Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (896 page)

   Urinary cortisol excretion should fall to <10 μg/24 hours on the 2nd day of dexamethasone administration.

   Serum cortisol concentration is <5 μg/dL, a plasma ACTH concentration is <5 pg/mL, and a serum dexamethasone concentration is between 2.0 and 6.5 ng/mL.

   In a recent meta-analysis, the 1-mg test and the 2-day 2-mg test were both accurate, but the 2-mg test had slightly less diagnostic accuracy.

HIGH-DOSE TEST: OVERNIGHT (8-mg) TEST

   Use

   Dexamethasone (8 mg) is taken orally between 11
PM
and midnight, and a single blood sample is drawn at 8
AM
the next day for measurement of serum cortisol.

   With this protocol, the 8
AM
serum cortisol concentration is <5 μg/dL in most patients with Cushing disease (i.e., a pituitary tumor) and is usually undetectable in normals.

HIGH-DOSE TEST: STANDARD 2-DAY (8-mg) TEST

   Use

   The patient collects at least one baseline 24-hour urine, at 8
AM
.

   The patient begins taking 2 mg of dexamethasone orally every 6 hours for a total of 8 doses, usually at 8
AM
, 2
PM
, 8
PM
, and 2
AM
, and the urine collections are continued.

   In practice, this test is often performed immediately after completing the low-dose dexamethasone suppression test (if the test is positive).

   The urine collections are assayed for urinary free cortisol and creatinine. In addition, a blood specimen can be collected 6 hours after the last dose of dexamethasone for measurement of cortisol, dexamethasone, and ACTH.

   This protocol leads to the following values in normal subjects:

   Urinary free cortisol excretion is <5 μg/24 hours.