Development of a Cost-effective Clinical Assay to Simultaneously Screen, Quantitate and Isotype M-proteins in Real-Time Using MALDI-TOF

Abstract

Background: Current laboratory detection of monoclonal gammopathies includes a panel of serum protein electrophoresis (SPEP), immunofixation electrophoresis (IFE) and serum free light chain quantitation (FLC). In this study we sought to evaluate an emerging clinical platform: matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry (MS) as a cost effective, sensitive method to detect, isotype and quantitate M-proteins. Methods: We developed an immuno-enrichment MALDI-TOF MS assay utilizing nanobodies to detect, quantitate and isotype Mproteins. Residual patient serum was enriched for IgG, IgA, IgM, kappa and lambda using nanobodies. After disassociating the heavy or light chains, five separate samples were spotted onto a Bruker Microflex MALDI plate. Automated acquisition (w10 seconds/ sample) was performed and the mass spectra corresponding to light chains from each nanobody were overlaid. M-proteins were detected, quantitated and isotyped by the presence of distinct peaks (mspike) within light chain m/z regions. Results: In M-protein positive patient samples, serial dilution revealed MALDI-TOF MS to have w10-fold lower limit of detection than IFE. Screening concordance for the MALDI-TOF MS was 97.8% for IFE(+) and 100% for SPEP(+) samples. For IFE(-) samples, the concordance was 88.6% with the MALDI-TOF identifying 4 additional positive patients. In samples positive by both methods (N1⁄487), isotyping concordance was 98.9%. The M-Spike quantitation correlation between SPEP and MALDI-TOF was linear with the MALDI giving slightly lower values. The MALDI-TOF method also had good agreement with the Hevy-LiteTM assay for the all three Ig classes. Conclusion: This study demonstrates that detection, quantitation and isotyping of M-proteins by MALDI-TOF MS is robust and exceeds the sensitivity of IFE. Unlike current methods, the MALDI can also document isotype suppression. This method is amendable to automation, is rapid and in its current format is costcompetitive with current clinical isotyping assays.