K Index is a Reliable Marker of Intrathecal Synthesis, and an Alternative to IgG Index in Multiple Sclerosis Diagnostic Work-Up

The K free light chain (K) index has been suggested as a reliable marker of intrathecal synthesis,despite the 2017 McDonald criteria for multiple sclerosis (MS) suggesting to "interpret with caution positiveimmunoglobulin G (IgG) index when testing for oligoclonal bands (OB) is negative or not performed". Theaim of this study was to compare the performance of K and IgG indexes for MS diagnosis and OB detectionin a cohort of Italian patients. We enrolled 385 patients (127 MS, 258 non-MS) who had cerebrospinal fluid(CSF) analysis, including isoelectric focusing (IEF), to detect OB in the diagnostic work-up. Albumin, IgGand free light chains were measured by nephelometry and used to calculate IgG and K indexes. Althoughthe two markers were highly related (r = 0.75, r2 = 0.55, p < 0.0001), the K index showed greater sensitivity andnegative predictive value (versus the IgG index) for OB detection (97% versus 48% and 97% versus 71%) andMS diagnosis (96% versus 50% and 98% versus 78%). These results support K index (and not IgG index) as afirst-line marker for MS, followed by IEF, according to a sequential testing approach in CSF analysis.