Chemosensitivity and resistance assays (CSRAs) aim to direct therapy based upon ex vivo response of patient tumor cells to chemotherapeutic drugs. However, successful CSRAs have yet to be developed. Here, we exposed primary CD138+ multiple myeloma (MM) cells to bortezomib, a clinical proteasome inhibitor, in microfluidic-cis-coculture (MicroC3) incorporating patientâs own CD138â tumor-companion mononuclear cells to integrate some of the patientsâ own tumor microenvironment components in CSRA design. Statistical clustering techniques segregated MicroC3 responses into two groups which correctly identified all seventeen patients as either clinically responsive or non-responsive to bortezomib-containing therapies. In contrast, when the same patient MM samples were analyzed in the absence of the CD138â cells (monoculture), the tumor cell responses did not segregate into clinical response clusters. Thus, MicroC3 identified bortezomib-therapy MM patient responses making it a viable CSRA candidate toward enabling personalized therapy.