Progress has been made in the treatment of multiple myeloma (MM), and a series of novel therapeutic agents, including antibody-based drugs such as elotuzumab and daratumumab, are available in the clinic. While fl ow cytometry (FCM) is a major method for MM diagnosis and evaluation of therapeutic effects, detecting MM cells after antibodybased therapies is challenging, as antibodies used for FCM sometimes recognize the same epitopes that are targeted by the therapeutic ones. As a result, FCM could fail to detect true MM clones. In this study, we examined the effi cacy and accuracy of the FCM-based diagnostic methods using an antibody targeting multiple epitopes of CD38 (CD38ME) and intracellular p63 as well as those targeting CD138 and CD38high. When we defi ned MM cells using antibodies against CD38ME and intracellular p63, proportions of MM cells were highly correlated with those defined by the conventional FCM methods using anti-CD38high and / or CD138 antibodies (r2 = 0.9967-0.9991). Interestingly, expression levels of CD38high and CD138 were signifi cantly low in MM cells obtained from antibody-treated individuals. In contrast, MM clones were accurately detected using antibodies against CD38ME and intracellular p63. Our data suggest that extra caution should be taken when MM cells obtained from patients treated with antibody-based therapies were evaluated by FCM. We propose that antibodies targeting CD38ME and/or intracellular p63 should be included in the antibody mixture for FCM-based detection of MM cells.
