Table 3.
Secondary testing to differentiate pathologic Cushing syndrome from nonneoplastic physiologic hypercortisolism (pseudo-Cushing syndrome)
| Test | Basis | Technique | Interpretation of results | Sensitivity | Specificity |
|---|---|---|---|---|---|
| DDAVP stimulation | Corticotroph adenomas have vasopressin receptors. DDAVP will stimulate ACTH secretion in Cushing disease, but response typically absent in physiologic hypercortisolism | Measure ACTH and cortisol levels before and after DDAVP stimulation (10 μg IV) | Increase in ACTH > 6 pmol/L supports diagnosis of Cushing disease (note: there are no universally agreed criteria partly because of different ACTH assays) | 75%-87% | 90%-91% |
| Dexamethasone-CRH testing | Hypercortisolism in pseudo-Cushing states is thought to be mediated by CRH and has diminished response to administration of CRH and greater inhibition of cortisol production by glucocorticoids compared with Cushing syndrome | Dexamethasone (0.5 mg every 6 h) given orally for 8 doses, with subsequent administration of CRH (1 μg/kg) in the morning 2 hours after last dexamethasone dose, with measurement of ACTH and cortisol levels pre- and post-CRH administration | Serum cortisol > 1.4 μg/dL (or 38 nmol/L) in response to CRH supports diagnosis of Cushing syndrome | 88%-100% | 50%-100% |