Background: The prognostic value of κ-free light chain (κ-FLC) index over the long term is unknown.

Objective: The objective of the study was to determine whether κ-FLC index determined at disease onset predicts relapse activity and disability accrual during long-term follow-up.

Methods: Patients with a first demyelinating event of the central nervous system who had cerebrospinal fluid and serum sampling were eligible for inclusion. At baseline, demographics, clinical data, number of T2 hyperintense (T2L) and contrast-enhancing lesions (CEL) on MRI were assessed. During follow-up occurrence of relapses, Expanded Disability Status Scale (EDSS) scores and disease-modifying treatments (DMT) were registered. κ-FLC was measured by nephelometry and κ-FLC index calculated as (CSF κ-FLC/serum κ-FLC)/albumin quotient.

Results: Sixty-four patients with a median age at onset of 32 years (25th-75th percentile: 27-39) and a female predominance of 75% were followed over a median of 113 (90-129) months. Forty-six (72%) patients experienced relapse, 30 (47%) showed disability accrual. Multivariable Cox regression analysis adjusted for age, sex, disease duration, T2L, CEL and DMT revealed that κ-FLC index independently predicts time to relapse (per increase of 10: hazard ratio (HR) = 1.04, lower limit (LL)-confidence interval (CI) = 1.001, p = 0.044) and disability accrual (per increase of 10: HR = 1.05, LL-CI = 1.009, p = 0.022).

Conclusions: κ-FLC index predicts long-term disease activity independently of other risk factors.