Minimal residual disease testing in multiple myeloma by flow cytometry: major heterogeneity

To the editor:

As more effective therapies become routinely used to treat multiple myeloma,¹  standardized methods (such as flow cytometry) to determine the presence/absence of minimal residual disease (MRD) will likely play an increasingly important role in the clinical, research, and regulatory settings. Indeed, prior studies show that among patients who obtain a complete response, those who are MRD negative in their bone marrow by flow cytometry have better survival than those who are MRD negative.²  At this time, there are no established consensus criteria for MRD by flow cytometry of the bone marrow in multiple myeloma. To improve our understanding of MRD assessment practices, we conducted a survey of 30 major medical institutions in the United States. Directors of flow cytometry at each institution were sent an e-mail with 14 questions regarding their measurement of MRD by flow cytometry of the bone marrow in multiple myeloma patients.

Twenty-six institutions responded; 11 (42%) said they perform MRD testing (Table 1). The number of events acquired for MRD testing by flow cytometry varied from 100 000 to 4 000 000, with most institutions (6/11; 55%) obtaining 100 000 to 500 000 events. The number of abnormal plasma cells needed to define MRD positivity ranged from 20 to 50. In 2008, the European Myeloma Network organized 2 flow cytometry workshops to identify specific indications for flow cytometry in patients with monoclonal gammopathies and to facilitate the development of consensus technical approaches.³  In our survey, we found the definition of abnormal plasma cells to differ substantially between institutions (Table 1), with some relying on CD19 and CD45 negativity with or without CD56 positivity to determine the extent of MRD despite previous studies showing that normal plasma cell subpopulations can be negative for CD19 and CD45 or CD56 positive.^4,5  More specific antigens such as CD27, CD81, and CD117 were used by fewer than half of the institutions.

Our survey illustrates the heterogeneity in MRD testing of multiple myeloma by flow cytometry. There is considerable variation in the number of bone marrow cells analyzed (events) and number of abnormal plasma cells needed to define the presence of MRD, which affects maximum possible sensitivity. Furthermore, the maximum detection sensitivity ranged from 0.0005% to 0.02%, a 100-fold difference in sensitivity (Table 1). The variation in antibodies studied and definition of an abnormal plasma cell by flow cytometry affects ability to differentiate normal from neoplastic plasma cells.

As antimyeloma therapies become more effective, standardization of MRD testing by flow cytometry of the bone marrow will become increasingly important to allow better assessment of response and clinical prognostication. In addition to technical aspects, future clinical studies need to define the optimal timing to assess for MRD testing in multiple myeloma. At the current time, to our knowledge, most centers assess MRD status at the time of complete response. Collaborative efforts are needed to develop standardized criteria for MRD testing in multiple myeloma.