The cerebrospinal fluid amyloid beta42/40 ratio in the differentiation of Alzheimer's disease from non-Alzheimer's dementia.

Background: Amyloid beta(40) (Abeta(40)) is the most abundant Abeta peptide in the brain. The cerebrospinal fluid (CSF) level of Abeta(40) might therefore be considered to most closely reflect the total Abeta load in the brain. Both in Alzheimer's disease (AD) and in normal aging the Abeta load in the brain has a large inter-individual variability. Relating Abeta(42) to Abeta(40) levels might consequently provide a more valid measure for reflecting the change in Abeta metabolism in dementia patients than the CSF Abeta(42) concentrations alone. This measure may also improve differential diagnosis between AD and other dementia syndromes, such as vascular dementia (VaD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD).
Objective: To investigate the diagnostic value of the CSF Abeta(42)/Abeta(40) ratio in differentiating AD from controls, VaD, DLB and FTD.
Methods: We analysed the CSF Abeta(42)/Abeta(40) ratio, phosphorylated tau(181) and total tau in 69 patients with AD, 26 patients with VaD, 16 patients with DLB, 27 patients with FTD, and 47 controls.
Results: Mean Abeta(40) levels were 2850 pg/ml in VaD and 2830 pg/ml in DLB patients, both significantly lower than in AD patients (3698 pg/ml; p<0.01). Abeta(40) levels in AD patients were not significantly different from those in controls (4035 pg/ml; p=0.384). The Abeta(42)/Abeta(40) ratio was significantly lower in AD patients than in all other groups (p <0.001, ANCOVA). Differentiating AD from VaD, DLB and non-AD dementia improved when the Abeta(42)/Abeta(40) ratio was used instead of Abeta(42) concentrations alone (p<0.01) The Abeta(42)/Abeta(40) ratio performed equally well as the combination of Abeta(42), phosphorylated tau(181) and total tau in differentiating AD from FTD and non-AD dementia. The diagnostic performance of the latter combination was not improved when the Abeta(42)/Abeta(40) ratio was used instead of Abeta(42) alone.
Conclusion: The CSF Abeta42/Abeta40 ratio improves differentiation of AD patients from VaD, DLB and non-AD dementia patients, when compared to Abeta42 alone, and is a more easily interpretable alternative to the combination of Abeta42, p-tau and t-tau when differentiating AD from either FTD or non-AD dementia.