Streamlining Alzheimer's disease diagnosis: real-world validation of two-cut-off diagnostic models based on plasma p-tau/Aβ42 ratios.

INTRODUCTION: Anti-amyloid monoclonal antibodies have increased the need for scalable, minimally invasive biomarkers for Alzheimer's disease (AD). In this single-center cohort study, we evaluated plasma biomarkers performance in detecting biologically defined AD, assessing diagnostic accuracy and generalizability outside dedicated laboratory settings and exploring suitability for clinical implementation. METHODS: We enrolled 204 outpatients referred to the memory clinic of Policlinico "Rome Tor Vergata" who underwent standard work-up, lumbar puncture for cerebrospinal fluid (CSF) biomarkers and paired blood sampling. Among plasma biomarkers, phosphorylated tau (p-tau) 181, p-tau217, and their ratios adjusted for Aβ42 were measured on the Lumipulse platform. AD pathology was defined by CSF p-tau181/Aβ42 ≥ 0.069. ROC analyses estimated AUCs, and a two-cut-off approach targeting 90% sensitivity and specificity classified individuals as low, intermediate, or high AD risk. Subgroup analyses examined the impact of sex, age (< 75, ≥ 75 years), chronic kidney disease, and cognitive impairment (MMSE ≥ 26/30, < 26/30) on plasma biomarker levels. RESULTS: Among single analytes, plasma p-tau217 showed the highest discriminative capacity (AUC 0.883). Combined ratios improved overall performance (p-tau181/Aβ42, AUC 0.928; p-tau217/Aβ42, AUC 0.894) and reduced intermediate-risk classifications to < 15%, with slightly better performance in women, patients < 75 and cognitively unimpaired. The two-cut-off model improved accuracy and rule-in ability. DISCUSSION: Plasma p-tau/Aβ42 ratios show high and robust accuracy for detecting CSF-defined AD pathology. A two-step approach based on p-tau181/Aβ42 or p-tau217/Aβ42 could streamline diagnostic workflows in memory clinics, reserving second-line assessments to indeterminate cases and supporting selection of candidates for disease-modifying anti-amyloid therapies.