Inter-assay diagnostic accuracy of cerebrospinal fluid kappa free light chains for the diagnosis of multiple sclerosis.

Background: Cerebrospinal fluid (CSF) kappa free light chain (κFLC) measures gained increasing interest as diagnostic markers in multiple sclerosis (MS). However, the lack of studies comparing assay-dependent diagnostic cutoff values hinders their use in clinical practice. Additionally, the optimal κFLC parameter for identifying MS remains a subject of ongoing debate. Objectives: The aim of this study was to compare same-sample diagnostic accuracies of the κFLC index, κIgG index, CSF κFLC/IgG ratio, and isolated CSF κFLC (iCSF-κFLC) between two reference centers using different methods. Methods: Paired serum and CSF samples were analyzed for κFLC and albumin concentrations by Freelite®-Optilite (Sint-Jan Bruges hospital) and N Latex®-BNII (Ghent University hospital). Diagnostic performance to differentiate MS from controls was assessed using ROC curve analysis. Results: A total of 263 participants were included (MS, n = 80). Optimal diagnostic cutoff values for the κFLC index (Freelite®-Optilite: 7.7; N Latex®-BNII: 4.71), κIgG index (Freelite®-Optilite: 14.15, N Latex®-BNII: 12.19), and CSF κFLC/IgG ratio (Freelite®-Optilite: 2.27; N Latex®-BNII: 1.44) differed between the two methods. Sensitivities related to optimal cutoff values were 89.9% (Freelite®-Optilite) versus 94.6% (N Latex®-BNII) for the κFLC index, 91% (Freelite®-Optilite) versus 92.2% (N Latex®-BNII) for the κIgG index, and 81.3% (Freelite®-Optilite) versus 91.4% (N Latex®-BNII) for the CSF κFLC/IgG ratio. However, for iCSF-κFLC, optimal diagnostic cutoff values (0.36 mg/L) and related specificities (81.8%) were identical with a related diagnostic sensitivity of 89.9% for Freelite®-Optilite and 90.5% for N Latex®-BNII. The diagnostic performance of the κFLC index [area under the curve (AUC) Freelite®-Optilite: 0.924; N Latex®-BNII: 0.962] and κIgG index (AUC Freelite®-Optilite: 0.929; N Latex®-BNII: 0.961) was superior compared to CSF oligoclonal bands (AUC: 0.898, sensitivity: 83.8%, specificity: 95.9%). Conclusions: The κFLC index and the κIgG index seem to be excellent markers for identifying MS, irrespective of the method used for κFLC quantification. Based on the AUC, they appear to be the measures of choice. For all measures, optimal cutoff values differed between methods except for iCSF-κFLC. iCSF-κFLC might therefore serve as a method-independent, more cost-efficient, initial screening measure for MS. These findings are particularly relevant for clinical practice given the potential future implementation of intrathecal κFLC synthesis in MS diagnostic criteria and for future multicentre studies pooling data on κFLC measures.

High CDT without clinical context: Beware of the variant.

Carbohydrate-deficient transferrin (CDT) is a performant biomarker used for the diagnosis of chronic alcohol abuse. Here, we describe the case of a 39-year-old male of Tamil ethnicity who had extremely elevated (20%) CDT using capillary electrophoresis (but without glycoforms profile analysis), putting his driving license regranting at risk. However, the patient had no symptoms of chronic alcohol abuse, normal mean corpuscular volume and gamma-glutamyl transferase, and did not admit to any alcohol consumption. Re-analysis by N-Latex CDT immunoassay revealed a CDT at 1.7%. Further investigation by whole-exome sequencing revealed a c.1295A>G missense variant at the heterozygous state on the TFgene. This variant is characterized by an amino-acid change at a consensus sequence forN-glycosylation. Therefore, half of the patient transferrin proteins were lacking a completeN-glycan chain out of two, despite no alcohol consumption. This also explains the discrepancies between the techniques, as the NLatex antibodies did not recognize the mutated sequence. In conclusion, this case highlights the importance of comparing laboratory results between themselves and the clinical description, the absolute requirement for glycoforms profile analysis before delivering results, and the necessity to confirm intriguing results by another technique in a specialized laboratory.

Evaluation of the N-latex serum free light chain assay on the Siemens BNII analyzer and agreement with The Binding Site FreeLite assay on the SPAPlus.

OBJECTIVES: To evaluate the Siemens N-latex kappa free light chain (κFLC) and lambda FLC (λFLC) assays on the BNII nephelometer and assess agreement with The Binding Site Freelite FLC assays on the SPAPlus. DESIGN AND METHODS: Over 180 patient serum samples from routine analysis of κFLC and λFLC measured by the Freelite assay were collected for the study and measured with the N-latex κFLC and λFLC assays to assess precision, linearity, method comparison and dilutional effects. RESULTS: Complex precision showed coefficients of variation of 4.8-7.2% for the κFLC assay and 3.6-6.0% for the λFLC assay. Linearity assessment showed both assays were linear (κFLC, y=1.00x-0.09 and λFLC, y=1.050x-1.252). Qualitative method comparison showed 87.9% (116/132) agreement and Cohen's kappa of 80.4% between the κFLC assays and 72.6% (98/135) agreement and Cohen's kappa of 55.4% for the λFLC assays. Quantitative method comparison for κFLC<150mg/L was y=0.92x+2.21, R=0.661 and for λFLC<150mg/L was y=7.90x-137.96, R=0.526. Dilutional effects including antigen excess and non-linearity were also examined. CONCLUSIONS: The N-latex assay showed good precision and linearity with reasonable agreement to the Freelite assay. However, the assays should not be used interchangeably to monitor patients.

Comparison of Free Light Chain Assays: Freelite and N Latex in Diagnosis, Monitoring, and Predicting Survival in Light Chain Amyloidosis.

OBJECTIVES: Measurement of serum free light chains (FLCs) is critical in diagnosis, prognosis, and monitoring treatment responses in light chain (AL) amyloidosis. We compare the Freelite assay (polyclonal antibodies to hidden light chain epitopes), which is the current gold standard, with a new assay: a mixture of monoclonal antibodies to light chain epitopes (N Latex). METHODS: We collected 240 serum samples from 94 consecutive patients with newly diagnosed AL amyloidosis (at least three serial serum samples during the first 6 months) analyzed at the National Amyloidosis Centre, London, from January 2011 to April 2012. Concordance in detecting abnormal light chain components and hematologic response was assessed at 2, 4, and 6 months. RESULTS: The κ and λ clonal light chain involvement was 21% and 79%, respectively, with an abnormal κ/λ ratio or detectable protein in 78.7%. Median κ, λ, and difference in involved and uninvolved FLCs by Freelite and N Latex assays were 17.3 vs 16 mg/L (R(2 ) = 0.91), 48.8 vs 52.6 mg/L (R(2) = 0.52), and 43.2 vs 39.1 mg/L, respectively. Discordant κ/λ ratios at presentation were as follows: 10 of 90 abnormal by Freelite/normal by N Latex and 11 of 90 abnormal by N Latex/normal by Freelite. CONCLUSIONS: Both FLC assays show good correlation in detecting the abnormal light chain subtype with discordance in absolute values and thus are not interchangeable.