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.

Table 1.

Flow cytometry of the bone marrow to detect MRD in multiple myeloma: current practice based on a survey of 30 major institutions in the United States

					Definition of MRD in bone marrow
Institution	Number of events acquired in MRD testing	Minimum number of abnormal plasma cells needed to call MRD	Maximum possible sensitivity, %*	Number of antigens studied	CD38+, CD138+ with monoclonal light chains	CD38+, CD138+, CD19−, CD45−	CD38+, CD138+, CD19−, CD45−, CD56+	CD38+, CD138+ with dim or negative CD27	Other
1	3 000 000-4 000 000	20	0.0005	12	×			×	×
2	1 800 000	50	0.003	9	×				×
3	1 000 000	50	0.005	8		×	×	×
4	500 000	25	0.005	6	×				×
5	500 000	NA		8	×
6	300 000-500 000	30-35	0.006	8	×	×	×
7	250 000-500 000	50	0.01	10	×	×	×	×
8	100 000	20	0.02	10	×	×	×		×
9	100 000	Variable†	NA	8		×			×
10	Variable	20	NA	8		×	×
11	NA	30	NA	8	×		×
12-26	Do not perform MRD testing				Do not perform MRD testing
27-30	Did not respond				Did not respond

NA, not applicable.

^*Maximum possible sensitivity determined by dividing the minimum number of abnormal plasma cells needs to call MRD by the number of events acquired in MRD testing (ie, a lower percentage indicates a more sensitive approach). For institutions with a range, the lowest number of minimum abnormal plasma cells needed and the highest number of events acquired were used for the calculation.

^†Pathologist dependent.

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.

Authorship

Acknowledgments: This research was supported by the Intramural Research Program of the National Cancer Institute of the National Institutes of Health.

Contribution: M.S.-S. designed and distributed the questionnaire and collected the data from participating institutions; A.F. tabulated the data; A.F. and O.L. gave input and drafted the manuscript; and all authors interpreted the data, reviewed, gave intellectual input, and approved the submitted version of the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Ola Landgren, National Cancer Institute, National Institutes of Health, Center for Cancer Research, Metabolism Branch, Multiple Myeloma Section, 9000 Rockville Pike, Building 10/Room 13N240, Bethesda, MD, 20892; e-mail: landgreo@mail.nih.gov.