The interesting question with longitudinal plasma NfL tracking isn’t whether it’s a valid biomarker. It clearly correlates with axonal injury across multiple conditions. The question is what intra-individual variability looks like in a healthy person with no active neurological disease. Most reference range data comes from cross-sectional studies. Week-to-week CV in a clinically stable healthy individual isn’t well characterized. I’ve seen ~10-20% cited, but those figures come from studies not designed to answer that question. With 22 tests, you’d need to know whether any observed trend actually exceeds expected biological noise. The confounders are real: endurance exercise transiently elevates NfL (there’s data on marathon runners specifically), acute illness does too, maybe sleep deprivation. Those sources of variation rarely get discussed when people post serial results. Has anyone actually done autocorrelation on their series to separate signal from drift?