The biological diagnosis of AD is actually performed by analysing Aβ1-40 and Aβ1-42 peptides, total tau and tau phosphorylated at Thr181 (p-Tau181) from cerebrospinal fluid (CSF) samples obtained by lumbar puncture (LP). Phospho-Tau 217 (p-Tau217) is a new biomarker that could be measured in plasma. The aim of this study is to compare the performances of plasma p-Tau217 with those of the reference CSF biomarkers in 150 patients recruited in the memory center of CHU Amiens (France). The study will be conducted during 18 months. The inclusion will be proposed to all patients for whom an indication of lumbar puncture / CSF is raised in a context of clinical suspicion of AD. During the daily hospitalization during which CSF will be collected for the "classical" biomarkers testing, a single tube of blood will be collected (anticoagulant EDTA, collected for p-Tau 217 analysis). The performances of CSF and plasma pTau 217 will be compared with the clinical diagnosis of AD (+, -, or indeterminate).

Inclusion Criteria:

• All patients addressed to the memory center of CHU Amiens with a clinical suspicion of AD, for whom lumbar puncture / CSF biomarkers of AD determination is proposed.

Exclusion Criteria:

• Minor patients
• pregnant women
• patients under guardianship