Abstract
Protein and immunofixation (IFIX) electrophoresis are used to diagnose and monitor monoclonal gammopathies. While IFIX detects clonal production of intact immunoglobulins and free light chains (FLC), the latter can also be quantified using a serum free light chain (SFLC) assay, in which polyclonal antisera detects epitopes specific for free kappa (KFLC) or lambda light chains (LFLC). An abnormal KFLC: LFLC ratio (KLR) serves as a surrogate for clonality. While the SFLC assay is highly sensitive, normal LFLC (<2.63mg/dL) and KLR results (>0.26 & <1.65) were found in samples with distinct lambda monoclonal free light chains visualized by IFIX (X-LMFLC). To investigate this discordance, contemporaneous SFLC or KLR values were evaluated for their ability to accurately classify monoclonal FLCs identified by IFIX. We performed a retrospective analysis of serum and urine IFIX (Sebia Hydrasys) and SFLC (Freelite®, Binding Site) results from our institution between July 2010 through December 2020, using R 4.0.2 and Tidyverse packages. From among 9,594 encounters in which a single monoclonal component was initially identified by IFIX, 157 X-LMFLC and 131 X-KMFLC samples were analyzed. Elevated LFLC with normal KFLC was identified in 105/157 X-LMFLC samples (67%), while both LFLC and KFLC were elevated in 42/157 samples (27%). Concordance between X-KMFLC and KFLC was markedly higher, where 122/131 samples (93%) displayed elevated kappa FLC (>1.94mg/dL) with normal LFLC, and only 7/131 X-KMFLC samples (5%) possessed both elevated KFLC and LFLC. The use of KLR to identify pathogenic monoclonal free light chains improved lambda concordance to 85%; however, 19/157 (12%) of X-LMFLC samples still exhibited normal KLR. High concordance of 98% was again observed for X-KMFLC with abnormal KLR. When samples were segregated according to normal or impaired renal function (eGFR > or ≤60mL/min/1.73m², respectively), this disparate identification of X-LMFLC and X-KMFLC by the SFLC assay persisted, suggesting that renal dysfunction (as measured by eGFR) does not underlie this phenomenon. Lastly, we corroborated the above findings in a larger sample population by examining patients with urine Bence Jones FLC identified by IFIX who had free or intact monoclonal components in serum (N=724), grouped by lambda or kappa light chain involvement. The cause(s) of the discrepant performance by the Freelite® SFLC assay, relative to the Sebia Hydrasys IFIX assay, for identifying lambda FLC components is currently unclear. Possible contributory factors include assay reference range cutoffs, other patient disease parameters, and differences in assay-specific polyclonal antisera. Future analyses of these factors will help to further characterize SFLC assay performance and elucidate how interpretation of composite serum FLC test results can be improved to better guide patient management.
Long-Term Biological Variation of Serum Protein Electrophoresis M-Spike, Urine M-Spike, and Monoclonal Serum Free Light Chain Quantification: Implications for Monitoring Monoclonal Gammopathies
