Diagnostic performance of plasma pTau217/Aβ42 ratio and a three-zone threshold model for Alzheimer's disease

Early and accurate diagnosis of Alzheimer's disease (AD) typically relies on invasive or expensive methods like cerebrospinal fluid (CSF) biomarkers and amyloid PET imaging. Blood-based biomarkers, particularly plasma phosphorylated tau (pTau181, pTau217) and amyloid-beta ratios (Aβ42/40), offer a more accessible diagnostic alternative. This study assessed the diagnostic accuracy of plasma biomarkers and developed a three-zone classification model to reduce reliance on invasive confirmatory tests. We retrospectively evaluated 109 participants referred to a tertiary memory clinic. Participants underwent cognitive assessments, brain MRI, CSF biomarker analyses (pTau181, Aβ42/40), and plasma biomarker measurements (pTau181, pTau217, Aβ42/40, pTau217/Aβ42 ratio). Diagnostic performance was evaluated using ROC analyses, and thresholds achieving ≥ 95 % sensitivity and specificity were used to define low, intermediate and high-risk zones. Plasma biomarkers correlated significantly with CSF biomarkers. For identifying AD pathology (A+/T + vs. others), plasma pTau217 and the pTau217/Aβ42 ratio demonstrated the highest accuracy (both AUC=0.95), outperforming plasma pTau181 (AUC=0.88) and Aβ42/40 ratio (AUC=0.73). At optimal thresholds, plasma pTau217 showed 87.5 % sensitivity and 93.4 % specificity, whereas the pTau217/Aβ42 ratio showed higher sensitivity (95.8 %) but lower specificity (85.2 %). Using the three-zone model, plasma pTau217 enabled definitive classification in 80.7 % of patients, increasing to 84.4 % with the pTau217/Aβ42 ratio. Among patients with mild cognitive impairment, plasma pTau217 achieved excellent accuracy (AUC=0.98). Plasma pTau217, alone or combined with Aβ42, provides highly accurate and scalable identification of AD pathology, substantially reducing the need for invasive diagnostic procedures.