The combined measurement of Ab42 and tau protein (total and phosphorylated) in the spinal fluid has been shown to be promising in the diagnosis of Alzheimer's disease (AD), and has justified its inclusion new diagnostic criteria. However, it can sometimes yield discordant results that are not discriminant (isolated variation in Ab42 or P-181 Tau). To answer this challenge, a new marker has been developed in recent years, namely amyloid beta-peptide 1-40 (Aβ40). This marker reflects the patient's total amyloid deposits and is used to calculate the Aβ42/Aβ40 ratio. This ratio measures the relative variation of Aβ42 as compared to the total amyloid burden. Literature data on this topic are sparse and to date, no report has been published evaluating the utility of this marker in the diagnostic strategy for AD.

Inclusion Criteria:

• Patients attending the Memory Clinic of Champagne-Ardennes for memory disorders, and
• Whose diagnostic work-up requires assessment of biomarkers present in the cerebrospinal fluid are considered eligible for inclusion in this study.

Exclusion Criteria:

• Patients who do not have any social security coverage will not be included.