Laboratory: Clinical Biochemistry  (AUTOMATED)
Test Name:
CORTISOL (OVERNIGHT DEXAMETHASONE SUPPRESSION TEST) - (P) and (S)
Test Code: None
Clinical Information:
Test Indications: The Dexamethasone Suppression Test (DST) is used in the evaluation of Cushing syndrome by assessing suppression of the hypothalamic-pituitary-adrenal (HPA) axis by exogenous corticosteroid (i.e. dexamethasone).


Procedure: This test is performed on an outpatient basis. Patient should take oral dexamethasone 1 mg at 2300h. The patient should be fasting after midnight (nothing to eat, drink or chew, except water). Obtain plasma cortisol level between 0700h and 0900h. Dexamethasone levels may be ordered by endocrinologist at the same time, please see Dexamethasone by LC-MS/MS
Collection Devices:
Specimen Required:


Plasma: 1.0 mL

Collection Information: 

1. Draw blood between 7:00 a.m. and 9:00 a.m. the morning following an evening dose of dexamethasone.

2. Centrifuge plasma/serum within one hour of collection.

3. Aliquot (if not tested onsite)

4. Freeze for shipping if analysis will not be completed in 24 hrs.

Special Processing: Laboratory must add comment to the test: “Cortisol post Dexamethasone.”

Specimen Stability:

Ambient: 24 hours

Refrigerated: 4 days

Frozen: 12 months, freeze only once

Test Code: None (see Cortisol – (P) entry)
Referral:
Serum: 1.0 mL

Shipping & Storage: Store and ship frozen. Avoid repeated freeze-thaw cycles.
Requisition:
Reference Values:
Reference Intervals: 

<50 nmol/L autonomous cortisol secretion excluded (95% Sensitivity, 80% Specificity for CS)

 50-138 nmol/L possible autonomous cortisol secretion; further investigation required

 >138 nmol/L autonomous cortisol secretion confirmed

Method of Analysis: Competitive ECLIA Roche immunoassay
Availability:
Daily
See Also:
More Information:
Interpretation & Assay Interferences: Multivitamins (45–125 µg biotin) or biotin-only supplements up to 1 mg per day do not interfere with cortisol assay. Samples should not be taken from patients receiving therapy with high biotin doses (i.e. >5 mg/day) until at least 8 hours after the last dose.

False-positive results might be seen with rapid absorption or malabsorption of dexamethasone due to increased gut transit time, chronic diarrhea, coeliac disease, increased cortisol-binding globulin or CYP3A4 inducing medications such as phenobarbital & carbamazepine. Measuring dexamethasone concomitantly with cortisol can reduce the risk for false-positive results.

False-negative results are less common and typically result from decreased CBG due to nephrotic syndrome or the presence of medications such as fluoxetine or cimetidine that inhibit dexamethasone metabolism.