The prevalence of Mild Cognitive Impairment (MCI) is about 15%-17%. 10%-15% of MCI progresses to Alzheimer's disease (AD) every year. The annual incidence of MCI in the normal elderly is about 1%. Peripheral Blood biomarkers is the key and difficult points in AD research. Except expensive brain β amyloid plaque imaging, few breakthroughs of early diagnosis technology of MCI due to AD can be made to facilitate clinical application. Even Tau-181 and Tau-217 were reported in this year on Lancet neurology and JAMA. We also need to study on the biomarkers upstream of pathological changes about senile plaque. The purpose of this program is to study the reliability and validity of plasma miRNAs for early diagnosis of MCI due to AD and other dementia such as DLB and FTLD. The clinical diagnosis of AD and MCI due to AD are according to the National Institute of Aging and the Alzheimer's Disease Association (NIA-AA) diagnostic criteria in 2011. Plaque imaging is used to be golden criteria for the diagnosis of AD and MCI due to AD.

Inclusion Criteria:

• 2011 NIA-AA criteria of MCI due to AD or AD or - 2017 Diagnosis and management of dementia with Lewy bodies: Fourth consensus report of the DLB Consortium or - 2001 the McKhann consensus clinical criteria for FTD (McKhann et al.)

Exclusion Criteria:

• Dementia caused by infection or substance
• Thyroid disease

NC, AD, DLB, FTD and MCI due to AD

According to the approval of IRB.