Dexamethasone/Thyrotropin-Releasing Hormone Test (DST/TRH), Combined
Basic Information 
• Collect baseline blood sample (first sample).
• Administer dexamethasone (40 µg/kg IV) between 8 and 10 am and collect a blood sample 3 hours later (second sample).
• Administer 1 mg IV thyrotropin-releasing hormone (TRH) and collect an additional blood sample 30 minutes latter (third sample, 3.5 hours from dexamethasone administration).
• Collect last blood sample 24 hours after dexamethasone administration (fourth sample).
• The test is considered positive if cortisol concentration is less than 1 µg/dL at 24 hours or cortisol concentration increases by more than 66% 30 minutes after TRH administration.
• This test has a sensitivity of 88% and a specificity of 76%.
• Advanges: This method has increased sensitivity and specificity compared with the dexamethasone suppression test (DST) and TRH test administered separately.
• Disadvantages: Additional expenses and collection of multiple samples.
• Blood samples can be collected in ethylenediamine tetraacetic acid (EDTA) or plain blood collection tubes.
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