The 2-day test is used to assess suppressibility in patients with an equivocal overnight test or in patients who have not had an overnight test.
Dexamethasone, 0.5 mg, is taken orally every 6 hours, usually at 8 am, 2 pm, 8 pm, and 2 am, for a total of eight doses.
Blood is drawn 2 or 6 hours after the last dose for measurement of cortisol.
Interpretation
The normal response to the 2-day test consists of the following:
Urinary cortisol excretion should fall to <10 Ž ¼g/24 hours on the 2nd day of dexamethasone administration.
Serum cortisol concentration is <5 Ž ¼g/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.