Abstract
Background Electrophoretic methods to detect, characterize and quantify M-proteins play an important role in the management of patients with monoclonal gammopathies (MGs). Significant uncertainty in the quantification and limit of detection (LOD) is documented when M-proteins are <10 g/L. Using spiked sera, we aimed to assess the variability in intact M-protein quantification and LOD across 16 laboratories. Methods Sera with normal, hypo- or hyper-gammaglobulinemia were spiked with daratumumab or elotuzumab, with concentrations from 0.125 to 10 g/L (n = 62) along with a beta-migrating sample (n = 9). Laboratories blindly analyzed samples according to their serum protein electrophoresis (SPEP)/isotyping standard operating procedures. LOD and intra-laboratory percent coefficient of variation (%CV) were calculated and further specified with regard to the method (gel/capillary electrophoresis [CZE]), gating strategy (perpendicular drop [PD]/tangent skimming [TS]), isotyping (immunofixation/immunosubtraction [ISUB]) and manufacturer (Helena/Sebia). Results All M-proteins ≥1 g/L were detected by SPEP. With isotyping the LOD was moderately more sensitive than with SPEP. The intensity of polyclonal background had the biggest negative impact on LOD. Independent of the method used, the intra-laboratory imprecision of M-protein quantification was small (mean CV = 5.0%). Low M-protein concentration and high polyclonal background had the strongest negative impact on intra-laboratory precision. All laboratories were able to follow trend of M-protein concentrations down to 1 g/L. Conclusions In this study, we describe a large variation in the reported LOD for both SPEP and isotyping; overall LOD is most affected by the polyclonal immunoglobulin background. Satisfactory intra-laboratory precision was demonstrated. This indicates that the quantification of small M-proteins to monitor patients over time is appropriate, when subsequent testing is performed within the same laboratory.
Highlights
Monoclonal gammopathies (MGs) are defined by the clonal expansion of plasma cells, resulting in the characteristic excretion of a monoclonal immunoglobulin (M-protein)
monoclonal gammopathies (MGs) encompass a broad spectrum of clinical disorders ranging from asymptomatic, MG of undetermined significance (MGUS) to life-threatening diseases, such as multiple myeloma (MM) and amyloid light chain (AL) amyloidosis [1, 2]
Limit of detection for serum protein electrophoresis (SPEP) and immunofixation electrophoresis (IFE)/ISUB. Both Daratumumab (Dara) and Elotuzumab (Elo) are of the IgGκ isotype and represent M-proteins in the gamma fraction. Both biologics have differing migration patterns, as Dara migrates at the cathodal end of the gamma fraction, whereas Elo migrates in the center of the gamma fraction on gel electrophoresis and at the anodal end of the gamma region by CZE
Summary
Monoclonal gammopathies (MGs) are defined by the clonal expansion of plasma cells, resulting in the characteristic excretion of a monoclonal immunoglobulin (M-protein). SPEP is further used for quantification, in which the M-protein is gated (M-spike) either by perpendicular drop (PD) or tangent skimming (TS) [3]. Significant uncertainty in the quantification and limit of detection (LOD) is documented when M-proteins are
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