Relevance of kappa free light chains index in patients with aquaporin-4 or myelin-oligodendrocyte-glycoprotein antibodies.

BACKGROUND: The kappa free light chains index (κ-index) is increasing in importance as a fast, easy, cost-effective, and quantitative biomarker in multiple sclerosis (MS), which can replace cerebrospinal fluid (CSF)-restricted oligoclonal bands (OCB) detection. In previous studies, controls often included mixed patients with several inflammatory central nervous system disorders. The aim of the present study was to assess the κ-index in patients with serum aquaporin-4 (AQP4)-IgG or myelin-oligodendrocyte-glycoprotein (MOG)-IgG. METHODS: We analyzed CSF/serum samples of patients with AQP4-IgG or MOG-Ig and evaluated distinct κ-index cut-offs. We described clinical and magnetic resonance imaging (MRI) features of patients with the highest κ-index values. RESULTS: In 11 patients with AQP4-IgG, median κ-index was 16.8 (range 0.2; 63) and 6/11 (54.5%) had κ-index >12. Among 42 patients with MOG-IgG, 2 had low positive MOG-IgG titers, were ultimately diagnosed with MS, and had a markedly increased κ-index (54.1 and 102.5 respectively). For the remaining 40 MOG-IgG-positive patients the median κ-index was 0.3 (range 0.1; 15.5). Some 6/40 (15%) and 1/40 (2.5%) patients had a κ-index >6 and >12, respectively. None fulfilled MRI dissemination in space and dissemination in time (DIS/DIT) criteria and the final diagnosis was MOG-IgG-associated disease (MOGAD) for these 40 patients. Four of the 40 (10%) MOG-IgG-positive patients had OCB. CONCLUSION: While a marked increase in κ-index could discriminate MS from MOGAD, a low κ-index threshold could lead to confusion between MS and MOGAD or AQP4 antibody-positive neuromyelitis optica spectrum disorder.