Background: Sports-related concussion (SRC) is a subset of mild traumatic brain injuries occurring in contact sports. Most people recover spontaneously, but in retired professional players, the risk for neurodegenerative diseases is increased. A biomarker, such as neurofilament light chains (NfL), would help to address this issue and demonstrate sports’ safety. Assessing NfL in professional soccer players may be the best way to investigate if repetitive head-impact exposure in the typical lower and asymptomatic range is harmful. Objective: To evaluate whether the NfL in serum is a sensitive biomarker to detect mild brain injury in professional soccer players. Methods: Thirty-six soccer players belonging to a professional Italian team underwent serum NfL assessment using ultrasensitive single-molecule array technology. Sixteen healthy nonathletic controls were also enrolled. Differences between groups and changes over time, considering pre-season vs. season, were considered. Results: Serum NfL concentrations were comparable in the soccer professional players (median [interquartile range], 6.44 pg/mL [4.60–8.27] and controls (6.50 pg/mL [5.26–7.04]), with a median difference of − 0.06 pg/mL (95% CI -1.36 to 1.18), p = 0.957. No significant differences according to players’ role (goalkeeper, defender, midfielder or forward) or according to timing of sampling (pre-season vs. season) were found. Conclusions: These results suggest that professional soccer, even when played at the highest level of competition, may be considered safe. Future studies assessing serum NfL levels after soccer-related concussions should be carried out, to evaluate their usefulness as a return-to-play marker avoiding second impact syndrome.
