A Two-Stage Plasma Biomarker Algorithm Reduces Amyloid PET Use and the p-Tau217 Gray Zone

Plasma p-Tau217 leaves 30%-50% of patients in an indeterminate "gray zone" often requiring Aβ-PET. We developed GRAD (Gatekeeper-Reflex for Alzheimer's Disease), a two-stage Aβ-positivity prediction model to improve classification while reserving Aβ-PET for residual uncertainty.

GRAD was developed in 145 cognitively impaired ADNI participants and externally validated in 1644 cognitively unimpaired A4/LEARN participants. Stage 1 used p-Tau217 alone; probabilities between 0.25 and 0.75 were considered indeterminate. Stage 2 used L2-regularized logistic regression combining plasma-biomarkers, genetic and demographic features. Aβ-PET served as the reference standard (ADNI florbetapir SUVR > 1.11; A4 Centiloid ≥ 20).

Full model AUC was 0.915 (95% CI, 0.859-0.961) internally and 0.879 (95% CI, 0.861-0.895) externally. In the external gray zone (n = 630), Stage 2 improved discrimination over p-Tau217 alone (AUC 0.793 vs. 0.719; p < 0.001), increased correct classification without PET from 41.4% to 56.2%, and reduced PET referral from 35.9% to 24.3%. In the p-Tau181 subset (n = 256), Stage 2 outperformed p-Tau181 (AUC 0.799 vs. 0.597; p < 0.001). Overall, 89.0% of ADNI and 90.7% of A4/LEARN participants were classified without PET; modeled testing costs were 44%-52% lower than p-Tau217 screening with PET confirmation of the gray zone.

Reflex multimarker testing improves indeterminate p-Tau217 classification while retaining Aβ-PET for residual uncertainty. GRAD may reduce PET use and testing costs in memory-clinic triage, prevention-trial enrichment, and disease-modifying therapy workup.