Neurofilament Light Chain (NfL) as a Biomarker in PLS — Body of Evidence
Neurofilament light chain is the closest thing PLS currently has to an objective, blood-based measure of neurodegeneration. It does not diagnose PLS — it is not specific to PLS or to any particular disease. But when axons degrade, NfL is released into the cerebrospinal fluid and then into the blood, where it can be measured. In ALS, this measurement has been validated extensively as a marker of disease activity and prognosis. In PLS, the evidence is earlier stage but growing. The most important finding to date: in the PLS Natural History Study (Mayo Clinic), baseline NfL predicted the rate of functional decline over the following year with a p-value of 0.001.
What NfL is
Neurofilaments are structural proteins that form the internal scaffold (cytoskeleton) of neuronal axons. The light chain (NfL) is one component of this scaffold. When an axon is damaged or dies — from any cause, including motor neuron disease, traumatic brain injury, multiple sclerosis, or normal aging — NfL is released. It enters the cerebrospinal fluid (CSF) first, then diffuses into the bloodstream, where it can be measured by highly sensitive assays.
The key technological development that made blood NfL clinically useful was the Simoa platform (single molecule array immunoassay), which can detect proteins at concentrations thousands of times below what conventional immunoassays can measure. NfL in healthy adults circulates at very low concentrations (roughly 5–15 pg/mL, varying with age); in neurological disease, it rises — sometimes dramatically.
An important caveat: NfL is not specific. It rises with any cause of neuronal injury. Age and BMI affect baseline levels. It cannot distinguish PLS from ALS, or from other neurological conditions. Its value is not as a diagnostic test but as a measure of neurodegeneration rate — how much axonal damage is occurring, and how quickly.
NfL in ALS: the established evidence base
The evidence for NfL in ALS is substantially more developed than in PLS, and the ALS evidence is the foundation on which PLS researchers are building.
In ALS, serum NfL averages approximately 81.49 ± 47.06 pg/mL versus 9.21 ± 3.05 pg/mL in healthy controls (p < 0.001), measured by Simoa (Correlation Analysis of Serum NfL in ALS, Scientific Reports 2025). This roughly 9-fold elevation in ALS makes NfL highly sensitive for distinguishing ALS patients from healthy individuals, though not from other neurological conditions with axonal injury.
Critically, NfL correlates with how fast ALS progresses. Fast progressors — patients who decline rapidly on the ALSFRS-R — have higher plasma and CSF NfL than slow progressors (Frontiers Aging Neuroscience 2021). The correlation with disease progression rate (ALSFRS-R slope) has been documented across multiple independent cohorts (Poesen & Van Damme, Frontiers 2019; Gagliardi, IJMS 2019). Population-based evidence from TU Munich confirmed that NfL is valid as an individual prognostic biomarker in real-world ALS populations, not just in selected academic cohorts (Annals of Neurology).
Tofersen: regulatory validation of NfL as an endpoint
The most important regulatory milestone for NfL in motor neuron disease was the FDA approval of tofersen (Biogen) for SOD1-ALS in 2023. Tofersen is an antisense oligonucleotide that reduces production of mutant SOD1 protein. The clinical trial demonstrated that tofersen significantly reduced plasma NfL — and the FDA accepted this as a meaningful pharmacodynamic endpoint, contributing to the approval. This established NfL as a biomarker that regulatory agencies will consider when evaluating motor neuron disease treatments. For any future PLS trial, this precedent is important: a drug that reduces NfL in PLS patients would have a well-established argument for regulatory acceptance.
NfL in PLS: the key evidence
PNHS Mayo Clinic 2025 — the landmark finding
The PLS Natural History Study (Mayo Clinic / Johns Hopkins, published Annals of Neurology September 2025) enrolled 76 participants with confirmed or probable PLS and measured serum NfL at baseline. Participants were then followed prospectively with PLSFRS assessments.
The finding: baseline serum NfL level was significantly associated with rate of PLSFRS decline over the following year (p = 0.001). Patients with higher NfL at enrollment declined faster on the PLSFRS over the next 12 months.
This is the most direct PLS-specific evidence for NfL as a prognostic biomarker. It demonstrates that a single blood draw at the time of diagnosis predicts, with statistical significance, how fast a patient will decline over the next year. The implications are concrete:
- For patients: NfL at diagnosis may eventually inform prognostic counseling — whether a patient is likely to be a faster or slower progressor relative to the PLS average.
- For clinical trials: Baseline NfL can stratify patients into fast and slow progressors, reducing the variance that obscures treatment effects and enabling smaller, more efficient trials.
- As an endpoint: If a treatment reduces NfL in PLS patients, that reduction would be expected — based on the p=0.001 correlation — to translate into slower functional decline. This makes NfL a candidate surrogate endpoint for future PLS trials.
Prior population-based evidence
Before the PNHS finding, population-based studies validated NfL in ALS specifically (Forgrave et al. and the TU Munich population cohort confirmed diagnostic and prognostic performance across real-world ALS populations). In PLS specifically, the biospecimen data from earlier cohorts was limited — PLS patients were enrolled in some ALS registries but rarely in sufficient numbers for PLS-specific NfL analysis. The PNHS was the first study powered to examine NfL as a prognostic biomarker specifically in PLS.
Measurement and reference ranges
Blood NfL is measured by Simoa from serum or plasma samples. Healthy adult reference ranges are age-adjusted: NfL rises approximately 2–3% per year with normal aging, which means a 75-year-old has higher baseline NfL than a 45-year-old even without neurological disease. Reference databases adjusted for age and BMI were developed and validated (Nat Rev Neurol) and are a prerequisite for meaningful clinical interpretation. Without age-adjustment, NfL elevation in an older PLS patient would be partially attributed to normal aging rather than disease.
CSF NfL can also be measured and is often more sensitive than serum NfL for detecting central nervous system pathology, but requires lumbar puncture — a limitation for routine monitoring. Serum NfL from a simple blood draw is now the standard for clinical and research use.
Limitations and uncertainties in PLS
Several questions remain open in PLS specifically. NfL levels in PLS may be lower than in ALS (given PLS's slower rate of neurodegeneration), potentially approaching the boundary of clinical interpretability when disease is stable. The relationship between NfL and long-term (5-10 year) functional outcomes in PLS has not been established — the PNHS finding is for 1-year outcomes. Age-adjusted reference ranges validated specifically for PLS populations do not yet exist. And NfL cannot distinguish PLS from ALS or other UMN conditions — its role is prognostic, not diagnostic.
How this connects
The key PLS NfL finding comes from the PNHS Mayo Clinic study. NfL's role in the broader biomarker landscape — alongside pNfH and GFAP — is covered on the Biomarker Research hub and the multi-biomarker panel page. The Scirocco 2025 review identifies NfL as the most important biomarker candidate for PLS trial design. How NfL and other tests contribute to diagnosis is on the Diagnosis page.
Key studies referenced
PLS Natural History Study Group. Primary lateral sclerosis natural history study: PLSFRS and
NfL findings. Annals of Neurology. September 2025.
Correlation Analysis of Serum NfL and GFAP in ALS. Scientific Reports. 2025.
Poesen K, Van Damme P. Diagnostic and prognostic performance of neurofilaments in ALS.
Frontiers in Neurology. 2019.
Neurofilaments as biomarkers in neurological disorders — towards clinical application.
Nature Reviews Neurology. [landmark reference for Simoa validation and reference ranges]
FDA approval of tofersen (Qalsody) for SOD1-ALS including NfL pharmacodynamic endpoint. 2023.