BACKGROUND: Cerebrospinal fluid (CSF) free light kappa chains (kappa FLC) may be a more sensitive marker of intrathecal IgG synthesis compared to oligoclonal bands (OCBs). Our aim was to retrospectively determine the additional value of the kappa and lambda index (CSF FLC/serum FLC)/(CSF albumin/serum albumin) in predicting a Multiple Sclerosis (MS) diagnosis in a group of OCB-negative patients with suspected MS.
METHODS: CSF and serum kappa and lambda FLC were tested using Freelite kit (serum) and Freelite Mx (CSF) assay in 391 OCB-negative patients with suspected/possible MS and in 54 OCB-positive MS patients.
RESULTS: CSF kappa FLC levels were below the detection limit (0.27mg/L) in 61% of patients. Using quantitative data, we found the best kappa index cut-off value for the prediction of MS to be 5.8. A kappa index ≥5.8 was present in 25% of OCB-negative MS (23/92) and in 98% of OCB-positive MS patients. Using a qualitative approach and a kappa index cut-off of 5.9, based on literature data, we likewise found that 24% of OCB-negative MS patients had a kappa index ≥5.9, compared to 5.4% of OCB-negative non-MS patients (p<0.001). No reliable data could be obtained for the lambda index: lambda FLC were below the detection limit (0.68mg/L) in 90% of CSF samples.
CONCLUSIONS: The kappa index could contribute to the identification of OCB-negative patients with a high probability of a MS diagnosis. Using more sensitive techniques might even improve the diagnostic performance of the kappa index, and better define the role of the lambda index.