Positive Single-Antigen Bead Assay with Negative Flow Crossmatch in a Renal Transplant - A Case Report

Abstract

The occasion is when there is discordance between the results of the cell-based and virtual crossmatch and requires further workup before considering a patient for renal transplant. A case of the positive single-antigen bead (SAB) with donor-specific antibodies (DSA) against HLA-A*33:01 with negative complement-dependent cytotoxicity crossmatch and flow crossmatch was identified. The acid treatments of the beads were done to denature the antigen to understand the nature of identified DSA. There was variation in the Mean fluorescence intensity (MFI) observed in between patient sera with acid-treated beads (significantly increased) in contrast to regularly untreated SAB. This indicated additional antigens become available by the denaturation process. Hence, before making a final decision about the transplant in cases of ambiguity, SAB results should be interpreted in light of other compatibility results.

Enhancing Precision of the Single-antigen Bead (SAB) Assay: Considerations and Challenges

“…In contrast, if an antibody targets a unique, private epitope exclusively present on a single HLA (antigen bead), it would exclusively bind to that specific bead, yielding a much more robust fluorescent signal. 14,15 As antigens consist of many epitopes, understanding this concept is crucial when interpreting test results. Furthermore, it is important to understand that the SAB assay evaluates how antibodies interact with antigens, rather than directly quantifying antibody levels.…”

“…The existence of these structural variations within the bead may impede the accurate assessment of anti-HLA antibodies. [15][16][17] Numerous strategies have been suggested to address this challenge, such as the application of a pretreatment reagent that captures nonspecific antibodies and the utilization of acid treatment to differentiate specific antibodies targeting denatured class I HLA. The adoption of acid treatment in combination with iBead has proven highly effective in mitigating issues associated with antibodies against denatured class I HLAs.…”