 0.5 mg tablets

One gold top tube for cortisol

figure a
FormalPara Assay for Cortisol:

Chemiluminescence Immunoassay (CLIA) .

FormalPara Precautions:

None.

FormalPara Interpretation:

Serum cortisol concentration > 1.4–1.8 µg/dl after 2 day low dose dex is strongly suggestive of Cushing’s syndrome [1].

FormalPara Caveats:

  • Use of the 2 mg 2-day test has greater specificity at high sensitivity compared to the 1 mg overnight test. However, it requires more patience on the part of the patient [2, 3].

  • We do not recommend 24 h urine cortisol measurement during 2 mg dexamethasone suppression test (DST) because measurement of serum cortisol concentration during the low dose dexamethasone test is simpler and more reliable than measurements of urinary steroids [3].

  • Do not use this test if the patient is on estrogens which increase cortisol binding globulin (CBG) and falsely elevate cortisol levels [4].

  • Drugs such as phenytoin, phenobarbital, phenobarbitone, carbamazepine, rifampicin, and alcohol induce hepatic enzymatic clearance of dexamethasone, mediated through CYP 3A4, thereby reducing the plasma dexamethasone concentrations and may be associated with a false positive result [5].

  • To evaluate for false-positive and negative responses, some experts have advocated simultaneous measurement of both cortisol and dexamethasone during dexamethasone suppression tests to ensure adequate plasma dexamethasone concentrations.

FormalPara Procedure:

Completed as outpatient

  1. 1.

    Instruct patient to begin dexamethasone tablets. Patient takes one tablet every 6 h for a total of 8 doses (8 am, 2 pm, 8 pm, and 2 am). Some clinicians prefer a different schedule such as 6 am, 12 pm, 6 pm, and 12 am as a more convenient alternative. Studies were performed in the former schedule.

  2. 2.

    6 h after the last dose, draw blood for cortisol (8 am).

    • Cortisol level at the end of the test: ______________________

    • Date and time of the cortisol: ___________________________

Physician name and signature: ▁

RN performing the procedure: ▁

Additional orders by physician: ▁