Quantification of bone-marrow plasma cell levels using various International Myeloma Working Group response criteria in patients with multiple myeloma.

We examined the association of residual bone-marrow plasma cells (PCs) and International Myeloma Working Group (IMWG) response assessment in light-chain-only multiple myeloma (LCMM) and intact immunoglobulin multiple myeloma (IIMM) using multicolour flow cytometry (MFC). We identify considerable differences in bone-marrow neoplastic PC levels between IIMM and LCMM for the same IMWG response categories. Furthermore, even in the same IMWG response category, residual neoplastic PC levels differed considerably over a logarithmic scale range However, normalization of the free light-chain ratio is associated with deeper response (< 10-4) in LCMM, but not in IIMM. Our observations highlight the importance of quantifying BM PC by MFC for response assessment, especially for relapsed disease when there is suspected discrepancy in paraprotein levels and disease progression.