Mass spectrometry is poised to transform the serological diagnosis and follow-up of multiple myeloma. By eliminating or reducing the impact of the challenges of current state of the art electrophoretic methods (such as co-migration of monoclonal immunoglobulins (M proteins) with other serum proteins and being masked by the polyclonal immunoglobulin background), it has been shown to achieve enhanced analytical sensitivity (1); and by identifying monoclonal immunoglobulins by their molecular mass, it enables personalized, patient-specific monitoring of M proteins.

The EXENT® solution (in development by The Binding Site, part of Thermo Fisher Scientific) uses matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry for the automated, algorithm-based detection, immunotyping and quantitation of M proteins. In a recent study, we aimed to assess the prevalence of multiple M proteins in monoclonal gammopathies with the EXENT solution, in a cohort of 271 patients of monoclonal gammopathy of undetermined significance, smoldering multiple myeloma, multiple myeloma, Waldenström’s macroglobulinemia and AL amyloidosis. Biclonal multiple myeloma or precursor conditions are rare (< 5%) when assessed with traditional technologies, but we have hypothesized that current electrophoretic methods may have insufficient analytical sensitivity to detect small additional M proteins or have insufficient resolution to discriminate between two M spikes or resolve M proteins from co-migrating serum proteins.

We found that the EXENT solution has identified a second M protein in the majority of the 271 patients, and in some cases, it detected a third or additional M proteins. Many of the M proteins were small, below the limit of quantitation, but 13.3% of the samples contained a second M protein that was an IgG or IgA or IgM with a concentration of ≥ 0.200 g/L. In one case, a second IgG kappa peak (16.9 g/L), not distinguishable from the primary peak by serum protein electrophoresis, was identified in a patient with a primary IgG kappa M protein, and in another case an additional IgA lambda peak (2.44 g/L) was found that was missed by electrophoresis due to its migration pattern. Thus, the frequency of two or more M proteins was higher than previously reported based on traditional technologies.

The other interesting observation was the high prevalence of minor M proteins in the majority of monoclonal gammopathy patients, in addition to their primary clone. Recent publications (2) and our internal data suggest that many non-monoclonal gammopathy patients, and even apparently healthy subjects have detectable, sometimes multiple, albeit small M proteins when their samples are analyzed with the EXENT solution. As these M proteins are not currently detected with electrophoretic techniques, their clinical significance is unknown. In other words, our analytical capabilities are ahead of our clinical understanding. We do not know if these M proteins are transient or permanent, or if part of the normal immune response or the start of a process that may lead to an actual monoclonal gammopathy. We do know, however, that the ability to detect and quantify M proteins that are present in very low concentration is of utmost importance for response assessment in patients who undergo treatment, therefore, the EXENT solution is expected to have a major impact on patient management. On the other hand, the clinical significance of small M proteins for the diagnosis on monoclonal gammopathies has yet to be determined. The threshold (0.200 g/L) that we have chosen is the approximate limit of detection of immunofixation electrophoresis, and it reflects the acknowledgement that M proteins that are smaller than that currently go unnoticed.

Technological advances always lead to deeper understanding in science; applying mass spectrometry for monoclonal gammopathy diagnosis and management will not be an exception to this. A lot of work needs to be done, but it will lead to better understanding of the behavior of the immune system and the origin and evolution of monoclonal gammopathies.

isclaimer: The EXENT solution has not been cleared for sale in the USA and is not commercially available. Future commercial availability cannot be guaranteed. The performance characteristics for this product are currently undergoing 510K review.


  1. Mills JR, Kohlhagen MC, Dasari S, Vanderboom PM, Kyle RA, Katzmann JA, Willrich MA, Barnidge DR, Dispenzieri A, Murray DL. Comprehensive Assessment of M-Proteins Using Nanobody Enrichment Coupled to MALDI-TOF Mass Spectrometry. Clin Chem. 2016 Oct;62(10):1334-44.
  2. El-Khoury H, Lee DJ, Alberge JB, Redd R, Cea-Curry CJ, Perry J, Barr H, Murphy C, Sakrikar D, Barnidge D, Bustoros M, Leblebjian H, Cowan A, Davis MI, Amstutz J, Boehner CJ, Lightbody ED, Sklavenitis-Pistofidis R, Perkins MC, Harding S, Mo CC, Kapoor P, Mikhael J, Borrello IM, Fonseca R, Weiss ST, Karlson E, Trippa L, Rebbeck TR, Getz G, Marinac CR, Ghobrial IM. Prevalence of monoclonal gammopathies and clinical outcomes in a high-risk US population screened by mass spectrometry: a multicentre cohort study. Lancet Haematol. 2022 May;9(5):e340-e349.