The kappa free light chains index is an accurate diagnostic biomarker for paediatric multiple sclerosis.

Background: Multiple sclerosis (MS) may occur before the age of 18. Differentiation between paediatric MS (PedMS) and other demyelinating syndromes (ODSs) is challenging. In adult with MS, the kappa free light chain (KFLC) index has proven to be a reliable marker of intrathecal Ig synthesis.

Objective: To assess the diagnostic value of the KFLC index in a cohort of patients with paediatric-onset, inflammatory disorders of the CNS.

Methods: We included 73 patients and divided them into four groups: PedMS ( = 16), ODS ( = 17), encephalitis and/or inflammatory epilepsy (EE, = 15), and controls without inflammatory CNS diseases ( = 25). The KFLC index was calculated and compared with the results of the oligoclonal bands determination.

Results: The KFLC index was higher in the PedMS group (median (interquartile range (IQR)): 150.9 (41.02-310.6)) than in the ODS (3.37 (2.22-8.11)), the EE (5.53 (2.31-25.81)) and the control group (3.41 (2.27-5.08)), respectively. The best KFLC index cut-off for differentiating between patients with PedMS and controls was 6.83 (sensitivity: 100%; specificity: 92%). A KFLC index over 93.77 indicated that the patient is very likely to have PedMS (sensitivity: 68%; specificity: 100%).

Conclusion: The KFLC index is a reliable tool for the diagnosis of MS in a paediatric population.