There is no single “normal” number for a neurofilament light chain (NfL) blood test that applies to everyone. NfL concentrations in blood rise steadily with age, so what looks normal for a 30-year-old would be flagged as unusually low for a 75-year-old. Most laboratories using current ultrasensitive technology report healthy adult values somewhere in the single digits of picograms per milliliter for younger adults, climbing into the teens or twenties for older adults. But the raw number on a lab report tells only part of the story, and how clinicians interpret it is shifting fast.
What NfL Actually Measures
Neurofilaments are structural proteins that live inside nerve cell axons, where they help maintain the axon’s shape and support the transmission of electrical signals.1PMC (National Institutes of Health / National Library of Medicine). Neurofilament-Light, a Promising Biomarker: Analytical, Metrological and Clinical Challenges When nerve cells are damaged or destroyed, the protein spills out. NfL first leaks into the cerebrospinal fluid surrounding the brain and spinal cord, then seeps into the bloodstream at much lower concentrations.2Frontiers in Aging Neuroscience. Neurofilament Light Chain: A Candidate Biomarker of Perioperative Stroke The amount that ends up in your blood reflects how much nerve damage is happening. Small amounts of NfL are released even in healthy people during normal brain development, maturation, and aging, which is why everyone has a detectable baseline.
Think of NfL as a smoke detector for the nervous system. It does not tell you which room the fire is in or what started it. It tells you that somewhere, nerve fibers are breaking down faster than expected. That nonspecificity is both a strength (it catches problems across many diseases) and a limitation (it cannot, by itself, tell you which disease you have).
Why Age Dominates Normal Ranges
The single biggest factor shaping your NfL result is your age. A large study using a nationally representative U.S. sample found a strong positive association between age and serum NfL levels.3PubMed Central. Normative Values for Serum Neurofilament Light Chain in US Adults Separate work establishing reference values in people aged 5 to 90 confirmed that plasma NfL concentrations climb significantly with age, and the rise becomes steeper after roughly 65 years.4Brain Communications. Establishment of reference values for plasma neurofilament light based on healthy individuals aged 5–90 years That accelerating curve means a 70-year-old with an NfL of 20 pg/mL could be perfectly healthy, while the same value in a 35-year-old would warrant attention.
This age effect is not a flaw in the test. Some low-grade nerve turnover is a normal part of aging. But it creates a real problem if a doctor simply compares your number against a single universal cutoff. A flat cutoff would over-diagnose older adults (whose baseline is naturally higher) and potentially miss pathology in younger people (whose naturally low baseline makes even a modest rise meaningful).
Kidney Function and Body Weight Also Move the Needle
Beyond age, kidney function is the most important non-neurological factor affecting NfL levels. Your kidneys are responsible for clearing NfL from the bloodstream. When kidney function declines, NfL accumulates even if no new nerve damage is occurring. Research in older adults found that kidney function, measured by estimated glomerular filtration rate (eGFR), had a strong inverse relationship with NfL levels. Adding eGFR to an age-only model substantially increased how well the model explained the variation in NfL concentrations.5Frontiers in Neuroscience. Renal Function and Body Mass Index Contribute to Serum Neurofilament Light Chain Levels in Elderly Patients With Atrial Fibrillation A separate study confirmed that the link between NfL and creatinine (a standard marker of kidney function) held up even after adjusting for age, sex, and body mass index.6PubMed Central. Renal function is associated with blood neurofilament light chain level in older adults
Body mass index plays a smaller but real role. Higher BMI tends to be associated with slightly lower NfL values, possibly because a larger blood volume dilutes the protein’s concentration. The effect is modest compared to age or kidney function, but it is another reason two people with the same neurological status can get different numbers on the same test.
