Our study demonstrates that nearly all patients (84%) with autoimmune GFAP‐astrocytopathy exhibit κ‐FLC intrathecal synthesis and that the positivity of the κ‐FLC index alone cannot reliably distinguish patients with autoimmune GFAP‐astrocytopathy from age‐ and sex‐matched MS patients. This evidence concerns the gene GFAP and myeloid sarcoma.