Increased tau protein differentiates mild cognitive impairment from geriatric depression and predicts conversion to dementia.

Significantly increased cerebrospinal fluid (CSF) total tau (t-tau) and phosphorylated at threonine 181 tau (p-tau) levels were frequently found in patients with mild cognitive impairment (MCI) who have an increased risk of developing Alzheimer's disease (AD). Though MCI often overlaps with depressive symptoms making early diagnosis difficult, to date no CSF marker has been probed to support the differential diagnosis of geriatric major depressive disorder and MCI eventually converting to AD. CSF levels of t-tau and p-tau were determined by ELISA in 80 subjects with MCI (aging associated cognitive decline criteria), 54 patients with major depression and 24 cognitively healthy controls. Patients were reassessed after a follow-up period of at least 12 month. During follow-up, 29% of the MCI patients but only one patient with major depression converted to AD. Already at baseline converters to AD were characterized by significantly higher t-tau and p-tau levels compared to non-converters and the other diagnostic groups. Our findings demonstrate that both, CSF t-tau and p-tau levels facilitate the differential diagnosis of MCI and are of prognostic value.