NEFL and myeloid sarcoma: In MS, the concentration of CSF NFL is increased during relapse and in conjunction with contrast-enhancing lesions on MRI.3, –, 6 The concentration is decreased by effective treatment with disease-modifying therapies (DMTs).3,4,7 The CSF NFL concentration at disease onset may predict disease severity.8,9 The sensitivity of the immunoassay has been improved, making it possible to determine NFL levels as low as those found in the CSF of healthy controls (HCs).1