Validation and meta-analysis of kappa index biomarker in multiple sclerosis diagnosis

The importance of studying the cerebrospinal fluid (CSF) in Multiple Sclerosis (MS) is included in the last McDonald criteria (2018). The study of oligoclonal IgG bands (OCGB) assay is strongly recommended in some situations in which MS diagnosis is uncertain. New biomarkers are developed during the last years. Kappa free light chains (FLC) can predict conversion to MS in patients with Clinically Isolated Syndrome (CIS). The aim of this work is to validate the clinical usefulness of the kappa index, and to establish the actual state of knowledge for kappa index as a biomarker of conversion in CIS patients by a meta-analysis. Kappa index seems more relevant than the mere concentration of kappa FLC in CSF. In the validation study, 334 patients were included; in which 100 were CIS patients. Patients were divided in two groups according kappa index cut-off of 10.62: group 1 (kappa index>10.62); group 2 (kappa index<10.62). In group 1 more patients had positive OCGB, IgG index>0.56 and fulfilled magnetic resonance imaging (MRI) criteria. In contrast, in group 2, more patients showed negative OCGB, IgG index<0.56 and did not fulfilled MRI criteria. While 67.6% of patients from group 1 converted to MS, only 12.5% of patients from group 2 converted to MS. An HR of 6.02 was obtained in the Kaplan-Meier analysis. In the meta-analysis, 8 studies were finally included. The SROC curve revealed a high diagnostic performance for the kappa index as a MS diagnostic biomarker. Despite heterogeneity found between studies, the global OR revealed a good discriminatory capacity of kappa index. In conclusion, kappa index has a great clinical sensitivity and specificity as a support in MS diagnosis. High kappa index increase the probability of CIS to MS conversion. A correct sample processing in the preanalytical stage is key to obtain right results and to allow establishing comparison between laboratories.