Dexamethasone suppression test and cortisol circadian rhythm in primary degenerative dementia.

Before the dexamethasone suppression test (DST) can be accepted as a valid diagnostic tool for differentiating depression from dementia, it must be demonstrated that dementing illnesses per se are not associated with a positive DST. The authors studied cortisol circadian rhythm and the overnight DST in 15 nondepressed patients with advanced primary degenerative dementia and 15 normal control subjects. Seven dementia patients and no control patients were DST positive. The DST-positive dementia patients had a blunted predexamethasone circadian cortisol rhythm. These results cast doubt on the utility of the DST in diagnosing depression that complicates advanced primary degenerative dementia.