Abstract Background MALDI-ToF mass spectrometry is an analytically sensitive and specific method for the detection of monoclonal immunoglobulins (M proteins) in patients’ sera. While detecting low concentration M proteins aids in monitoring multiple myeloma (MM) patients into deeper response during therapy, there is concern that identifying low-level M proteins in the routine patient population may lead to unnecessary investigation and follow-up, patient anxiety and potential overdiagnosis of monoclonal gammopathy of undetermined significance (MGUS). The objective of this study was to establish clinical cut-off concentrations for M proteins measured by the Immunoglobulin Isotypes (GAM) for the EXENT® analyzer mass spectrometry assay (EXENT GAM) (The Binding Site, part of Thermo Fisher Scientific), to be used in the initial evaluation of patients with suspected monoclonal gammopathy. Methods Serum samples from 364 apparently healthy, ethnically diverse US subjects (188 males, 176 females, median age 57 years) were tested. All M proteins detected by the EXENT GAM assay were ranked according to their concentrations and their isotype, and a cut-off value was established by the nonparametric percentile method separately for IgG, IgA and IgM M proteins. The 95th percentile limit was used for IgG, and the 99th percentile limit for IgA and IgM to establish an M protein cut-off, to mirror the natural isotype distribution of MGUS: about 70% of MGUS is reported by the literature as of IgG isotype, with the rest consisting of IgM, IgA and bi-clonal MGUS. Results One or more M protein (any type and concentration) was detected by the EXENT GAM assay in 216 subjects (59.3%); 27.4% of the M proteins were below the lower limit of the measuring interval (LLMI). The cut-off was 0.359 g/L (95% CI: 0.224-0.866) for IgG, 0.325 g/L (0.133-2.320) for IgA and 0.227 g/L (0.132-0.829) for IgM. The number (%) of apparently healthy subjects with at least one intact immunoglobulin M protein above the established cut-off or total Kappa or Lambda M protein was 40 out of 364 (11.0%). The isotype distribution of M proteins in the 40 individuals is shown in the Table. 251 out of the 364 samples had serum protein electrophoresis (SPE) and immunofixation electrophoresis (IFE) results available, and 27 samples had positive or suspicious SPE. Among these 27 samples, 10 were positive and 3 indeterminate with IFE, and 12 were positive with the EXENT GAM assay, resulting in 85%/81% agreement with IFE (when IFE indeterminate results were considered negative, or positive, respectively). Conversely, out of the 23 samples that had intact immunoglobulin (IgG, or IgA or IgM) M protein above the cut-off by the EXENT GAM assay, 14 had SPE and IFE results available. Ten SPE results were positive or suspicious for M protein, and 8 were confirmed by IFE; for one sample IFE was indeterminate. Conclusion Population-based clinical cut-off values were established for IgG, IgA and IgM M proteins measured by MALDI-ToF mass spectrometry. Using these cut-off values for the interpretation of EXENT GAM assay data resulted in high level of agreement with IFE and would safeguard against overdiagnosis of MGUS
Lakos et al. (Wed,) studied this question.