Abstract Context Diagnostic accuracy of testing currently used for the differential diagnosis of Cushing disease (CD) vs ectopic adrenocorticotropic hormone secretion (EAS) is difficult to interpret. Objective The present study aimed to identify and evaluate the diagnostic accuracy of the corticotropin-releasing hormone (CRH) test, the desmopressin test, and the high-dose dexamethasone suppression test (HDDST) when used to establish a CD or EAS diagnosis. Methods This study is a systematic review of the literature and meta-analysis. MEDLINE, OVID, and Web of Science databases were searched for articles published between 1990 and 2021. Articles included described at least 1 test(s) (CRH, desmopressin, or HDDST) and the diagnostic reference standard(s) (histopathology, petrosal sinus sampling, surgical remission, imaging, and long-term follow-up) used to establish a CD or EAS diagnosis. Results Sixty-two studies were included: 43 reported the use of the HDDST; 32, the CRH test; and the 21, the desmopressin test. The CRH test was found to have the highest sensitivity in detecting CD (ACTH 86.9%, 95% CI 82.1-90.6, cortisol 86.2%, 95% CI 78.3-91.5) and the highest specificity in detecting EAS (ACTH 93.9%, 95% CI 87-98.3, cortisol 89.4%, 95% CI 82.8-93.7). This resulted in a high diagnostic odds ratio (58, 95% CI 43.25-77.47), large area under the curve, and a receiver operating characteristic of 0.934. The diagnostic accuracy of the HDDST and desmopressin test was lower than that of the CRH test. Conclusion The meta-analysis indicates that a patient with a positive ACTH response after a CRH test is highly likely to have CD. Further studies analyzing role of dynamic testing in addition to imaging are needed.