Abstract Background Mass spectrometry, an emerging alternative to traditional electrophoretic methods, shows promise for routine analysis of M-proteins in peripheral blood. The Immunoglobulin Isotypes (GAM) Assay for the EXENT Analyser (The Binding Site, part of Thermo Fisher Scientific) offers sensitive detection, isotyping, and quantification of M proteins by identifying their unique mass/charge ratio (m/z). This study aimed to evaluate the performance of the EXENT system in identifying and quantifying M proteins in baseline samples from patients with monoclonal gammopathies. Methods The study encompassed 718 patients with monoclonal gammopathies: 215 with Multiple Myeloma (MM), 152 with smoldering MM (SMM), 243 with monoclonal gammopathy of undetermined significance (MGUS), 53 with Waldenström’s Macroglobulinemia (WM), and 55 with AL amyloidosis. The isotype distribution included 359 IgG, 97 IgA, 98 IgM, 2 IgD, 89 light chain only (kappa or lambda) and 64 biclonal patients. Serum samples were analyzed at three sites using EXENT. A positive EXENT result was defined as the presence of an M protein, either an intact immunoglobulin (IgG /= 0.359 g/L; IgA /= 0.325 g/L; IgM /= 0.227 g/L) or a light chain-only. These isotype-specific cut-off values were established using serum samples from 364 healthy US subjects by the nonparametric percentile method, with the 95th percentile limit for IgG and the 99th percentile limit for IgA and IgM. In the 520 specimens with detectable IgG, IgA, or IgM M-protein by routine SPE and a positive EXENT result, concordance between methods for M-protein concentration was assessed using Pearson’s correlation and Passing-Bablok regression. Comparisons were conducted on all monoclonal gammopathy samples and separately for each disease condition and M-protein isotype (IgG, IgA, IgM). Isotype comparison included 601 samples that were immunofixation (IFE) positive, had monoclonal IgG, IgA or IgM M-protein /= the isotype-specific cut-off or a kappa or lambda M-protein by the EXENT System, and were not-biclonal. Results Quantitative comparisons included 520 patients (153 MM, 175 MGUS, 123 SMM, 50 WM, and 19 AL amyloidosis); 335 had monoclonal IgG, 75 IgA, and 82 IgM. Median M-protein levels were 10.4 g/L (SPE) and 10.9 g/L (EXENT). Passing-Bablok slopes ranged from 0.79 to 1.14 for each disease group and M-protein isotype, except for IgM (slope=1.55), WM patients (slope=1.56), and MGUS patients (slope=1.32). Pearson’s correlation coefficient was between 0.84 and 0.94 for the various disease groups and M-protein isotypes, indicating a strong linear relationship between methods. Overall concordance with IFE for the M-protein isotype was 97% (585/601 patients). Concordance per isotype was 93% for IgG, 99% for IgA, 97% for IgM, 81% for kappa, and 97% for lambda. The EXENT System identified a lambda monoclonal peak in one IgD lambda patient. Conclusion The EXENT System demonstrated high concordance with IFE for M-protein isotype and good quantitative agreement with SPE for M-protein quantitation. The higher IgM values reported by EXENT System are attributed to reliance on turbidimetric immunoglobulin measurement for quantitation, which is known to report systematically higher IgM M-protein values than densitometry.
Lakos et al. (Wed,) studied this question.