Background

The kappa index, calculated by dividing the cerebrospinal (CSF)/serum kappa free light chain (KFLC) ratio by the CSF/serum albumin ratio, is gaining increasing interest as an indirect marker of intrathecal activation of the humoral immune response. The demonstration of intrathecal synthesis is of particular relevance in the diagnostic work-up of suspected Multiple Sclerosis. However, the lack of consistent data on inter-laboratory agreement in CSF and serum KFLC measurements is one of the factors that hamper the use of kappa index in routine practice.

Objectives

Aim of this study was to assess agreement in CSF and serum KFLC measurements and kappa index values across different laboratories.

Methods

Fifteen paired CSF and serum samples were analyzed in all participating laboratories (nr=8). Four centers used Binding Site instruments and assays, 3 centers used Siemens instruments and assays, and one center used a Siemens instrument and a Binding Site assay.

Absolute individual agreement between laboratories was calculated using a two-way mixed effects intraclass correlation coefficient (ICC). Cohen's kappa coefficient was used to measure inter-laboratory agreement on positive (≥5.8) kappa index values.

Results

Within Binding Site laboratories, ICC for KFLC measurements was 0.96 (95%CI: 0.9-0.98) for CSF, 0.93 (95%CI: 0.63-0.98) for serum and 0.97 (95%CI: 0.94-0.99) for kappa index values. Within Siemens laboratories, ICC for KFLC measurements was 0.99 (95%CI: 0.97-100) for CSF, 0.93 (95%CI: 0.48-0.98) for serum and 0.95 (95%CI: 0.89-0.98) for kappa index values. ICC calculated for all laboratories was 0.93 (95%CI: 0.87-0.97) for CSF KFLC, 0.81 (95%CI: 0.53-0.93) for serum KFLC and 0.65 (95%CI: 0.43-0.84) for kappa index. Cohen's kappa coefficient for a positive kappa index was 0.89 across Binding Site laboratories, 0.70 across Siemens laboratories, and 0.77 across all laboratories.

Conclusions

There was an excellent agreement in CSF KFLC measurements and in kappa index values within laboratories using the same instrument and assay (Binding Site or Siemens), while serum KFLC measurements were less concordant. Agreement across all laboratories was decreased when including the laboratory using a Siemens instrument coupled with a Binding Site assay in the analyses. Concordance for a positive kappa index was substantial across all laboratories and within Siemens laboratories, and very good within Binding Site laboratories.

Background

Detection of IgG Oligoclonal bands (OCB) in CSF by Isoelectric focusing (IEF) is an important criteria for multiple sclerosis (MS) diagnosis. Recently, quantitative measurement of CSF Free Light Chains (FLC), has been proposed as a faster, standardized and cheaper alternative to detect intrathecal Immunoglobulin (Ig) synthesis. However, FLC indexes (FLCI) cut off often varies depending on the selected population, with a lack of consensus.

Objectives

To assess, in a large and heterogeneous population, the diagnostic accuracy of CSF k and λ FLC in MS and other neurological diseases.

Methods

406 patients were selected in 5 Italian centres. OCB were detected by IEF, followed by immunodetection, FLCs were measured in paired CSF (LFLC) and serum (SFLC) using Freelite MX assays on the Optilite turbidimeter (Binding Site), as well as serum (SAlb) and CSF Albumin (LAlb). FLCI= (LFLC/SFLC)/(LAlb/SAlb).

Results

Patients: 174 MS, 149 non-inflammatory neurological disorders (NIND), 50 inflammatory CNS disorders (IND), 33 peripheral neurological disorders (PIND). KFLCI was significantly higher in patients with MS compared to the other groups (p <0.0001). The best kFLCI cut off for the prediction of MS was 6.4 (kFLCI+) with a diagnostic accuracy comparable to OCB (sensitivity 82.2% vs 82,7%; specificity 88.4% vs 90,5%). 7% MS patients were kFLCI+ and OCB negative (-), 7% were kFLCI- and OCB positive (+), with 86% concordance of both tests. 40% IND were kFLCI+, 35% of which OCB-. 4% NIND were kFLCI+, with only 1 patient OBC+. 27% PIND were kFLCI+ and 21% OBC+. λFLCI values were higher in the MS group, but gave few additional information (optimal cut off 13.5). In MS group, OCB results were highly variable across the centres, while kFLC were more consistent, confirming that FLCI is less subject to personal interpretation.

Conclusions

Our findings support the combined use of KFLCI and OCB to detect intrathecal Ig synthesis. KFLCI can accurately discriminate MS from NIND and PIND patients. IND patients showed high KFLCI in higher proportion than OCB, hence clinical, imaging and other laboratory data are necessary for a correct diagnosis. On the other hand, OCB has technical limitations and showed high variability across the centres to support MS diagnosis. In conclusion, KFLC performances in our heterogeneous population, with patients coming from 5 geographically distinct centres, support the robustness of this test implying that it can be easily used across our country.

Background

Detection of IgG Oligoclonal bands (OCB) in CSF by Isoelectric focusing (IEF) is an important criteria for multiple sclerosis (MS) diagnosis. Recently, quantitative measurement of CSF Free Light Chains (FLC), has been proposed as a faster, standardized and cheaper alternative to detect intrathecal Immunoglobulin (Ig) synthesis. However, FLC indexes (FLCI) cut off often varies depending on the selected population, with a lack of consensus.

Objectives

To assess, in a large and heterogeneous population, the diagnostic accuracy of CSF k and λ FLC in MS and other neurological diseases.

Methods

406 patients were selected in 5 Italian centres. OCB were detected by IEF, followed by immunodetection, FLCs were measured in paired CSF (LFLC) and serum (SFLC) using Freelite MX assays on the Optilite turbidimeter (Binding Site), as well as serum (SAlb) and CSF Albumin (LAlb). FLCI= (LFLC/SFLC)/(LAlb/SAlb).

Results

Patients: 174 MS, 149 non-inflammatory neurological disorders (NIND), 50 inflammatory CNS disorders (IND), 33 peripheral neurological disorders (PIND). KFLCI was significantly higher in patients with MS compared to the other groups (p <0.0001). The best kFLCI cut off for the prediction of MS was 6.4 (kFLCI+) with a diagnostic accuracy comparable to OCB (sensitivity 82.2% vs 82,7%; specificity 88.4% vs 90,5%). 7% MS patients were kFLCI+ and OCB negative (-), 7% were kFLCI- and OCB positive (+), with 86% concordance of both tests. 40% IND were kFLCI+, 35% of which OCB-. 4% NIND were kFLCI+, with only 1 patient OBC+. 27% PIND were kFLCI+ and 21% OBC+. λFLCI values were higher in the MS group, but gave few additional information (optimal cut off 13.5). In MS group, OCB results were highly variable across the centres, while kFLC were more consistent, confirming that FLCI is less subject to personal interpretation.

Conclusions

Our findings support the combined use of KFLCI and OCB to detect intrathecal Ig synthesis. KFLCI can accurately discriminate MS from NIND and PIND patients. IND patients showed high KFLCI in higher proportion than OCB, hence clinical, imaging and other laboratory data are necessary for a correct diagnosis. On the other hand, OCB has technical limitations and showed high variability across the centres to support MS diagnosis. In conclusion, KFLC performances in our heterogeneous population, with patients coming from 5 geographically distinct centres, support the robustness of this test implying that it can be easily used across our country.