Postmortem Associations Between Alzheimer Disease Pathology and Plasma pTau217, GFAP, and NfL in AD and AD-Related...

Alzheimer disease (AD) and its related disorders (ADRDs) are characterized by a high frequency of copathologies. We aimed to determine the specificity of plasma pTau217, glial fibrillary acidic protein (GFAP), and neurofilament light chain (NfL) for AD neuropathological change (ADNC) in the presence of common ADRD copathologies.

pTau217, GFAP, and NfL were measured using S-PLEX immunoassays from Meso Scale Discovery in banked plasma samples from 2 groups of participants in the Massachusetts Alzheimer's Disease Research Center (MADRC) Longitudinal Cohort study: (1) participants spanning the cognitive spectrum, who underwent brain autopsy, and blood collection within 6 years before death, and (2) participants with normal cognition and no neurologic diagnosis during 5 years of follow-up, but no autopsy data (normal controls [NCs]). Cross-sectional associations between biomarker levels and ADNC, primary neuropathologic diagnosis (NPDx1), and presence of non-AD copathologies were evaluated using linear regression models controlling for age, sex, and time to death.

One hundred eighty-seven participants with brain autopsy (NPDx1: AD n = 85; other n = 102; mean age: 74.3 years, 38.5% female; interval blood collection-death [mean ± SD]: 2.8 ± 1.6 years) and 67 NC without brain autopsy (mean age: 66.5 years, 71.6% female) were included. pTau217, but not GFAP, levels increased stepwise with increasing Thal phases (β = 0.61; 95% CI [0.24-0.97] to β = 0.91 [0.55-1.27]) and Braak stages (β = 0.59; [0.16-1.01] to β = 0.74 [0.33-1.15]). Although 23% of individuals with a non-AD NPDx1 had increased pTau217 levels using a cutoff defined by the contrast between ADNC and NC, the majority (62%) had intermediate/high ADNC copathology and the remaining pTau217+ individuals had borderline increased levels. By contrast, 48% of individuals without ADNC had increased GFAP levels. pTau217 and GFAP were not different in the presence or absence of cerebral amyloid angiopathy, α-synuclein or TDP-43 proteinopathies, or primary tauopathies. NfL was not specifically associated with ADNC.

Plasma pTau217, but not GFAP or NfL, levels accurately reflect the presence of ADNC in the brain even in individuals with an NPDx1 of a non-AD dementia. Thus, a positive plasma pTau217 test in an individual with a suspected non-AD dementia should not necessarily be considered a misdiagnosis of the presumed non-AD dementia or as a false positive, but rather as evidence of ADNC copathology.