A Rapid MALDI-TOF Method for Isotyping and Quantitating M-Proteins in a Single Assay: Longitudinal Comparison to Serum Protein Electrophoresis and Hevylite for Monitoring Patients with Monoclonal Gammopathies

Background: Current laboratory methods for detection of monoclonal gammopathies include a panel of serum protein electrophoresis (SPEP), immunofixation electrophoresis (IFE) and serum free light chain quantitation (FLC).  Our group has recently described a new assay based on matrix assisted laser desorption ionization time of flight (MALDI-TOF)  mass spectrometry (MS) which is capable of detecting, isotyping and quantitating M-proteins in a single assay. The basic principle of the method involves leveraging the unique mass resulting from light chain (LC) Ig gene rearrangement in B-cells.  In a similar fashion to SPEP, the mass distribution of LCs after dissociation from their heavy chains can be examined for the presence of an over expressed light chain.  In this study we retrospectively compared the MALDI-TOF MS method to SPEP, IFE and Hevylite ratios for monitoring patients with monoclonal gammopathies.

Methods:  Serum from patients who had at least 1 diagnostic and 4 serial follow-up samples was enriched for IgG, IgA, IgM, kappa and lambda using nanobodies.  After disassociating the heavy and light chains by reduction, the five purified samples were spotted onto a Bruker Microflex MALDI plate.  Automated acquisition (~10 seconds/sample) was performed and the five LC mass spectra from each enrichment were overlaid.   M-proteins were detected, quantitated and isotyped by the presence of distinct peaks (m-spike) within LC mass to charge regions.   The serial patient samples were then measured by MALDI-TOF and a comparison of results was made to previous aquired data.  The results were then analyze in light of clinical and lab data in order to evaluate the ability of the MALDI-TOF MS method to monitor patients with monoclonal gammopathies.

Results:  In M-protein positive patient samples, serial dilution revealed MALDI-TOF MS to have ~10-times lower limit of detection than IFE.  M-protein isotype in the cohort by MALDI-TOF MS was 100 percent concordant with the isotype from IFE.   The MALDI TOF M-protein quantitation was linear over a clinically acceptable range (0.1 to 6 g/dL) and had improved linearity over SPEP at lower M-protein levels.   M-proteins quantitation by MALDI-TOF MS compared well with SPEP with a slope of 1.16 (R²-0.93).  Hevylite Ig ratios for each isotype were statistically similar to those from MALDI-TOF MS demonstrating the ability of the MALDI to measure isotype suppression.   For some patients, the MALDI method was able to detect M-proteins in patients with normal Hevylite ratios.   Finally, the results for each patient were then compared over the course of monitoring.  The time evolution change in M-protein concentration was similar among the three methods with the exception of a few samples in which the MALDI-TOF detected residual M-proteins in treated patients not detected by the other methods.          

Conclusion: This preliminary study demonstrates that MALDI-TOF MS method can provide detection, quantitation and isotyping data suitable for monitoring patients.  This is a significant finding since the MALDI-TOF method is amendable to automation, is rapid and in its current format is cost-competitive with current clinical assays.