Kappa Free Light Chains in Cerebrospinal Fluid as Markers of Intrathecal Immunoglobulin Synthesis

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Abstract
Background: Intrathecal immunoglobulin synthesis is observed in several inflammatory disorders of the central nervous system, but its detection by current laboratory tests is either tedious or relatively insensitive. We assessed the diagnostic accuracy of an assay for κ free light chains (κFLC) in cerebrospinal fluid (CSF) and serum, and compared it with traditional tests for intrathecal immunoglobulin synthesis. Methods: κFLCs were measured by nephelometry in CSF/serum pairs from 112 patients. Samples were excluded if blood contamination of CSF as a result of traumatic lumbar puncture (n = 12) or monoclonal bands in both CSF and serum (n = 5) were present. The remaining sample pairs were grouped according to the presence (n = 71) or absence (n = 24) of oligoclonal bands. Data were analyzed as κFLC concentrations in CSF, as κFLC CSF/serum ratios, and by use of the quotient diagram described previously for immunoglobulins. Results: Both κFLC concentrations in CSF and the κFLC CSF/serum ratio identified patients with oligoclonal bands with high specificity and sensitivity. The areas under the ROC curves were 0.991 (95% confidence interval, 0.944–0.998) and 0.978 (0.924–0.996), respectively. Exclusion of patients with impaired blood–CSF barrier function further improved diagnostic accuracy. To account for patients with impaired blood–CSF barrier function, data were also analyzed in a quotient diagram. Only two patients without detectable oligoclonal bands would have been misclassified by this approach. Conclusions: Our data indicate that the nephelometric assay for κFLCs in CSF reliably detects intrathecal immunoglobulin synthesis. This automated and quantitative method could simplify the diagnostic procedure for CSF analysis in the routine laboratory.

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