Why Your Doctor Should Not Rely on a Single Cutoff Number
The field is moving away from using a raw concentration threshold like “above 10 pg/mL is abnormal.” Instead, the more informative approach is to compare your result against what would be expected for someone your age, sex, and BMI, then express the difference as a Z-score (how many standard deviations you fall from the population average) or a percentile. A study in people with multiple sclerosis directly compared these two approaches and found that age- and BMI-corrected Z-scores predicted future relapses and new brain lesions far better than a flat concentration cutoff. Patients whose Z-score was at or above the 88.5th percentile had roughly 80 percent higher odds of new disease activity, while an absolute cutoff of 10.8 pg/mL failed to identify the same risk.7PubMed Central. Serum Neurofilament Light Chain in Multiple Sclerosis: Superiority of Age- and BMI-Corrected Z Scores/Percentiles Over Absolute Cutoff Values for Prediction of Treatment Response
Similar findings exist in children. A study building a pediatric reference database found that Z-scores adjusted for age produced larger effect sizes and more accurate estimates of ongoing nerve damage in individual patients compared to raw NfL numbers.8PubMed. Serum neurofilament light chain reference database for individual application in paediatric care: a retrospective modelling and validation study If your lab report gives you only a raw pg/mL value with no age-based reference, the number is hard to interpret on its own. Online tools and reference databases now exist that allow clinicians to plug in your age and convert the raw result into a percentile, and this conversion turns out to matter a great deal for whether the result looks worrisome.
How the Test Became Possible
NfL concentrations in blood are extremely low, roughly 50 to 100 times lower than in cerebrospinal fluid.9PubMed. Development and multi-center validation of a fully automated digital immunoassay for neurofilament light chain: toward a clinical blood test for neuronal injury Older lab methods like traditional ELISA simply were not sensitive enough to pick up such tiny amounts reliably. The game-changer was a technology called single molecule array (Simoa), which counts individual protein molecules one at a time. Simoa is roughly 125 times more sensitive than traditional ELISA and about 26 times more sensitive than the next-best method, electrochemiluminescence.10PubMed Central. SIMOA-based analysis of plasma NFL levels in MCI and AD patients: a systematic review and meta-analysis Without that leap in sensitivity, measuring NfL from a simple blood draw would not have been practical.
Newer fully automated platforms are now being validated across multiple centers, which is important for standardization. When different hospitals use different assay platforms, results can shift slightly, so knowing which platform your lab uses matters if you are tracking your number over time.
Sample Handling and Stability
One practical advantage of the NfL blood test is that the protein is remarkably stable once collected. Research found that NfL concentrations in both serum and plasma stayed essentially unchanged when samples sat at room temperature for up to seven days before being frozen, with variation of only about one percent.11Scientific Reports. Seven day pre-analytical stability of serum and plasma neurofilament light chain That stability means the test is not especially finicky about shipping times or minor delays in processing. Results from serum and lithium-heparin plasma track very closely, with correlations above 0.98, though a separate study noted a small decline of about 7 percent when frozen serum was stored at −20°C for 12 months.12PubMed. Neurofilament light chain – matrix comparison and long-term stability in frozen serum For most clinical purposes, that drift is minor, but it matters for researchers tracking stored biobank samples over long periods.
What NfL Looks Like in Specific Diseases
Because NfL rises whenever nerve fibers are damaged, it is elevated across a wide range of neurological conditions. The size of the elevation and the way it behaves over time vary in ways that are clinically useful.
Multiple Sclerosis
MS is probably the disease where NfL testing has gained the most clinical traction. Serum NfL can be drawn frequently and inexpensively compared to repeated MRI scans, and it provides an independent read on whether a treatment is suppressing disease activity.13PubMed Central. Guidance for use of neurofilament light chain as a cerebrospinal fluid and blood biomarker in multiple sclerosis management Guidance documents now support using NfL alongside MRI and clinical exams as a decision-making tool. One large study found that patients whose NfL Z-score did not drop during the first year of treatment had roughly three times the odds of further disease activity in the second year. In treatment-naive patients, that risk was even more dramatic. Importantly, these associations held up even in patients who appeared clinically stable by other measures.14PubMed. Refining Treatment Monitoring in Patients With Relapsing Multiple Sclerosis Based on Changes on Neurofilament Light Chain Z-Scores For people living with MS, a persistently elevated or rising NfL can signal the need to reconsider therapy before relapses or new lesions appear on imaging.
