Implication of a positive virtual crossmatch with negative flow crossmatch: A mind-boggler

There are occasions when tests performed before considering a patient for transplant are ambiguous and require further workup. One such condition is the presence of a positive virtual crossmatch (VXm) (anti-human leukocyte antigen [HLA-A]*26: 01 antibody in this case) with a negative complement-dependent cytotoxicity, Luminex, and flow crossmatch. To ascertain the nature of the antibody, the beads used in single-antigen bead assay (SAB) were treated by acid to denature the antigens and retested with the control and test sample. The mean fluorescence intensities (MFIs) from the patient sera with acid-treated beads increased considerably as compared to the regularly untreated SAB indicating additional antigen epitopes become available by the denaturation process. The MFIs of the antibodies from that of the control sera were reduced to half on testing with the acid-treated SAB assay, indicating that HLA antigen HLA-A*26 was susceptible to acid treatment. Therefore, results of VXm should be interpreted with caution.