The cerebrospinal fluid (CSF) biochemical markers (biomarkers) Amyloidβ 42 (Aβ ), total Tau (T-tau) and Tau phosphorylated at threonine 181 (P-tau ) have proven diagnostic accuracy for mild cognitive impairment and dementia due to Alzheimer's Disease (AD). In an effort to improve the accuracy of an AD diagnosis, it is important to be able to distinguish between AD and other types of dementia (non-AD). The concentration ratio of Aβ to Aβ (Aβ Ratio) has been suggested to be superior to the concentration of Aβ alone when identifying patients with AD. This article reviews the available evidence on the use of the CSF Aβ ratio in the diagnosis of AD. Based on the body of evidence presented herein, it is the conclusion of the current working group that the CSF Aβ ratio, rather than the absolute value of CSF Aβ , should be used when analysing CSF AD biomarkers to improve the percentage of appropriately diagnosed patients.