Daratumumab and isatuximab differentially affect <scp>CD38</scp> detection on plasma cells in myeloma: Anti‐<scp>CD38</scp> nanobody (clone <scp>JK36</scp>) and <scp>CD319</scp> combination improve flow cytometric identification of plasma cells after

Inclusion of daratumumab and isatuximab in therapeutic regimens has improved outcomes in multiple myeloma patients. As these agents block CD38 detection, they may result in difficulties in flow cytometric (FC) identification of plasma cells (PCs). In addition, the heterogeneity of myeloma immunophenotype and the introduction of novel therapeutic agents necessitate FC panels with robust gating markers. To this end, we validated an 11-colour PC panel that in addition to commonly used gating markers (CD45, CD38, CD138) includes CD319 and anti-CD38 single variable heavy domain nanobody (clone JK36). A Boolean "or" function was applied to gate PCs using a combination of the above markers. Patterns and intensity of antigen expression were compared in samples from patients receiving daratumumab or isatuximab therapy, or neither. Daratumumab resulted in loss of detectable CD38 expression (clone HB-7) on abnormal PCs, and JK36 overcame daratumumab blockage in almost all these cases. Interestingly, isatuximab showed an opposite pattern, with complete loss of CD38 detection by JK36, while retaining dim positivity by HB-7. This combination of markers proved highly effective in identifying abnormal PCs after CD38-directed therapies, supporting a need for redundancy of gating markers in FC panels.