Given the reported high accuracy of the κ‐FLC index for diagnosing MS and the limited information on this biomarker in other CNS inflammatory disorders, we proposed measuring κ‐FLC and λ‐FLC in a cohort of patients with autoimmune GFAP‐astrocytopathy, alongside age‐ and sex‐matched MS patients (the positive control group) and age‐ and sex‐matched symptomatic controls (the negative control group). Here, GFAP is linked to myeloid sarcoma.