Differentiating Cushing disease (CD) from non-neoplastic physiologic hypercortisolism remains a challenge[2]. The endocrine society guidelines in 2008 included the 2-day LDDST along with 24 hr UFC, late-night salivary cortisol, and 1 mg DST as one of the initial tests to evaluate patients suspected to have CS[1]. No single test was recommended over the others except for special situations [1, 14]. The same guidelines commented that the addition of CRH to 2-day DST may be considered in patients with equivocal results. A Cushing consensus guideline recommends using the Dex-CRH test in patients suspected of having non-neoplastic physiologic hypercortisolism [14].
The original 2-day LDDST was first described by Liddle[3] in 1960, where dexamethasone 0.5 mg is administered every six hours for eight doses, then measurement of 24 hour urinary 17-hydroxycorticosteroid is obtained. However, the use of serum cortisol is simpler and provides a higher diagnostic accuracy [6, 7]. A serum cortisol cutoff ranging from 1.4–1.8 ug/dL has been recommended as the pass criteria for this test, providing a diagnosis sensitivity of 90–95% [4, 8, 15]. The Dex-CRH test was first described by Yanovski et al.[8] in 1993, where he followed the original 2-day LDDST with CRH stimulation, and it showed 100% specificity, sensitivity, and diagnostic accuracy to differentiate CD from non-neoplastic physiologic hypercortisolism. The utility of the Dex-CRH test in this setting is based on the assumption that only patients with ACTH-dependent Cushing's syndrome will show a cortisol response to CRH after dexamethasone intake. The same group later revealed that the Dex-CRH test also differentiated patients with mild CD from healthy volunteers using the same criteria[16]. Subsequently, multiple other studies debated the diagnostic accuracy of the Dex-CRH test and showed varying degrees of specificity and sensitivity[9–13]. However, it is important to note that, in these studies, the authors used protocols for the Dex-CRH test that were different than the original description by Yanovski et al. [8]. The differences included timing of Dex administration starting at 09:00 instead of noon[9], using different cutoff levels for cortisol as a criterion 1.8 µg/dL[9, 11, 13] or 2.5 µg/dL [10] instead of 1.4 µg/dL, using human instead of ovine CRH[9, 13] or using ovine CRH but at a fixed dose of 100 µg instead of 1 µg/Kg with max dose 100 µg[11, 12]. These variations may have contributed to the differences in results observed[14]. In addition, some of the differences may be related to the cortisol assay used[9–13]. Furthermore, none of the above mentioned studies had dexamethasone measurement.
Our study protocol for Dex-CRH was identical to the initial description by Yanovski et al.[8]. Dex 0.5 mg was administered every six hours with last dose given at 6 AM, then serum cortisol was measured, and ovine CRH (1 µg/kg, max dose 100 µg) administered at 8 AM (2 hours after the last dose of Dex) and we used the same serum cortisol cutoff of > 1.4 µg/dL as a criterion in our study. Our study showed the following test performance for LDDST: sensitivity 92%, specificity 93%, and diagnostic accuracy 93%. The test performance was improved after adding CRH to the LDDST: sensitivity 100%, specificity 93%, and diagnostic accuracy 97%.
In our study, among 65 patients with Cushing disease (CD), 5 patients (7.7%) had a suppressed cortisol level < 1.4 µg/dL after the LDDST but were appropriately classified as Cushing disease with a cortisol level > 1.4 µg/dL at 15-min post Dex-CRH test. In contrast, 3/42 patients (7.1%) in the group where Cushing could not be confirmed (NCD) had an abnormal Dex-CRH test. In only one of these three patients, the LDDST was borderline normal (cortisol post-DEX was 1.4 µg/dL and increased to 3.1 µg/dL 15-min post CRH).
All patients had a Dex level > 167 ng/dL during the Dex-CRH test (range 167–1200 ng/dL). There were four patients who had a Dex level < 220 ng/dL, the minimum level suggested by the Endocrine Society as being adequate for the test to be valid [1]. One patient had a cortisol level of 12.9 and 16.1 µg/dL after LDDST and 15-min post Dex-CRH, respectively, and had pathology-proven CD. There were three other patients with Dex levels < 220 ng/dL. All had low cortisol levels < 1.4 µg/dL after both LDDST and Dex-CRH tests.
When only patients with available Dex levels were included in the analysis (n = 74), the sensitivity did not change, but the specificity and diagnostic accuracy of the Dex-CRH test further increased to 97% and 99%, respectively. This supports the value of measuring dexamethasone level during the Dex-CRH test. Others have shown an improvement in the diagnostic accuracy of dexamethasone suppression test [17], as false positive results can be seen in different settings related to dexamethasone such as rapid or malabsorption due to increased gut transit, chronic diarrhea or celiac disease, rapid metabolism due to concomitant use of CYP3A4 inducers (eg, phenobarbital, carbamazepine, St. John's wort) [14]. False positive results can also be seen due to increased cortisol binding globulin in the setting of estrogen use, pregnancy, or hepatitis which can increase total cortisol concentrations [14, 18].
Our study has limitations inherent to its retrospective design and a lack of dexamethasone measurement in all patients. No uniform protocol was used in the diagnostic workup of the patients suspected to have Cushing syndrome and their follow-up. The strength of the study includes the evaluation of the Dex-CRH test in more than 100 patients in a referral center, the long follow-up of at least 14 months in patients in whom Cushing disease could not be confirmed, and using the same protocol that was originally described by the NIH.