The limited utility of the dexamethasone suppression test for the diagnostic process in psychiatry

In 231 psychiatric in-patients, the 1 mg or 1.5 mg DST with blood samples at 0900 h, 1600 h and 2300 h and a post-dexamethasone plasma cortisol threshold of greater than or equal to 5 micrograms/dl were tested for their differential diagnostic utility in clinical psychiatry. Neither test significantly separated endogenous depressed patients from patients with other depressive or non-depressive psychiatric disorders. Studies of the 1 mg or 1.5 mg DST in 75 healthy subjects revealed about 12% of cortisol non-suppressors, when a post-dexamethasone cortisol threshold of greater than or equal to 5 micrograms/dl was used. This seemed to be an unacceptably low specificity of the test in normal subjects. A threshold criterion of greater than or equal to 8 micrograms/dl, however, yielded only 2.7% of non-suppressed normal subjects. Analyses of the DST data of the psychiatric patients, using a cortisol threshold of greater than or equal to 8 micrograms/dl, also failed to reveal a significantly higher specificity of the DST for endogenous depression. However, it was demonstrated that intervening variables such as stress due to hospital admission, drug withdrawal, suicidal turmoil, weight loss, as well as a low dexamethasone plasma level, enhance the rate of abnormal DST results in psychiatric in-patients, regardless of their diagnostic classification.