Comparison of two serum free light chain assays for the diagnosis of primary plasma cell malignant proliferative disease

Background and aims: The serum free light chain assay (sFLC) is well established for aiding in the diagnosis, prognosis, and monitoring of plasma cell proliferative disorders. There are currently two commercially available sFLC immunoassays, Freelite, based on polyclonal antibody technology, and N Latex FLC, based on monoclonal antibodies. This study aimed to compare the analytical and clinical performance of these two assays in a Chinese population.

Methods: This study included 74 consecutive patients who were newly diagnosed with monoclonal gammopathies (MGs) including multiple myeloma (MM), AL amyloidosis, and light chain deposition disease (LCDD) between January 2014 and May 2015 at the First Affiliated Hospital of Zhejiang University. Alongside serum and urine electrophoresis analysis, the serum samples were retrospectively tested with both sFLC assays according to the manufacturers' instructions.

Results: The two sFLC assays showed a moderate correlation for κFLC (Passing-Bablok slope = 0.645, coefficient of determination (R ²) = 0.83, and Spearman coefficient = 0.904). However, for λFLC, a poor correlation was found (Passing-Bablok slope = 0.690, R ² = 0.39, and Spearman coefficient = 0.852). The concordance rate of κFLC, λFLC, and κ/λ FLC ratio were 83.78%, 75.68%, and 86.49%, respectively. The clinical sensitivity of the κ/λ ratios were 83.8% for the Freelite assay and 75.7% for the N Latex FLC assay.

Conclusion: Although the concordance and the clinical sensitivity of the two assays appeared comparable, a number of discrepancies were observed. There is a low correlation between the two assays in clinical practice, suggesting that the assays are not equivalent and, thus, current IMWG guidelines, based on Freelite, cannot be cross-applied to N Latex FLC.