| Laboratory: | Clinical Biochemistry (AUTOMATED) | ||||
| Test Name: |
CORTISOL (OVERNIGHT DEXAMETHASONE SUPPRESSION TEST) - (P) and (S)
Test Code:
None
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| 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).
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| Collection Devices: |
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| 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) |
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| Referral: |
Serum: 1.0 mL
Shipping & Storage: Store and ship frozen. Avoid repeated freeze-thaw cycles.
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| 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 |
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| Availability: |
Daily
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| 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. |