The immunophenotypic stability of plasma cell myeloma by flow cytometry

Introduction: Flow cytometry (FC) has become increasingly utilized in the diagnosis and monitoring of plasma cell myeloma (PCM), though few studies have evaluated the longitudinal stability of antigen expression. Methods: We studied 45 PCM patients by four-color FC for shifts in CD19, CD20, CD38, CD45, CD56, and cytoplasmic light chain expression, between diagnostic/first encounter and positive follow-up analyses. An immunophenotypic (IP) change was defined as gain, loss, or 1/2 log shift of antigen expression. Results: An IP change was observed in 14/45 (31%) patients, with single IP changes in 9/14, two changes in 2/14, and three changes in 3/14. 3/14 reverted from an aberrant to a normal plasma cell IP, while remaining light chain-restricted. Changes in expression of CD45 occurred in 9/45 (20%), CD19 in 5/45 (11.1%), CD20 in 2/45 (4.4%), and CD56 in 5/45 (11.1%). Conclusion: Approximately 1/3 of PCM cases show IP changes over time, with CD45 the least stable antigen. Recognition of this relative instability is important to avoid narrow targeting of follow-up FC analyses, especially for minimal residual disease monitoring.