Electrochemical impedance-based biosensor for label-free determination of plasma P-tau181 levels for clinically accurate diagnosis of mild cognitive impairment and Alzheimer's disease.

Plasma phosphorylated-tau threonine 181 (p-tau181) is a promising biomarker for predicting Alzheimer's disease (AD) and mild cognitive impairment (MCI), which is the symptomatic pre-dementia stage of AD. To date, there are limitations in the current diagnosis and classification of the two stages of MCI and AD in clinical practice remain a dilemma. In this study, we aimed to discriminate and diagnose patients with MCI, AD, and healthy participants based on the accurate, label-free, and ultrasensitive detection of p-tau181 levels in human clinical plasma samples using our developed electrochemical impedance-based biosensor, which allows to detect p-tau181 at a very low concentration of 0.92 fg mL. Human plasma samples were collected from 20 patients with AD, 20 patients with MCI, and 20 individuals with healthy control. The change in charge-transfer resistance of the developed impedance-based biosensor caused by capturing p-tau181 in plasma samples was recorded to evaluate the determination of plasma p-tau181 levels in human clinical samples for discrimination and diagnosis of AD, MCI, and healthy control individuals, respectively. Receiver operating characteristic (ROC) curve, a standard analysis to judge the clinically diagnostic capability of our biosensor platform based on the estimated levels of plasma p-tau181, resulted a sensitivity of 95%, a specificity of 85%, the area under the ROC curve (AUC) value of 0.94 of the accuracy for discriminating AD patients from healthy controls; a sensitivity of 70%, a specificity of 70%, the AUC of 0.75 to discriminate MCI patients from healthy controls. Statistical analysis (one-way analysis of variance (ANOVA)) was used to compare the estimated plasma p-tau181 levels in clinical samples, indicated significantly higher for AD patients with healthy controls (***p ≤ 0.001), AD with MCI patients (***p ≤ 0.001), and MCI patients with healthy controls (*p ≤ 0.05), respectively. In addition, we compared our sensor to the global cognitive function scales and discovered that it performed noticeably improvement in diagnosing the stages of AD. These results demonstrated the good application of our developed electrochemical impedance-based biosensor in the identification of clinical disease stages. Moreover, in this study, a small dissociation constant (K) of 0.533 pM was first determined to evaluate the high binding affinity between the p-tau181 biomarker and its antibody, providing a reference parameter for future studies of the p-tau181 biomarker and AD.