Introduction: Several recent research studies show high performance of blood biomarkers to identify Alzheimer's disease also in the pre-dementia mild cognitive impairment (MCI) stage, but data from the routine clinical care memory clinic setting are needed. Methods: We examined plasma samples of 144 memory clinic patients, including dementia of Alzheimer type (DAT, n = 54), MCI (n = 57), and subjective cognitive decline (SCD, n = 33), who either presented as self-referrals or were referred by general practitioners or neurologists or psychiatrists. The plasma biomarkers, amyloid-beta42 (Ass42), amyloid-beta40 (Ass40), phospho-Tau181 (pTau181), total-tau (tTau), and neurofilament light (NFL), as well as different ratios, were measured using the ultrasensitive single molecule array (Simoa) immunoassay technology. Statistical analysis including Kruskal-Wallis test, linear regression, and receiver operating characteristics analyses was performed. Results: Of the single markers, we observed statistically significant group effects of pTau181 (H(2) = 34.43, p < 0.001) and NFL (H(2) = 27.66, p < 0.001). All individual group comparisons of pTau181 were significant, while the contrast of SCD versus MCI for NFL was not significant. In addition, the ratios of Ass42/Ass40 (H(2) = 7.50, p = 0.02) and pTau181/Ass42 (H(2) = 25.26, p < 0.001) showed significant group effects with significant difference between all groups for pTau181/Ass42 and an SCD versus MCI difference for Ass42/Ass40. PTau181 showed the highest area under the curve of 0.85 for the discrimination of SCD and DAT with a sensitivity of 80% and a specificity of 79% at a cut-off of 12.2 pg/mL. Age influenced Ass42, Ass40, and NFL concentrations. Conclusion: Plasma pTau181 and NFL, as well as the ratios Ass42/Ass40 and pTau181/Ass42, are biomarkers, which can differentiate diagnostic groups in a memory clinic setting outside of research studies.
