Comparison of the Freelite serum free light chain (SFLC) assay with serum and urine electrophoresis/immunofixation and the N Latex FLC assay

• Published in: Pathology Vol. 47; no. 6; pp. 564 - 569
• Main Authors: Sasson, S.C., McGill, K., Wienholt, L., Carr, A., Brown, D.A., Kelleher, A.D., Breit, S.N., Sewell, W.A.
• Format: Journal Article
• Language: English
• Published: England Elsevier B.V 01.10.2015
• Subjects: Aged; Electrophoresis; Electrophoresis - methods; Female; Humans; Immunoassay - methods; immunofixation; Immunoglobulin Light Chains - blood; Immunoglobulin Light Chains - urine; Latex; Male; Middle Aged; myeloma; Paraproteinemias - diagnosis; plasma cell dyscrasias; Reproducibility of Results; Retrospective Studies; serum free light chains; SFLC; Electrophoresis; immunofixation; myeloma; serum free light chains; SFLC; plasma cell dyscrasias
• ISSN: 0031-3025; 1465-3931
• DOI: 10.1097/PAT.0000000000000316

Few reports have compared available serum free light chain (SFLC) assays. Here, a retrospective audit of the Freelite SFLC assay compared results to electrophoresis (EP)/immunofixation (IFX) and the N Latex FLC assay. A total of 244 samples collected over 3.5 months were studied using the Freelite and N Latex FLC nephelometry assays. Results were compared with serum and/or urine EP/IFX. The precision and linearity of the N Latex FLC assay was examined. Detectable paraprotein by serum or urine EP/IFX was present in 94% of samples with kappa and 100% with lambda FLC restriction. The correlation between the assays was higher for kappa (rho=0.97) than lambda (rho=0.89) especially when lambda results were above the upper limit of normal (rho=0.62). Agreement in the categorical diagnosis as measured by the Cohen’s kappa statistic was good (0.70). The N Latex FLC assay displayed good precision and linearity. In discordant samples the Freelite and N Latex FLC assays had equivalent agreement with IFX. Traditional methods of EP/IFX detected paraproteins in the majority of cases. Correlation between the Freelite and N Latex FLC assay is better for kappa than lambda FLC. The two assays are not entirely equivalent. Care should be taken by interpreting physicians and laboratories considering switching assays.