MAPT and Alzheimer disease: Phosphorylated tau (p-tau) is the leading blood biomarker candidate, demonstrating superior diagnostic accuracy and disease specificity compared with other candidates. The amyloid β 42/40 (Aβ42/40) ratio, a validated CSF biomarker, has limitations in blood and lacks the robustness required for routine clinical testing. In contrast, high-performing p-tau blood test results exhibit a substantial increase in patients with AD, occurring concurrently with extracellular Aβ plaque deposition, an AD hallmark feature.