NEFL and myeloid sarcoma: In multiple sclerosis (MS), it mainly reflects disease activity but may also predict disease severity1 and response to treatment.2, –4 Thus, high NfL concentrations in cerebrospinal fluid (CSF) and serum are associated with relapses, lesion formation on magnetic resonance imaging (MRI), and disease worsening.5, –7 Over recent years, serum NfL (sNfL) measurements have been investigated extensively, and sNfL has become an outcome measure in several clinical trials.8,9 However, the utility of monitoring sNfL changes in individual patients warrants further investigation.