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

Abstract

AbstractBackgroundThe 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.MethodsWe 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.ResultsIn 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.ConclusionWhile 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.