Background and objectives: Plasma biomarkers provide new tools for evaluating patients with mild cognitive impairment (MCI) for Alzheimer disease (AD) pathology. Such tools are needed for anti-amyloid therapies that require efficient and accurate diagnostic evaluation to identify potential treatment candidates. This study sought to develop and evaluate the clinical performance of a multimarker combination of plasma beta-amyloid 42/40 (Aβ42/40), ptau-217, and APOE genotype to predict amyloid PET positivity in a diverse cohort of patients at a memory clinic and evaluate >4,000 results from "real-world" specimens submitted for high-throughput clinical testing.
Methods: Study participants were from the 1Florida AD Research Center. Demographics, clinical evaluations, and amyloid PET scan data were provided along with plasma specimens for model development in the intended-use cohort (MCI/AD: n = 215). Aβ42/40 and ApoE4 proteotype (reflecting high-risk APOE ɛ4 alleles) were measured by mass spectrometry and ptau-217 by immunoassay. A likelihood score model was determined for each biomarker separately and in combination. Model performance was optimized using 2 cutpoints, 1 for high and 1 for low likelihood of PET positivity, to attain ≥90% specificity and sensitivity. These cutpoints were applied to categorize 4,326 real-world specimens and an expanded cohort stratified by cognitive status (normal cognition [NC], MCI, AD).
Results: For the intended-use cohort (46.0% prevalence of PET positivity), a combination of Aβ42/40, ptau-217, and APOE4 allele count provided the best model with a receiver operating characteristic area under the curve of 0.942 and with 2 cutpoints fixed at 91% sensitivity and 91% specificity, yielding a high cutpoint with 88% positive predictive value and 87% accuracy and a low cutpoint with 91% negative predictive value and 85% accuracy. Incorporating the APOE4 allele count also reduced the percentage of patients with indeterminate risk from 15% to 10%. The cutpoints categorized the real-world clinical specimens as having 42% high, 51% low, and 7% indeterminate likelihood of PET positivity and differentiated between NC, MCI, and AD dementia cognitive status in the expanded cohort.
Discussion: Combining plasma biomarkers Aβ42/40, ptau-217, and APOE4 allele count is a scalable approach for evaluating patients with MCI for suspected AD pathology.
Copyright © 2025 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology.