Neurofilaments are key axonal proteins, with neurofilament light (NfL) and heavy (NfH) chain recognised as promising biomarkers for neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). However, neurofilament medium chain (NfM) remained previously underexplored due to a lack of quantitative assays. In this study, we developed a sensitive immunoassay to measure NfM in cerebrospinal fluid (CSF) and analysed its levels in ALS, Alzheimer's disease (AD), frontotemporal dementia (FTD), and Lewy body dementia (LBD). Correlations among neurofilaments and their diagnostic performance were also evaluated.

In this study CSF levels of three neurofilament proteins were measured in 305 participants, including patients with ALS (n = 91), AD (n = 59), FTD (n = 38), LBD (n = 18), non-neurodegenerative controls (CTRL, n = 51), and 48 individuals initially evaluated for ALS but ultimately diagnosed with other conditions (CTRL.DD). NfM levels were quantified using a homemade sandwich ELISA, while NfL and NfH were measured using commercialised Ella cartridges.

All three neurofilaments were significantly elevated in ALS compared to CTRL and CTRL.DD groups (p < 0.0001 for both), with NfM and NfL also increased in FTD (p < 0.0001 for both) and AD (NfM, p < 0.0001; NfL, p = 0.0001) compared to CTRL. NfH demonstrated the greatest distinction between ALS and FTD (p < 0.0001). Strong correlations were observed among neurofilament subunits, particularly between NfM and NfL (r = 0.93, 95% CI: 0.91–0.94, p < 0.0001). All neurofilaments effectively distinguished ALS from CTRL and CTRL.DD, with AUC values ranging from 0.92 to 0.99. NfM and NfL showed high accuracy in differentiating AD (NfM, AUC: 0.91; NfL, AUC: 0.89) and FTD (NfM, AUC: 0.91; NfL, AUC: 0.92) from CTRL, while NfH best separated ALS from FTD (AUC: 0.96).

This study provides a quantitative comparison of NfM with NfL and NfH in a neurodegenerative cohort, highlighting its potential diagnostic value. Further research with larger cohorts, longitudinal studies, and investigations into neurofilament distribution in different compartments is needed to clarify the distinct roles of NfM, NfL, and NfH in the diagnosis and treatment of neurological diseases.

The present study was supported by the Else Kroener-Fresenius Foundation (2024-EKEA.126) and Chemische Fabrik Karl Bucher GmbH.