NEFL and nervous system disorder: A better understanding of the longitudinal intra-individual biological and disease-associated variability; the potential impact of clinical confounders and biological factors, including race and ethnicity, peripheral neuropathies and other neurologic diseases, BMI, and kidney disease; and the relative effects on the clinical performance of plasma Aβ42/Aβ40, p-tau, NfL, and GFAP in large cohorts is needed.