Comparison of Free Light Chain Assay with Protein Electrophoresis for Screening and Monitoring PTLD

Background: Post-transplant lymphoproliferative disorder (PTLD) is primarily diagnosed histologically using tissue biopsy. Free light chain (FLC) assay and serum protein electrophoresis (SPE) have both been studied as tools to screen and monitor PTLD. However, limited data are available to compare these two assays in a well characterized patient population. It is also not clear what reference ranges should be adopted for the FLC assay in a post-transplant population. Method: Blood samples from 169 patients receiving a variety of solid organ transplants were analyzed for FLCs and screened for gammopathies by SPE/IFE. Results: Compared with non-PTLD patients, PTLD patients had higher mean, median and upper 95 percentile range of both κ and λ FLCs (p ranging from 0.0002 to 0.024). The mean, median and 95 percentile range of κ:λ ratio were similar between the two groups. PTLD patients were more likely to have polyclonal or monoclonal FLC elevations (p = 0.04). They also showed a higher frequency of gammopathy abnormalities (p = 0.0052). Nonetheless, neither FLC assay nor SPE demonstrated a clear association with the timing of PTLD diagnosis. FLC concentrations in non-PTLD recipients were higher than those in the general healthy population (95 percentile range: κ, 0.60-8.33 mg/dL vs. 0.33-1.94 mg/dL; λ, 0.77-7.08 mg/dL vs. 0.571-2.63 mg/dL) but the κ:λ ratio was similar to that of the healthy group (0.26-1.65). Conclusions: Our results suggested that elevated FLC concentrations and gammopathy abnormalities were both associated with PTLD. Therefore, FLC assay and SPE should be used conjunctively for screening PTLD among solid organ transplant recipients. For this application, the data showed that a higher upper limit of κ and λ FLC levels and normal κ:λ ratio should be used as diagnostic reference ranges. Additionally, neither method was clearly associated with the timing of PTLD diagnosis, indicating that they may be unsuitable for monitoring PTLD in the post-transplant population. Table 1. Longitudinal measurements of serum/plasma free light chains and SPE/IFE in eight PTLD cases. Type of transplant and type of PTLD Samples Days from PTLD diagnosisa κ FLC, mg/dL λ FLC, mg/dL κ /λ SPE/IFE Liver transplant, B-cell PTLD 1.1 1.2 – 616 2.33b 1.82 4.96 6.01 0.47 0.303 no band –c Liver transplant, B-cell PTLD 2.1 2.2 2.3 2.4 2.5 -145* -126 84 141 428 0.338 0.79 0.335 0.476 2.53 0.62 1.02 1.06 1.15 1.98 0.545 0.775 0.316 0.414 1.28 no band no band 1 IgG ©µ, 1 IgG λ 1 IgG ©µ, 1 IgG λ no band Liver transplant, polymorphic hyperplasia 3.1 3.2 -77 208 4.19 7.18 5.9 3.38 0.71 2.12 2 IgG ©µ, 2 λ FLC– Liver transplant, B-cell PTLD 4.1 4.2 4.3 61 272 537 4.64 3.25 7.1 11.1 6.65 10.96 0.418 0.489 0.648 no band no band no band Liver transplant, B-cell PTLD 5.1 5.2 5.3 5.4 5.5 9 12 393 429 476 305.5 957 0.721 1.01 1.24 74.25 192 1.67 1.72 2.56 4.11 4.98 0.432 0.587 0.484 1 IgG ©µ, 1 λ FLC 2 IgM ©µ no band no band no band Liver transplant, B-cell PTLD 6.1 6.2 6.3 6.4 62 153 174 188 3.16 5.58 3.97 2.14 2.92 3.91 3.81 3.37 1.08 1.43 1.04 0.635 – 1 IgG ©µ 1 IgG ©µ 1 IgG ©µ Kidney transplant, PTLD 7.1 7.2 15 30 1.4 1.68 1.58 2.06 0.89 0.82 no band no band Lung transplant, Non-Hodgkin lymphoma 8.1 -60 4.28 4.58 0.94 no band a Positive values indicate time points before PTLD diagnosis, while negative values indicate time points after PTLD diagnosis. b Numbers in bold format indicates values above ULN. c SPE/IFE results not available due to insufficient sample volume. Disclosures Kuhn: The Binding Site, Inc: Employment.