Amyotrophic Lateral Sclerosis
ALS produces some of the highest NfL elevations of any neurological disease. Blood NfL levels in ALS patients are typically about four times those of healthy controls, and higher levels at diagnosis strongly predict faster disease progression. In one study, patients with the highest baseline NfL had a mortality hazard nearly four times that of patients with the lowest levels.15PubMed Central. Neurofilament light chain: A prognostic biomarker in amyotrophic lateral sclerosis A more recent analysis found that ALS patients had a mean serum NfL of about 81 pg/mL, compared to about 9 pg/mL in healthy controls.16Scientific Reports. Correlation analysis of serum neurofilament light chain and glial fibrillary acidic protein levels with amyotrophic lateral sclerosis That nearly ninefold difference makes NfL a potentially useful tool for distinguishing ALS from conditions that mimic it early on, when clinical signs may be ambiguous.
Alzheimer’s Disease and Other Dementias
NfL rises along the Alzheimer’s disease continuum, from subjective cognitive decline to mild cognitive impairment (MCI) to full dementia. Patients with MCI who also had positive amyloid pathology showed NfL levels comparable to those already diagnosed with Alzheimer’s, suggesting the protein tracks the transition from preclinical to active neurodegeneration.17PubMed Central. Plasma neurofilament light chain as a biomarker of Alzheimer’s disease in Subjective Cognitive Decline and Mild Cognitive Impairment Longitudinal data show that higher baseline plasma NfL correlates with faster cognitive decline and accelerated brain volume loss over time.18JAMA Neurology. Association of Plasma Neurofilament Light With Neurodegeneration in Patients With Alzheimer Disease
One study tracking patients over several years found that the rate of NfL change over time was even more informative than a single snapshot. MCI patients who progressed to Alzheimer’s dementia had a mean NfL increase of roughly 3.5 pg/mL per year, while those who remained stable showed essentially no change. A rate of change below about 1.6 pg/mL per year was highly accurate at excluding progression from MCI to dementia.19PubMed Central. Plasma neurofilament light chain predicts Alzheimer’s disease in patients with subjective cognitive decline and mild cognitive impairment: A cross-sectional and longitudinal study
Stroke
NfL also rises after acute events like ischemic stroke, though the timing is different from chronic diseases. Levels tend to climb within the first 24 hours of symptom onset and correlate with stroke severity and the size of the brain tissue lost. In one study, NfL peaked about seven days after the stroke and tracked with infarct volume.20Neurology India. Advances on the Neurofilament Light Chain as a Biomarker in Ischemic Stroke For stroke, the clinical role of NfL is still emerging, but it may eventually help quantify how much nerve damage occurred, which can inform rehabilitation planning.
Peripheral Nerve Diseases
NfL is not limited to brain disorders. Because the protein lives in all nerve axons, diseases affecting peripheral nerves also cause elevations. Patients with Charcot-Marie-Tooth disease, the most common inherited peripheral neuropathy, had median plasma NfL of about 26 pg/mL compared to about 15 pg/mL in healthy controls, and levels correlated with clinical severity.21PubMed Central. Plasma neurofilament light chain concentration in the inherited peripheral neuropathies This makes NfL useful well beyond the brain diseases that dominate headlines about the test.
Pediatric Reference Ranges Are Different
In children, NfL behaves in the opposite direction from adults: levels are highest in the youngest children and decrease with age, presumably because growing brains undergo more active nerve remodeling and pruning. A study establishing pediatric reference intervals using a large Canadian cohort found that children aged 1 to 10 had a reference range of roughly 3 to 21 pg/mL, while those aged 10 to 18 had a narrower range of about 2 to 7 pg/mL. There was no significant difference between boys and girls.22PubMed Central. Pediatric reference intervals for serum neurofilament light and glial fibrillary acidic protein using the Canadian Laboratory Initiative on Pediatric Reference Intervals (CALIPER) cohort This declining-then-rising pattern across the lifespan means pediatric and adult reference values cannot be mixed.
