Intrathecal immunoglobulin M production: A promising high‐risk marker in clinically isolated syndrome patients

: Markers predicting the course of patients with clinically isolated syndrome (CIS) toward multiple sclerosis (MS) are urgently needed. We evaluated the predictive values of intrathecal immunoglobulin (Ig) G and IgM production with different methods for conversion to definite MS according to revised McDonald 2010 criteria in a cohort of 126 CIS patients. Intrathecal IgM production showed the highest likelihood ratio for the conversion of CIS patients to definite MS at 6.3, whereas it was 1.4 for oligoclonal IgG bands. We conclude that the determination of intrathecal IgM is a valuable tool to predict the disease course of patients with CIS. Ann Neurol 2018;83:1032-1036.