Kappa-index: Real-life evaluation of a new tool for multiple sclerosis diagnosis.

The intrathecal production of oligoclonal immunoglobulin bands (OCB) is a prognostic factor for multiple sclerosis (MS) evolution in clinically isolated syndrome (CIS) patients and a diagnostic factor for MS. The kappa free light chain (K)-index represents a quantitative automated alternative to OCB. We retrospectively evaluated OCB and K-index results in 274 patients with MS (n = 48) or CIS (n = 29) at diagnosis, non-MS inflammatory central nervous diseases (n = 35), and non-inflammatory central/peripheral nervous diseases (n = 162). Several cut-offs were established: a pathophysiological cut-off (K-index: 3.3) useful for differential diagnosis (negative predictive value for MS >99%), an optimised cut-off (K-index: 9.1) with better sensitivity and equivalent specificity than OCB for the diagnosis of MS, and a high-risk cut-off (K-index: >55.0) allowing prediction of MS (specificity 100%). We developed a scaled interpretation of the K-index and we discuss the usefulness of testing OCB only when the K-index is positive >3.3 to obtain a better specificity.