What are the tests available for differentiating pseudo-Cushing’s from Cushing’s syndrome?

The tests that are used to rule out pseudo–Cushing’s syndrome are essentially screening tests for Cushing’s syndrome with a relatively greater specificity for identifying Cushing’s syndrome.

• Low-dose dexamethasone test: In conditions where the probability of pseudo–Cushing’s disease is high, LDDST may be an optimal screening test as it has greater specificity as compared to overnight dexamethasone test or 24 hours urine cortisol levels. In the LDDST, dexamethasone (0.5 mg) is given every 6 hours for eight doses and the serum cortisol assessed after that. It is important to acknowledge that conditions that alter cortisol-binding globulin levels can interfere with the test results as are ingestion of drugs which alter dexamethasone metabolism. With a cut-off value of 1.8 mg/dL, the sensitivity ranges from 90% to 95%. Not all studies that have explored LDDST have reported similar results, but of the tests available, LDDST is preferred first line test.

• Midnight serum cortisol: Midnight serum cortisol value >7.5 ug/dL is diagnostic of Cushing’s syndrome. In a study by Papanicolaou, et al. using this cut-off correctly identified 225/234 patients with Cushing’s syndrome, while a value less than this cut-off was found in all 23 patients with pseudo-Cushing’s states. Thus, the specificity was 100% and the sensitivity was 96%. Where there is a low clinical index of suspicion, such as in simple obesity, but lack of suppression on dexamethasone testing and mildly elevated UFC, a sleeping midnight serum cortisol less than 1.8 mg/dL effectively excludes Cushing’s syndrome at the time of assessment.

• Midnight salivary cortisol: One study examined 11 PM salivary cortisol in 39 patients with proven Cushing’s syndrome. The average 11 PM salivary cortisol was 20 times higher in the patients with Cushing’s syndrome than the other two groups. Using a cut-off of 3.6 nmol/L (0.13 μg/dL), 36/39 patients with Cushing’s syndrome had an elevated value, while 38/39 of the normal volunteers had values less than the cut-off. The sensitivity and specificity of this test was 92% and 96%, respectively. The ability to assay cortisol from saliva is not widely available in India.

• Dexamethasone-CRH test: The dexamethasone test takes advantage of the fact that in patients with Cushing’s syndrome, dexamethasone ineffectively suppresses the production of pituitary ACTH. CRH stimulates the pituitary to secrete ACTH which leads to an increase in cortisol levels. Patients with Cushing’s syndrome have a larger increase in plasma ACTH and cortisol levels than in normal individuals or those patients with pseudo-Cushing’s states after injection of CRH. In one study 39 patients with Cushing’s syndrome and 19 patients with pseudo-Cushing’s state underwent the combined test. Patients received dexamethasone (0.5 mg) 4 times a day for 2 days starting at 12 noon (last dose at 6 am). At 8 am on the day of the last dose, the patients received intravenous ovine CRH (1 μg/kg) and cortisol and ACTH were measured at various times. 

A plasma cortisol greater than 1.4 μg/dL (38 nmol/L) measured 15 minutes after the CRH injection correctly identified all patients with Cushing’s syndrome, while a value less than 1.4 μg/dL identified all patients with pseudo-Cushing’s states (100% sensitivity and specificity). Of course, CRH is difficult to procure in India.

• Loperamide test: Loperamide decreases ACTH and cortisol levels. The opiate agonist is given at a dose of 16 mg at 8:30 AM and 3 samples (basal, 180 and 210 min after drug) are obtained. In 41 patients with confirmed Cushing’s syndrome, loperamide did not suppress the cortisol levels below 138 nmol/L (5 μg/dL), while in 104 of 110 patients referred for evaluation of Cushing’s syndrome, which was subsequently ruled out, the cortisol value suppressed to less than 138 nmol/L (5 μg/dL) at either 150 or 210 min. This test has not been validated nor routinely used in our clinical setting. Other tests that have been used but not recommended are Insulin tolerance test, IL-6 test and desmopressin test.