Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (844 page)

   Cortisol 1.4 μg/L is virtually 100% specific and 100% diagnostic for Cushing syndrome.

   Use

   Objectives

   To evaluate the cause of ACTH-dependent Cushing syndrome (with or without vasopressin analogs)

   To discriminate between pseudo-Cushing and Cushing syndrome

   To discriminate between primary and central adrenal insufficiency

   CRH stimulation test: The patient fasts for 4 hours or more, after which an intravenous access line is established and synthetic ovine CRH (1 μg/kg body weight or 100 μg total dose) is injected as an intravenous bolus. Blood samples for ACTH and cortisol are drawn 15 (or 5) and 0 minutes before and as often as 5, 10, 15, 30, 45, 60, 90, and 120 minutes after CRH injection. However, in Cushing syndrome, if one measures only the plasma ACTH response, the samples at −5, −0, 15, and 30 minutes are sufficient, and if one measures only the serum cortisol response, the samples at −15, 0, 45, and 60 minutes are sufficient. Normally both hormones should be measured, since the criteria for a positive response may include increases in either plasma ACTH or serum cortisol concentrations.

   CRH test after low-dose dexamethasone procedure: The patient takes 0.5 mg of dexamethasone every 6 hours for 2 days (a total of eight doses); 2 hours after the last dexamethasone dose is taken, 1 μg/kg of CRH is administered intravenously. Blood for a plasma cortisol measurement is drawn 15 minutes after the CRH injection.

   Interpretation

   Normal or exaggerated response: pituitary Cushing disease

   No response: ectopic ACTH-secreting tumor

   Limitations

   Responses to CRH are variable among subjects and from one time to another in the same subject.