Data analytics improves the diagnostic accuracy of serum free light chain results for detecting monoclonal gammopathy

Objectives: To evaluate the real-world performance and reference intervals of the Binding Site Freelite serum free light chain (SFLC) assay (Thermo Fisher Scientific), a global standard for diagnosis, prognostication, and response assessment for monoclonal gammopathies.Methods: An informatics-based approach was used to retrospectively evaluate concordance between SFLC and the orthogonal Sebia HYDRASYS immunofixation assay results in a large clinical data set consecutively reported between 2010 and 2020.Results: Among patients with monoclonal-negative results by both SFLC and Sebia HYDRASYS immunofixation assays, 25% (1226/5057) had kappa/lambda ratios (KLRs) outside the manufacturer-defined and International Myeloma Working Group-cited normal reference interval of 0.26 to 1.65. These results were consistent over the study period and were not affected by sex, age, impaired kidney function, or assay antisera lot variation. Assay drift, in addition to other potential factors, affected the KLR distribution. Using International Statistical Classification of Diseases (ICD) codes, kidney function data, and the central 95% of KLR values generated on the Optilite platform (Thermo Fisher Scientific), we derived a new reference interval of 0.67 to 2.13, reducing the KLR false-positive rate to 8%. However, normal KLR persisted among 16% (14/85) of samples with free lambda chains by immunofixation, warranting caution during interpretation.Conclusions: Our analysis indicated that revision of Freelite SFLC reference intervals improves assay interpretation and should prompt reconsideration of Freelite reference intervals worldwide.

Automated summary

Our study evaluates the real-world performance and reference intervals of the Binding Site Freelite serum free light chain (SFLC) assay and highlights the impact of assay drift and other potential factors on the kappa/lambda ratio (KLR) distribution. We propose a revised reference interval for KLR that improves assay interpretation and should prompt the reconsideration of Freelite reference intervals worldwide.