Delgado, Julio C.

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.

Abstract Background Serum protein electrophoresis (SPEP) is used to quantify the serum monoclonal component or M-protein, for diagnosis and monitoring of monoclonal gammopathies. Significant imprecision and inaccuracy pose challenges in reporting small M-proteins. Using therapeutic monoclonal antibody-spiked sera and a pooled beta-migrating M-protein, we aimed to assess SPEP limitations and variability across 16 laboratories in three continents. Methods Sera with normal, hypo- or hypergammaglobulinemia were spiked with daratumumab, Dara (cathodal migrating), or elotuzumab, Elo (central-gamma migrating), with concentrations from 0.125 to 10 g/L (n = 62) along with a beta-migrating sample (n = 9). Provided with total protein (reverse biuret, Siemens), laboratories blindly analyzed samples according to their SPEP and immunofixation (IFE) or immunosubtraction (ISUB) standard operating procedures. Sixteen laboratories reported the perpendicular drop (PD) method of gating the M-protein, while 10 used tangent skimming (TS). A mean percent recovery range of 80%–120% was set as acceptable. The inter-laboratory %CV was calculated. Results Gamma globulin background, migration pattern and concentration all affect the precision and accuracy of quantifying M-proteins by SPEP. As the background increases, imprecision increases and accuracy decreases leading to overestimation of M-protein quantitation especially evident in hypergamma samples, and more prominent with PD. Cathodal migrating M-proteins were associated with less imprecision and higher accuracy compared to central-gamma migrating M-proteins, which is attributed to the increased gamma background contribution in M-proteins migrating in the middle of the gamma fraction. There is greater imprecision and loss of accuracy at lower M-protein concentrations. Conclusions This study suggests that quantifying exceedingly low concentrations of M-proteins, although possible, may not yield adequate accuracy and precision between laboratories.

Context.— Serum tests used for the screening and diagnosis of monoclonal gammopathies include serum protein electrophoresis (SPE; agarose gel or capillary zone), immunofixation (IFE) and immunosubtraction capillary electrophoresis, serum free light chains, quantitative immunoglobulins, and heavy/light–chain combinations. Urine protein electrophoresis and urine IFE may also be used to identify Bence-Jones proteinuria. Objective.— To assess current laboratory practice for monoclonal gammopathy testing. Design.— In April 2016, a voluntary questionnaire was distributed to 923 laboratories participating in a protein electrophoresis proficiency testing survey. Results.— Seven hundred seventy-four laboratories from 38 countries and regions completed the questionnaire (83.9% response rate; 774 of 923). The majority of participants (68.6%; 520 of 758) used agarose gel electrophoresis as their SPE method, whereas 31.4% (238 of 758) used capillary zone electrophoresis. The most common test approaches used in screening were SPE with reflex to IFE/immunosubtraction capillary electrophoresis (39.3%; 299 of 760); SPE only (19.1%; 145 of 760); SPE and IFE or immunosubtraction capillary electrophoresis (13.9%; 106 of 760); and SPE with IFE, serum free light chain, and quantitative immunoglobulins (11.8%; 90 of 760). Only 39.8% (305 of 767) of laboratories offered panel testing for ordering convenience. Although SPE was used by most laboratories in diagnosing new cases of myeloma, when laboratories reported the primary test used to follow patients with monoclonal gammopathy, only 55.7% (403 of 724) chose SPE, with the next most common selections being IFE (18.9%; 137 of 724), serum free light chain (11.7%; 85 of 724), and immunosubtraction capillary electrophoresis (2.1%; 15 of 724). Conclusions.— Ordering and testing practices for the screening and diagnosis of monoclonal gammopathy vary widely across laboratories. Improving utilization management and report content, as well as recognition and development of laboratory-directed testing guidelines, may serve to enhance the clinical value of testing.