Intrathecal K free light chain synthesis in multiple sclerosis at clinical onset associates with local IgG production and improves the diagnostic value of cerebrospinal fluid examination

•Immunoglobulin free light chains (FLC) are intrathecally synthesized in MS.•A correlation between intrathecal IgG synthesis and κFLC, but not λFLC, was found.•Intrathecal IgM synthesis mildly associated with λFLC, but not with κFLC.•At clinical onset, 26% of MS patients without IgGOB had increased FLC Indexes.