Potential Pitfalls of Serum Free Light Chain Analysis to Assess Treatment Response for Multiple Myeloma
Authors: Abbi Kamal Kant Singh , Tricot Guido J , Silverman Margarida , Nadiminti Kalyan , Krasowski Matthew 
Published: October 2019

Abstract
Background
The serum free light chain (FLC test) allows measurement of low concentrations of FLC. Post-transplantation, especially after tandem autologous transplants and consolidation therapy, the immune system is often extremely suppressed and its recovery is disorganized.
Patients and Methods
This study was limited to patients with multiple myeloma, plasma cell leukemia and amyloidosis, who received autologous transplantation and consolidation therapy at the University of Iowa Hospitals and clinics. Most of these patients had already received some form of induction therapy with IMIDs or proteasome inhibitors or in combination. They then proceeded with a cycle of D-PACE followed by one or two autologous transplants. The preparative regimen for virtually all patients was VDT-MEL. Post-transplantation, most of the patients received consolidation therapy with VTD the first year and either VCD or Revlimid/dexamethasone for second year. Thereafter, all myeloma therapy was halted. Serum free light chains were measured with polyclonal FLC antisera according to Freelite, Binding site, UK. The kappa/ lambda ratio was calculated.
Results
In total 142 patients were evaluated; 12 % (17/142) of patients were found to have abnormal light chains ratio but no other evidence of active disease, including negative serum M-protein, serum IFE, urine M-protein and urine IFE. In addition, bone marrows showed no evidence of clonal plasma cells by both 10-color flow cytometry and FISH analysis of CD138 selected plasma cells; both methods have a sensitivity of ≥ 10-4. The κ/λ ratio was abnormal due to increase/decrease in the same light chain as the M- protein in 11/17 patients; 6/17 patients had abnormal κ/λ ratio due to increase in the opposite light chain as the M- protein (Table 1)
Table 1. Patient with abnormal light chains due changes in Involved light chain levels Age Gender M protein/Light chain Transplant M protein Abnormal light chain Immunofixation Elevated or decreased Duration of abnormal ratio 64 Male IgG Kappa Tandem 0.0 Kappa Negative Elevated 2 weeks 58 Female IgA Kappa Tandem 0.0 Kappa Negative Elevated 4 weeks 46 Female Kappa Tandem 0.0 Kappa Negative Elevated 4 weeks 58 Male IgG Kappa Tandem 0.0 Kappa Negative Elevated 20 weeks 55 Male IgG Kappa Tandem 0.0 Kappa Negative Elevated 4 weeks 60 Male IgG Kappa Tandem 0.0 Kappa Negative Elevated One week 60 Female IgA Kappa Single 0.0 Kappa Negative Elevated 8 weeks 68 Female IgG Kappa Single 0.0 Kappa Negative Elevated 26 weeks 42 Male kappa Tandem 0.0 Kappa Negative Elevated One week 67 Male Kappa Single 0.0 Kappa Negative Decreased 3 weeks 68 Male IgG Kappa Single 0.0 Kappa Negative Elevated 8 weeks Patient with abnormal light chain ratio due to changes in the opposite light chain levels 53 Female IgG Lambda Tandem 0.0 Kappa Negative Elevated 2 weeks 62 Female IgA lambda Tandem 0.0 Kappa Negative Elevated 6 weeks 50 Male Lambda Single 0.0 Kappa Negative Elevated One week 60 Male IgG Lambda Tandem 0.0 Kappa Negative Elevated 100 weeks 68 Female IgA Lambda Single 0.0 Kappa Negative Elevated 8 weeks 68 Male lambda Single 0.0 Kappa Negative Elevated 4 weeks
Conclusions According to the IMWG uniform response criteria, patients achieving CR for whom the involved FLC reduced sufficiently to normalize the FLC ratio (range, 0.26 to 1.65) in the absence of monoclonal BMPCs as assessed by immunohistochemistry or immunofluorescence are considered to have achieved stringent CR. However, patients can be in stringent complete remission with abnormal k/l ratios if 1) the ratio is abnormal because the non-involved free light chain is elevated while the involved free light chain is normal; 2) the ratio is abnormal because involved light chain is elevated, with no other evidence of disease, including multicolor flow cytometry and FISH analysis on selected plasma cells of the bone marrow and imaging by MRI and/or PET-CT scan. This occurred in > 10% of patients. It should be noted that the FLC causing the abnormal k/l ratio was always kappa. The IMWG criteria should be adjusted these potential pitfalls.

Disclosures
No relevant conflicts of interest to declare.