NEFL and myeloid sarcoma: In these situations, Nf is released into the interstitial fluid and into CSF.20 In MS, the CSF NfL concentrations increase after relapses, reaching their peak at 2 weeks after the beginning of symptoms, remaining elevated for at least 15 weeks after an exacerbation.21,22 Cerebrospinal fluid NfL levels are associated with the presence of gadolinium enhancing MRI lesions.23 These data indicates that increased CSF NfL levels correlate with clinical and MRI parameters, being a potential biomarker of MS clinical activity.