New SPRi Biosensors for Simultaneous Detection of Tau Protein Isoforms—The Importance of The ptau181/Total Tau Ratio in Alzheimer's Disease

Tau protein is a nonspecific marker of neurodegeneration, and its phosphorylated form, ptau-181, is specifically associated with Alzheimer's disease (AD). Calculating the ratio of the phosphorylated form to total tau protein can help distinguish AD from other tauopathies or neurodegeneration, as well as reduce the impact of individual differences in total tau protein levels. This also allows for monitoring and comparing the dynamics of changes within the same patient. For this purpose, two SPRi biosensors were constructed, sensitive to the proteins described: total tau and ptau-181 for plasma determinations. The use of these biosensors requires prior sensor validation, during which specific parameters of the analytical method are established. A study of the optimal concentration of the receptor layer in which particular antibodies were immobilized found that the optimal concentration for total tau protein determinations was 1000 ng/mL. For ptau-181, it was 90 ng/mL. Biosensor layer formation was confirmed by analysis over a wide angle range, which enabled the generation of SPR curves. The dynamic range of the sensors is 1–50 pg/mL for total tau and 1–100 pg/mL for ptau-181. The limits of detection are 0.18 pg/mL and 0.037 pg/mL, respectively. Low standard deviation (SD) and coefficient of variation (CV) values indicate good precision and accuracy of the results obtained using the SPRi biosensors. Specificity testing confirmed that no interferents influenced the assay. The method is therefore suitable for researching biological materials, such as blood plasma. Proteins were thus measured in the blood plasma of AD patients and controls. Statistical analysis revealed significant differences in the concentrations of tau and ptau-181 protein in both groups. The calculated ptau/total tau ratio for both sample groups also demonstrated high statistical significance. This suggests that a high ratio may be characteristic of AD. However, more extensive analysis is needed to obtain cutoff values. The ROC curves indicate that both biosensors have good diagnostic utility, with lower specificity for total tau.