BACKGROUND: Serum neurofilament light chain (sNFL) is a promising biomarker for neuroaxonal injury in multiple sclerosis (MS). Traditional clinical and radiological examinations often fail to capture the underlying neurodegeneration, particularly in the absence of clinical relapses or gadolinium-enhanced lesions. This study aims to assess sNFL levels in real-world MS patients who have no evidence of activity, to evaluate the potential of sNFL as a biomarker for smoldering-associated worsening (SAW). MATERIALS AND METHODS: A cross-sectional study, involved 162 MS patients without evidence of disease activity and 40 healthy, age, sex, and education matched controls (HCs). Patients were classified according to MS subtype, DMT status, and type. sNFL levels were measured using an enzyme-linked immunosorbent assay (ELISA) and levels were compared in each group. RESULTS: sNFL levels were significantly higher in MS patients compared to (HCs) (p <
0.001). Median sNFL levels were lowest in the clinically isolated (CIS) group and steady increase in RRMS and reaching the highest levels in the SPMS group (p <
0.001). Despite a slight decrease in sNFL levels in patients who started DMT for a year or less than in the naïve group, sNFL levels were highest in patients who were on DMTs for longer durations (p = 0.003). EDSS score was the sole independent predictor of sNFL levels (B = 0.415, p = 0.002). A cut-off value of 23.25 pg/ml was set to distinguish cases and HCs (92 % specificity and 90 % sensitivity), and 75.48 pg/ml was set to distinguish progressive forms (70.00 % sensitivity and 78.30 % specificity). CONCLUSION: sNFL is sensitive for detecting subclinical neurodegeneration in the absence of relapse or gadolinium-enhanced lesions, supporting the utility of sNFL measurements into routine clinical practice to improve monitoring and management of MS.