Amyotrophic lateral sclerosis (ALS) is a serious neurodegenerative disease, often difficult to diagnose due to symptoms similar to other neurological pathologies. Diagnosis can take up to 14 months, although the rapid progression of the disease requires early detection. At present, there is no validated biomarker to aid diagnosis. Serum neurofilaments light chain (NfL), markers of neuronal degeneration, show great potential to help diagnose ALS early and assess disease severity. Recent research has shown that measurement of NfL in the blood can differentiate ALS from other neurological disorders, and new technologies are increasingly making it possible to perform these tests clinically.

The study hypothesis is that NfL blood levels, measured using clinical analyzers, could improve early ALS diagnosis, optimize patient recruitment for therapeutic trials and accelerate the assessment of treatment efficacy.

The primary objective is to evaluate the sensitivity and specificity of serum NfL for the diagnosis and differential diagnosis of amyotrophic lateral sclerosis (ALS) in newly recruited patients referred to the ALS Reference Center at Montpellier University Hospital. The diagnosis is established according to the revised El Escorial diagnostic criteria (see Appendix). This diagnosis is determined independently of the serum NfL concentration.

Amyotrophic lateral sclerosis (ALS) is one of the most severe neurodegenerative diseases. It is characterized by the progressive degeneration of upper and lower motor neurons, leading to progressive paralysis and ultimately death from respiratory failure, with a median survival ranging from 30 to 36 months following symptom onset (Hardiman et al., 2011). To date, no curative treatment is available, and the exact etiology of ALS remains largely unknown, except for the familial forms, which account for approximately 10% of cases.

Establishing an early diagnosis is essential to optimize patient management and reduce diagnostic delay, which is currently estimated at an average of 12 to 14 months (Paganoni et al., 2014). The diagnostic workup includes clinical examination, electroneuromyography, and additional complementary investigations. However, this diagnostic pathway remains highly variable among patients and may require several years before a definitive diagnosis is reached, as evidence of both upper and lower motor neuron involvement is present in only approximately 50% of patients at the initial consultation.

Furthermore, reliable prognostic assessment is not currently possible during the early stages of the disease, even when the diagnosis has been established. Improving prognostic evaluation therefore represents a major clinical challenge to provide appropriate information to patients and their families.

To date, no validated biomarker is available to assist clinicians in the rapid differential diagnosis of ALS or to accurately predict disease severity at an early stage (Lenglet and Camdessanche, 2017).

Neurofilaments (Nf), which are major structural components of the neuronal cytoskeleton, are released into the cerebrospinal fluid (CSF) and subsequently into the bloodstream during neurodegenerative processes, including ALS (Bridel et al., 2019). The diagnostic value of neurofilament light chain (NfL) measurements in CSF for the differential diagnosis of ALS has already been demonstrated in various clinical settings, including prospective studies (Li et al., 2018).

The ultrasensitive Single Molecule Array (SIMOA) technology, initially available at the Department of Clinical Biochemistry and the Clinical Proteomics Platform (LBPC/PPC, Montpellier University Hospital), enabled the quantification of NfL concentrations in both CSF and blood samples. NfL levels measured in these biological fluids have been shown to correlate with patient survival (Poesen and Van Damme, 2018; Thouvenot et al., 2019; Brousse et al., 2022).

However, most published studies have been retrospective in nature. In addition, with the exception of the recent work by Brousse et al. (2022), blood samples were generally not collected during the early phase of the disease.

The present study offers several innovative features. It is conducted prospectively under real-world clinical conditions. All patients referred to the ALS Reference Center at Montpellier University Hospital for suspected ALS are included, regardless of the final diagnosis.

Historically, SIMOA technology was used for research purposes ("Research Use Only", RUO) to quantify serum NfL and glial fibrillary acidic protein (GFAP). Validated assays providing comparable analytical performance are now available on fully automated clinical analyzers, including Lumipulse and Cobas platforms, which have replaced SIMOA for routine laboratory use.

GFAP is predominantly expressed by astrocytes within the central nervous system and plays a key role in several biological processes, including cell communication and maintenance of the blood-brain barrier. Increased GFAP concentrations have been associated with astroglial activation, a mechanism implicated in ALS pathophysiology and linked to poor prognosis. However, recent findings (Mondesert et al., in preparation) suggest that GFAP concentrations do not significantly change during the course of ALS.

Accordingly, the present project focuses on the prospective clinical validation of blood NfL measurements obtained under routine clinical practice conditions. This objective represents an important step toward complementing existing retrospective evidence and confirming the clinical utility of NfL as a biomarker.

Ultimately, the prospective implementation of NfL measurements is expected to improve both the diagnostic and prognostic value of this biomarker while facilitating patient selection and monitoring in future therapeutic clinical trials for amyotrophic lateral sclerosis.

Inclusion Criteria:

• Be at least 18 years of age
• Be able to undergo blood sampling (however, blood sampling is part of the standard examination and will not be performed exclusively for this study).
• Patients with suspected ALS

Exclusion Criteria:

-• Patients with recent stroke

• Pregnant or breast-feeding women
• Patient deprived of liberty by judicial or administrative decision, or hospitalization under duress
• Adult protected by law (guardianship, curatorship)
• Patient unable to understand and read information and consent forms in French
• Person participating in another research study with an exclusion period still in progress.
• Failure to obtain written informed consent after a period of reflection
• Not affiliated to a social security scheme or beneficiary of such a scheme
• Person unable to give consent

The study population consists of all patients referred to the rare disease reference center for amyotrophic lateral sclerosis (ALS) at CHU Montpellier for suspected ALS, regardless of the final diagnosis. These patients will be included prospectively as part of the diagnostic evaluation. The inclusion will be based on clinical suspicion of ALS, and the final diagnosis will be established according to the revised El Escorial criteria, independent of the NfL serum levels. This population will be monitored and analyzed for diagnostic performance and prognostic value of serum NfL levels.