Establishing demographic-based normative values for neurofilament light chain and glial fibrillary acidic protein in serum and plasma samples

Background Interpretation of neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) levels in blood requires reference ranges from healthy cohorts, accounting for demographic features. Objective To compare serum and plasma NfL and GFAP in healthy individuals, accounting for age, sex, and body mass index (BMI). Methods Adolescents through older adults were recruited at Colorado community events and a senior clinic. A neurologist reviewed self-reported medical histories. Paired serum and EDTA plasma samples were assayed using the Quanterix SR-X Simoa Neuro 4-Plex. Log-linear models estimated percent change per year of age, per BMI unit, and sex ratios. GFAP used a piecewise age model (knot at 30 years). Spearman correlation assessed serum–plasma concordance. Results We enrolled 143 participants (ages = 16.4–85.9; 63.6% female; mean BMI = 26.7). Serum–plasma correlations were strong (NfL ρ = .923; GFAP ρ = .833; both p < .001). In plasma, age was associated with a 2.96% increase in NfL and 2.34% in GFAP per year for ages ≥30. In serum, NfL rose 2.90% and GFAP 2.69% per year (both p < .0001). Conclusion NfL and GFAP are strongly age-dependent and concordant across serum and plasma in healthy individuals. An online calculator provides age-, sex-, and BMI-adjusted reference estimates and serum/plasma conversions.