This study set out to better characterize donor-specific HLA antibodies (DSA), which are linked to kidney-graft rejection but which current clinical assays cannot reliably separate into harmful and harmless. After reproducing HLA-monoclonal-antibody affinities across SPR, BLI, Luminex single-antigen beads and Flow Induced Dispersion Analysis (FIDA), the authors applied FIDA to simultaneously determine both the affinity and the concentration of DSA directly in 20 pre-transplant patient sera, without immobilization — measuring KD from 0.055 to 24.7 nM and concentrations from 11.2 to 1223 nM. Being in-solution on native soluble HLA, FIDA reported true affinity rather than avidity. It is an example of using FIDA to measure antibody binding affinity and concentration together in complex serum.