BACKGROUND Daratumumab (DARA), a promising novel therapy for multiple myeloma, is an IgG1κ monoclonal antibody that recognizes CD38 on myeloma cells. During routine compatibility testing, we observed that the plasma of five of five DARA‐treated patients demonstrated a positive antibody screen and panreactivity on red blood cell (RBC) panel testing. We hypothesized that the observed panreactivity reflected DARA binding to CD38 on reagent RBCs, and we investigated methods to prevent this binding. STUDY DESIGN AND METHODS DARA binding to CD38+ or CD38– HL60 cells was assessed by flow cytometry. To remove cell surface CD38, cells were incubated with dithiothreitol (DTT) or trypsin. Soluble CD38 or anti‐DARA was used to neutralize DARA in solution. Routine blood bank serologic methods were used to test samples from DARA‐treated patients and normal plasma samples spiked with DARA and/or alloantibodies. RESULTS Normal plasma samples spiked with DARA (0.1‐10 µg/mL) and incubated with reagent RBCs recapitulated the interference observed with samples from DARA‐treated patients. Flow cytometry experiments confirmed DARA binding to CD38+ HL60 cells, but not to CD38– controls. DTT treatment of CD38+ HL60 cells reduced DARA binding by 92% by denaturing cell surface CD38. Treating DARA‐containing plasma with soluble CD38 or anti‐DARA idiotype also inhibited DARA binding. CONCLUSION DARA causes panreactivity in vitro by binding to CD38 on reagent RBCs. Treating reagent RBCs with DTT is a robust method to negate the DARA interference, enabling the safe provision of blood to DARA‐treated patients. Because DTT denatures Kell antigens, K– units are provided to these patients.

BACKGROUND Daratumumab (DARA) consistently interferes with routine blood bank serologic testing by directly binding to CD38 expressed on reagent red blood cells (RBCs). Treating RBCs with dithiothreitol (DTT) eliminates the DARA interference. We conducted an international, multicenter, blinded study aimed at validating the DTT method for use by blood bank laboratories worldwide. STUDY DESIGN AND METHODS Paired plasma sample unknowns were sent to 25 participating blood bank laboratories. Sample 1 was spiked with DARA only (10 µg/mL), and Sample 2 with DARA plus a clinically significant RBC antibody (anti‐D [n = 6], anti‐Fya [n = 9], or anti‐s [n = 10]). Sites were instructed to perform an antibody screen with and without DTT‐treated RBCs and to use a DTT‐treated RBC panel for antibody identification. Qualitative data about the DTT method were collected by online survey. The primary outcome was the proportion of study sites able to identify the antibody unknown in the presence of DARA. RESULTS All sites observed the DARA interference with the antibody screen. The DARA interference was seen with all testing methods (gel, tube, or solid phase). Using the DTT method, 25 of 25 sites (100%) successfully identified the antibody unknown in the presence of DARA. Feedback on the DTT method was positive, with 17 of 19 (90%) sites responding to the survey indicating that they planned to use the DTT method to test clinical samples from DARA‐treated patients. CONCLUSION The DTT method is robust and reproducible and can be implemented by transfusion services worldwide to help provide safe blood products to patients treated with DARA.

Introduction Daratumumab (DARA), an IgG1k human monoclonal antibody (Ab) against CD38, is a promising novel therapy for multiple myeloma. However, direct binding of DARA to endogenous CD38 on reagent red blood cells (RBCs) interferes with routine blood bank serologic testing. We recently showed that treating reagent RBCs with DTT eliminates the DARA interference by denaturing cell surface CD38, allowing the safe transfusion of patients on DARA.1 This multicenter international study was aimed at validating the DTT method for use by blood banks worldwide. Methods Participating blood banks received two plasma sample unknowns. Sample 1 was spiked with DARA alone (5 mcg/mL). Sample 2 was spiked with DARA plus a clinically significant RBC Ab (anti-D (Rh immune globulin) or monoclonal anti-Fya or anti-s). Sites were instructed to first perform an Ab screen using their usual method (tube, gel, or solid phase), then to repeat the Ab screen using DTT-treated RBCs (gel or tube). If the Ab screen remained positive with DTT-treated RBCs (Sample 2), sites were to identify the unknown Ab using a DTT-treated RBC panel (gel or tube.) The primary outcome measure was the proportion of sites able to successfully identify the unknown Ab in the presence of DARA. Qualitative data were collected by online survey. Results Paired plasma sample unknowns were shipped to 25 study sites in North America, South America, Europe, Asia, and Australia/New Zealand. Data were received from 23 sites to date (Table). For the initial Ab screen, 10 sites used tube testing, 7 sites used gel, and 6 sites used solid phase. All sites observed DARA interference with the Ab screen (false positive agglutination reactions). All sites reported no DARA interference using DTT-treated RBCs. For Ab identification (Sample 2), 13 sites used tube testing and 10 sites used gel. 23/23 sites (100%) were able to correctly identify the unknown Ab using the DTT method. The Abs identified were: anti-Fya (9/9), anti-s (8/8), and anti-D (6/6). Feedback on the DTT method was mainly positive, with 86% of sites that responded to the survey indicating that they planned to use the DTT method to manage clinical samples from DARA-treated patients. Conclusion DARA consistently interferes with all three Ab screening methods currently used by blood banks (tube, gel, and solid phase.) Using DTT-treated RBCs, 23/23 (100%) of blood bank laboratories from around the world were able to identify a clinically significant Ab initially masked by the presence of DARA. The DTT method is robust, reproducible, and can be implemented by blood banks globally to help provide safe blood products to patients on DARA. As DTT denatures Kell antigens, K- RBC units should be provided when using the DTT method. 1. Chapuy CI, Nicholson RT, Aguad MD, et al. Resolving the daratumumab interference with blood compatibility testing. Transfusion. 2015;55(6pt2):1545-1554. Disclosures Unger: Janssen: Employment. Doshi:Janssen: Employment. Kaufman:Janssen: Consultancy, Research Funding.