Quantitation of free light chains in the cerebrospinal fluid reliably predicts their intrathecal synthesis

Abstract

Background: The results of free light chains quantitation in the cerebrospinal fluid were recently compared with the presence of cerebrospinal fluid-restricted oligoclonal IgG, but not oligoclonal free kappa light chains and oligoclonal free lambda light chains. We therefore aimed to compare the performance of the quantitative tests with the qualitative one for the same molecule. Methods: Seventy-five paired cerebrospinal fluid and serum samples were analysed for oligoclonal IgG, oligoclonal free kappa light chains and oligoclonal free lambda light chains. Cerebrospinal fluid and serum free kappa and lambda light chains were quantified using Freelite TM kits on SPA Plus analyzer. ROC curves were analysed for the prediction of intrathecal synthesis and compared for cerebrospinal fluid concentration, cerebrospinal fluid/serum quotient (Q fLC ) and index (Q fLC /Q Albumin ). The presence of cerebrospinal fluid-restricted oligoclonal free kappa light chains and oligoclonal free lambda light chains bands was used as reference. Results: No statistically significant differences were observed among cerebrospinal fluid concentration, Q fLC and index for the prediction of free light chain intrathecal synthesis. Each parameter was able to predict the occurrence of cerebrospinal fluid-restricted oligoclonal free light chain bands (AUCs 0.932-0.999). However, we noted elevated cerebrospinal fluid free light chain concentrations in the absence of cerebrospinal fluid-restricted oligoclonal free light chain bands in two patients with very high serum free light chain values. Conclusions: Quantitation of cerebrospinal fluid free light chains reliably predicts their intrathecal synthesis. Yet, cerebrospinal fluid/serum quotient may still be preferred to correct for high serum free light chain concentrations. An appropriate formula should be sought to correct for blood-cerebrospinal fluid barrier status.

Assessment of Intrathecal Free Light Chain Synthesis: Comparison of Different Quantitative Methods with the Detection of Oligoclonal Free Light Chains by Isoelectric Focusing and Affinity-Mediated Immunoblotting

“…We confirmed our earlier observations [39] that for both fKLC and fLLC there is a clear correlation between fLC quotient and albumin quotient values in cases with a negative o-fLC test. Conversely, no such a correlation was observed in cases with CSF-restricted o-fLC bands.…”

“…There was no significant difference in the AUCs between CSF concentrations, CSF/Serum quotients or indices. When the cut-off for CSF-restricted o-fKLC bands was raised to ≥6 bands as in our previous studies [37, 39], all AUCs for CSF fKLC, fKLC quotient as well as fKLC index were >0.95. For the Freelite method on SPA PLUS , calculated cut-off values were >0.78 mg/L for CSF fKLC, >43.19 ∙ 10 3 for fKLC quotient and >6.07 for fKLC index, i. e. similar to those found in our preliminary study [39].…”

“…When the cut-off for CSF-restricted o-fKLC bands was raised to ≥6 bands as in our previous studies [37, 39], all AUCs for CSF fKLC, fKLC quotient as well as fKLC index were >0.95. For the Freelite method on SPA PLUS , calculated cut-off values were >0.78 mg/L for CSF fKLC, >43.19 ∙ 10 3 for fKLC quotient and >6.07 for fKLC index, i. e. similar to those found in our preliminary study [39]. However, we feel that while searching for optimal cut-offs may be important for diagnostic purposes, the canonical definition of ≥2 CSF-restricted oligoclonal bands is rather appropriate for a methodical study.…”

“…in patients with paraproteinaemia or systemic inflammation). The relationship between fLC quotient and albumin quotient appears to be non-linear and the intrathecal synthesis could be calculated using either inverted Reiber’s hyperbole, as we proposed recently [39], or the power function proposed by Presslauer et al [7, 8]. We demonstrated that almost all values lie above the line of equality, as expected for the CSF/Serum quotient of a smaller molecule plotted on the ordinate and that of a larger molecule plotted on the abscissa [43].…”

“…Our sandwich ELISA, which was developed during several years, is based on the same principle as the o-fLC test [38]. After being CE marked for CSF fLC analysis, we started to use the Freelite assay in our laboratory [39] and aimed to compare it with other commercially available or in-house developed tests. The present paper is intended to report the analysis of such a comparison and to point to some questions that arose during this research.…”