Kappa free light chains in cerebrospinal fluid as markers of intrathecal immunoglobulin synthesis.

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 kappa free light chains (kappaFLC) in cerebrospinal fluid (CSF) and serum, and compared it with traditional tests for intrathecal immunoglobulin synthesis. METHODS kappaFLCs 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 kappaFLC concentrations in CSF, as kappaFLC CSF/serum ratios, and by use of the quotient diagram described previously for immunoglobulins. RESULTS Both kappaFLC concentrations in CSF and the kappaFLC 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 kappaFLCs in CSF reliably detects intrathecal immunoglobulin synthesis. This automated and quantitative method could simplify the diagnostic procedure for CSF analysis in the routine laboratory.