47yo man presents to your clinic to establish care. He has a history of resistant hypertension, DMII, and sleep apnea. Vital signs are BP-159/101, HR-74, RR-16, O2-100%, and temp-98.9. Physical examination is also significant for multiple bruises on the lower extremities.
- What would be the next step in the diagnosis of this patient?
- What else would you need to order to determine the cause of this patient’s condition?
The initial SCREENING test of choice for Cushing Syndrome is a low-dose dexamethasone overnight suppression test. For this, 1mg dexamethasone is given around midnight and a serum cortisol is measured at 8am. A positive result is cortisol level of 1.8 mcg/dL.
The CONFIRMATORY test of choice for Cushing Syndrome is a 24-hour urinary cortisol excretion. A positive finding would be levels that are 3x the upper limit of normal for the assay used.
Once the diagnosis is made, the cause of the hypersecretion needs to be determined. For this, ordering a serum ACTH and high dose dexamethasone suppression test will help differentiate the various causes of the hypersecretion.
- Findling JW, Raff H, Aron DC. The low-dose dexamethasone suppression test: a reevaluation in patients with Cushing’s syndrome. The Journal of clinical endocrinology and metabolism. 2004; 89(3):1222-6. [pubmed]
- Dichek HL, Nieman LK, Oldfield EH, Pass HI, Malley JD, Cutler GB. A comparison of the standard high dose dexamethasone suppression test and the overnight 8-mg dexamethasone suppression test for the differential diagnosis of adrenocorticotropin-dependent Cushing’s syndrome. The Journal of clinical endocrinology and metabolism. 1994; 78(2):418-22. [pubmed]