Equivalence of Plasma and Serum for Clinical Measurement of p‐tau217: Comparative Analyses of Four Blood‐Based Assays

Phosphorylated tau (p-tau) 217 is a promising blood biomarker for Alzheimer's disease (AD). However, most p-tau217 assays have been validated solely in ethylenediaminetetraacetic acid (EDTA) plasma, leaving the clinical applicability of serum p-tau217 largely unexplored despite serum being a preferred matrix in many clinical laboratories. To address this gap, we compared p-tau217 concentrations and classification accuracies in matched plasma and serum samples in four research-use-only assays. Paired plasma and serum samples were processed from the same venipuncture collection and assessed with each of the four p-tau217 assays following manufacturer-recommended procedures in two research cohorts from the University of Pittsburgh Azheimer's Disease Research Center (Pitt-ADRC; n = 50) and the Human Connectome Project (n = 34). The four assays evaluated included three from commercial sources-Lumipulse (recently gained FDA approval), ALZpath, and NULISA-and another from the University of Pittsburgh (Pittsburgh plasma p-tau217). Plasma and serum p-tau217 levels varied across assays; the ALZpath, Pittsburgh, and NULISA methods showed significantly lower p-tau217 levels in serum compared with plasma (p < 0.0001) for both cohorts, while Lumipulse showed higher plasma levels in the Pitt-ADRC cohort but equivalent plasma and serum levels in the HCP cohort. Yet, strong correlations (rho > 0.8) were observed between plasma and serum p-tau217 pairs for all methods. Both plasma and serum p-tau217 demonstrated strong classification accuracies to differentiate clinical AD from normal controls, with high AUC (up to 0.963) for all methods. The exception was the Pittsburgh assay, where plasma p-tau217 had significantly superior AUC to serum p-tau217 (plasma: 0.912, serum: 0.844). The rest of the assays had equivalent accuracies in both matrices. Serum p-tau217 performs equivalently as plasma p-tau217 for most assessed assays. Serum can be used as a substitute for plasma in the use of most p-tau217 assays to evaluate Aβ pathology in AD for both research and clinical purposes.