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A 68 year old female with a past medical history of IgG Kappa Multiple myeloma RISS Stage II, presents for follow up with associated symptoms of chronic back pain. (RISS stage II, +1q and t(11;14), hyperdiploidy) 3 months prior the patient completed 6 cycles of daratumumab/velcade/dexamethasone.
No plasma cells were seen in the squared box on CD38 versus CD138 ungated dot plot (A). However, a gate was placed on CD38-, CD138+ area (B), the gated cells showed expression of cytoplasmic kappa, not lambda (C). Bone marrow biopsy showed presence of atypical plasma cells with eccentric nuclei as seen on hematoxylin and eosin stain (10X, D), and immunohistochemistry were positive for CD138 (10X, E) and MUM-1 (40X, F).
In summary, flow cytometry showed atypical plasma cells expressing CD138, cytoplasmic kappa and CD20, they were negative for CD19, CD38, CD56, CD117 and cytoplasmic lambda. Overall, these clinical and histopathological findings were consistent with a diagnosis of CD38 negative persistent myeloma status post daratumumab treatment. The learning point for this case was to be aware of daratumumab’s effect on flow cytometry interpretation. Daratumumab is the human monoclonal antibody that targets CD38. Treatment with the drug could lead to decrease or no expression of CD38. If flow cytometry analysis was gating only on CD38+/CD138+ plasma cells, the diagnosis of CD38 negative myeloma might be missed.