Endocrine Abstracts

The rising prevalence of obesity and type II diabetes makes the diagnosis of Cushing’s syndrome increasingly difficult. The dexamethasone-suppressed corticotrophin-releasing hormone (CRH) test, which involves CRH administration immediately following a standard low dose dexamethasone suppression test (LDDST), has been reported to diagnose Cushing’s syndrome with 100% sensitivity and specificity. We adapted our LDDST protocol to include CRH administration post-dexamethasone suppression. We investigated the diagnostic accuracy of this combined LDDST and CRH test (LDDST-CRH) and compared it to a standard LDDST by retrospectively studying 36 patients (including 12 confirmed cases of Cushing’s syndrome; 8 Cushing’s disease, 4 primary adrenal) who had undergone investigation for suspected Cushing’s syndrome.

The sensitivity of the standard LDDST in diagnosing Cushing’s syndrome was 100%, with a specificity of 88%. In contrast, although the LDDST-CRH diagnosed Cushing’s syndrome with 100% sensitivity, specificity was reduced at 67%. These results give a positive predictive value of 80% for the LDDST and 60% for the LDDST-CRH. Five subjects showed ‘discordant’ results whereby there was adequate suppression of serum cortisol following a standard LDDST, yet after CRH administration, serum cortisol exceeded the cut-off. Four subjects within this sub-group were amenable to repeat testing, and all subsequently ‘passed’ both the LDDST and LDDST-CRH tests without any intervention.

These data show that the combined LDDST-CRH test is less specific and accurate in the diagnosis of Cushing’s syndrome than was previously reported and does not improve the diagnostic accuracy of the LDDST.