The development of Freelite® assays that are validated for use in cerebrospinal fluid (CSF) provides an important tool to aid in the diagnosis of multiple sclerosis (MS). In this condition, κ FLC CSF concentrations are typically high, whilst λ FLC concentrations are only moderately elevated (Chapter 36). An elevated κ FLC index supports a diagnosis of MS, with similar diagnostic accuracy to oligoclonal band detection (Section 36.2).