Exercise, Sleep, and Short-Term Fluctuations
If you are getting an NfL test, you might wonder whether a hard workout or a bad night’s sleep could throw off the result. Research on this is still limited, but what exists is reassuring. A crossover study in young adults found that moderate- and high-intensity interval exercise produced only small, transient dips in plasma NfL immediately afterward, on the order of 1 to 1.5 pg/mL, which resolved quickly.23PubMed. Effects of interval exercise and diurnal variation on blood biomarkers for sport-related concussion: A crossover cohort study These shifts are tiny compared to the elevations seen in disease. That said, if you are being monitored serially and your clinician is watching for small changes, drawing blood at roughly the same time of day and under similar conditions is reasonable practice.
NfL Does Not Work Alone
For many conditions, NfL is most useful as part of a panel rather than as a standalone test. In Alzheimer’s disease research, combining NfL with GFAP (glial fibrillary acidic protein, a marker of astrocyte damage) and phosphorylated tau 181 (a marker more specific to Alzheimer’s pathology) improves diagnostic accuracy beyond what any single marker achieves. One study found that GFAP alone had strong ability to distinguish Alzheimer’s from controls, and combining it with NfL pushed accuracy even higher.24Frontiers in Neurology. Evaluation of serum neurofilament light chain and glial fibrillary acidic protein in the diagnosis of Alzheimer’s disease Another analysis found that all three markers differentiated preclinical stages of dementia from healthy aging.25PubMed Central. Plasma GFAP, NfL and pTau 181 detect preclinical stages of dementia
The practical implication is that NfL tells you nerve damage is happening, GFAP tells you whether the brain’s support cells are also reacting, and phosphorylated tau pins the damage to Alzheimer’s-type pathology specifically. If you see NfL on a lab order alongside these other markers, that is why. Each fills in a different piece of the diagnostic puzzle.
What an NfL Result Means for Your Care
If your NfL result comes back elevated, the first question is “elevated compared to what?” A raw number above the lab’s reference range is a starting point, but without adjusting for your age and kidney function, it can be misleading. A result that falls at, say, the 95th percentile for your age group is much more informative than one that simply exceeds a flat cutoff of 10 or 16 pg/mL.
A high result does not diagnose any specific disease. It signals that nerve damage is happening at a rate above what is expected for your demographic. Your clinician will combine it with imaging, clinical symptoms, and sometimes other blood markers to narrow down the cause. A normal result, on the other hand, can be genuinely reassuring, as conditions like ALS produce such dramatic elevations that a normal NfL makes the diagnosis much less likely.
For people already diagnosed with conditions like MS, the test’s real power is in monitoring over time. A rising NfL Z-score during treatment can flag subclinical disease activity before a relapse occurs, potentially prompting a therapy switch. Experts have noted that regular NfL monitoring could reduce reliance on expensive MRI scans and help avoid continuing treatments that are not working well.26Frontiers in Public Health. Value contribution of blood-based neurofilament light chain as a biomarker in multiple sclerosis using multi-criteria decision analysis That potential to make neurological care more proactive and less reactive is a large part of why NfL has generated so much enthusiasm in recent years.
NfL in Veterinary Neurology
An unexpected avenue of NfL research is in companion animals. Dogs develop age-related neurodegenerative diseases that resemble human conditions, and their plasma NfL rises with age in the same pattern seen in people. Dogs diagnosed with neurodegenerative disease show significantly elevated levels compared to healthy age-matched controls.27PubMed Central. Plasma Neurofilament Light Chain as a Translational Biomarker of Aging and Neurodegeneration in Dogs Because dogs share our households and age on a compressed timeline, they are being used as translational models to study the biology of NfL and to test interventions for neurodegeneration. The same blood-based assay that measures your NfL can measure a dog’s, making it one of the few biomarkers that crosses cleanly between species in both research and clinical practice.