AbstractBackground and objectivesAnti‐CD38 monoclonal antibodies, including daratumumab and isatuximab, often interfere with pretransfusion testing. Dithiothreitol (DTT) treatment of red blood cells (RBCs) negates this interference. However, the optimum DTT concentration and treatment time have not been well defined. Here, we quantified CD38 on RBCs before and after DTT treatment using a flow cytometric antibody binding assay (FABA) to specify the optimum conditions for CD38 inactivation.Materials and methodsFor FABA, untreated or DTT‐treated RBCs were incubated with fluorescein isothiocyanate‐labelled anti‐CD38 antibody, in the presence or absence of 100‐fold or more excess of unlabelled anti‐CD38 antibody, and then analysed by flow cytometry (FCM). Dissociation of CD38‐positive and control histograms was determined from the D‐value using the Kolmogorov–Smirnov test. The results from FABA were compared with those from conventional FCM, indirect antiglobulin test (IAT) and Western blotting.ResultsThe results from FABA were more consistent than those from conventional FCM. The D‐value was found to be reliable in the analysis of difference between CD38 before and after DTT treatment. Our data showed that 0·0075 mol/l DTT for 30 min is sufficient to inactivate CD38 on RBCs. These results were stable and consistent with the findings from IAT.ConclusionFlow cytometric antibody binding assay is an objective way of evaluating the efficacy of DTT treatment for CD38 on RBCs. This approach allows the detection of a small number of cell surface antigens and will be useful for assessing the various chemical treatments to denature RBC antigens.