Some informations are needed:

the ammount of plasma cells counted in the aspirate was via microscope, and it was around 95%.

the same material was used in the flow (bone marrow).

Also, the biopsi was investigated With immunohistochemistry, and it was found to have also around 95% of plasma cells ( I do not remember which markers, but they usually use 138, kappa/lambda

Flow cytometry typically underestimates the plasma cells in a bone marrow sample.  Some plasma cells are lost in processing of the flow cytometry specimen and this accounts for the discrepancy. The percentage of plasma cells should be based on morphologic assessment of CD138 on a bone marrow biopsy for the most accurate estimate.

thanks, I have seen that, but never such a big discrepancy, from 3% With flow to 95% With CD138 in the biopsi, that's a lot!

In the bone marrow the topographic distribution of plasma cells in cases of MM can be non homogeneous and in some cases "focal" aspects can be observed microscopically. The probability that one or more of these "hotbed" could constitute the prevalent material of a bone marrow aspiration must be considered. So, the case reported by Dr. Tacchi may be surprising for they entity but as much understandable and plausible

Which Ab is used for the detection of plasma cells in FACS?

If  anti-CD38 Ab is used, CD38-negative plasma cell components are not detected as clonal plasma cells by FACS.