Low-Dose Test: Overnight 1-mg SCREENING Test

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

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Definition

  • 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%.

 
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