Background: Cerebrospinal fluid (CSF) biomarkers have recently been included in the criteria for the diagnosis of Alzheimer's disease (AD). Since interpretation of CSF profile requires the combination of three parameters, biological data are not always conclusive and isolated elevation of phosphorylated tau (P-tau) or reduction of amyloid-beta (A beta)(42) alone can be observed. In these cases, A beta(42)/A beta(40) ratio could be more relevant than A beta(42) absolute values by considering inter-individual variations in the total amyloid load. Objective: The objective of this study was to assess the use of A beta(42)/A beta(40) ratio to improve the accuracy of biological conclusions in the diagnosis of patients with ambiguous CSF A beta(42) or tau results. Methods: Among 386 lumbar punctures analyzed in the lab in 2 years, 122 showed ambiguous biological data that were completed by CSF A beta(40) quantification and A beta(42)/A beta(40) ratio calculation. A biological conclusion was then made using 0.05 as the A beta(42)/A beta(40) ratio cut-off. Results: Our results showed that one-third of the biological profiles of patients with atypical dementia were ambiguous. The addition of A beta(42)/A beta(40) ratio increased the proportion of interpretable biological profiles from 69% to 87%, without changing the conclusion when usual biomarkers (A beta(42) and P-tau) were concordant. Conclusion: Our results support the use of the A beta(42)/A beta(40) ratio in addition to the usual CSF AD biomarkers for patients with ambiguous biological profiles. This method could be specifically directed to this population in order to improve the level of certainty for clinical routine practice.