Why someone might want to know their NfL level
The case for measuring NfL comes down to four practical points.
First, NfL responds to injuries and interventions on a timescale of weeks. After a traumatic brain injury or concussion, plasma NfL rises within days, peaks at roughly 10 days to 6 weeks, and can remain elevated for months (Shahim et al., Sci Transl Med 2021). This response curve is part of why serial NfL measurements are now being incorporated into recovery tracking after head injuries, including in sports medicine where biomarker trajectories are starting to inform return-to-play decisions in concussed athletes (McDonald et al., JAMA Netw Open 2024). After a multiple sclerosis relapse, NfL peaks at 3 to 4 weeks, and treatment effects on NfL typically show up within 3 to 6 months in MS clinical trials. MRI volume changes from interventions can also be detected within 3 to 4 months in some studies, particularly in clinical populations like mild cognitive impairment, but NfL gives a faster individual-level readout on whether the underlying damage process is responding to whatever someone is doing.
Second, NfL is more accessible than MRI for repeat monitoring. A direct-to-consumer NfL test runs roughly $200 to $400, compared with $600 to $8,000 for a brain MRI. A blood draw also works for people who cannot easily get into an MRI scanner, such as those with pacemakers or significant claustrophobia. For someone considering serial monitoring once or twice a year, the difference in cost and convenience adds up.
Third, NfL is non-specific in a useful way. The criticism sometimes leveled at NfL is that it does not diagnose any single disease, and that is correct. It is not an Alzheimer’s test or an MS test. What it does measure is the overall rate of nerve damage in your body. For a person who wants a single number to track brain health over time, that breadth is a feature rather than a flaw.
Fourth, the value of NfL is in the trajectory, not the snapshot. A single elevated NfL does not mean disease, and a single normal NfL does not mean safety. What matters is how your level compares to age-matched ranges and how it changes over time. People who track their NfL annually can see whether their brain aging is accelerating, holding steady, or slowing in response to whatever they are doing.