Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (895 page)

   Overnight screening test is a quick screening test for nonsuppressible cortisol production and subclinical or clinical Cushing syndrome and should not be used as the sole criterion for excluding the diagnosis of Cushing syndrome.

   Use

   Dexamethasone (1 mg) is taken orally between 11
PM
and midnight, and a single blood sample is drawn at 8
AM
the next morning for assay of serum cortisol.

   Interpretation

   The 2008 Endocrine Society Guidelines suggest a diagnostic serum cortisol criterion of 1.8 μg/dL.

   This test has a significant false-positive rate when sensitivity is maximized. Using a serum cortisol criterion of <3.6 μg/dL, the test has a 12–15% false-positive rate. If, however, the criterion for suppression of serum cortisol is increased to <7.2 μg/dL, the false-positive rate falls to 7%. This suggests that the multiple criteria may be useful in interpreting the test.

   The salivary cortisol concentration at 8
AM
after 1 mg dexamethasone given at midnight was 0.8 ± 0.4 ng/mL (range 0.6–1.1 ng/mL) in 101 normal subjects, a sensitivity and specificity of 100%.

LOW-DOSE TEST: STANDARD 2-DAY (2-mg) TEST

   Use

   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: