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Test Code S2923 Dexamethasone, Overnight, Suppression Test, Plasma

Performing Laboratory

Barnes-Jewish Hospital Laboratory


Chemiluminescent Microparticle Immunoassay (CMIA)

Literature References:

1. Package insert: Cortisol reagent. Abbott Diagnostics


Specimen Requirements

Specimen Type: Plasma

Container/Tube: Mint green top (lithium heparin)

Specimen Volume: Full tube

Collection Instructions:

1. Give patient 1 mg of dexamethasone by mouth between 2300 and 2400.

2. Draw specimen at 0800.

Additional Information:

1. Prednisone and prednisolone can interfere with this assay by causing a false-positive reaction.

2. Test performed is #2923 Cortisol, Plasma.

Day(s) Test Set Up

Monday through Sunday


Turnaround Time:
STAT: 2 hours after receipt in laboratory 

Routine: 4 hours after receipt in laboratory

Reference Values

0800: <5 mcg/dL
An abnormal test does not establish a diagnosis of Cushing’s syndrome.

In conditions with elevated levels of cortisol binding globulin (pregnancy, estrogen therapy, hyperthyroidism), a false-positive test is likely and use of another screening test (eg, #82948 Cortisol/Cortisone, Free, Urine) is preferable.

A false-positive test may occur because of:

1. Low dexamethasone levels—noncompliance or increased metabolism of dexamethasone (eg, phenytoin, phenobarbital)

2. Interference with cortisol assay (eg, prednisone)

3. Temporary loss of suppressibility (eg, depression, alcoholism)

Test Classification and CPT Coding


Additional Information

For BJH Laboratory Use Only
Alternate Tube:

1 red-top tube

Dark Green tube

Separator gel tube is acceptable

Analyte Stability:

See Analyte Stability Chart in Special Instructions.

Laboratory Processing Instructions:

Test performed in BJH Core Lab. Test performed is #2923 Cortisol, Plasma.  Processed by automated line for delivery to the Architect.  Completed specimens are stored refrigerated for 5 days.

Specimen Transport Temperature


Special Instructions