Formation of antibodies against polymorphic HLA molecules on donor endothelium is central to the pathogenesis of antibody-mediated rejection, the dominant cause of long-term kidney allograft loss. Although introduction of the single-antigen bead assay has greatly facilitated the immune risk assessment of transplant recipients, it is recognized that not all IgG HLA antibodies detected using this method are equally relevant. In recent years, novel assays (C4d, C1q, C3d) have been developed to interrogate the complement-activating potential of anti-HLA antibodies in vitro, with the hypothesis that complement-fixing antibodies are more immediately injurious to the graft compared with noncomplement-binding antibodies. Although initial studies demonstrated the potential of these assays to risk-stratify antibodies beyond the conventional limited metric of mean fluorescence intensity values, new data from recent analyses challenge some of these early findings. In this review, we examine the technical aspects of these assays and key studies that evaluated the discriminant capacity of these tests to predict numerous outcomes in kidney transplantation. We discuss conflicting data and emerging controversies in the context of recent experimental evidence which offer new insights into the major factors that influence complement activation. Finally, we provide our perspective on the current role and utility of complement diagnostic assays as 1 variable in the multifactorial risk assessment and management of kidney transplant recipients.