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Subject: Low-Dose Test: Overnight 1-mg SCREENING Test
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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.
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.
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%.