Aim To investigate observations of high rates of discrepancies between virtual crossmatch (VXM) positive and flow cytometry crossmatch (FCXM) negative cases in the presence of HLA-B13 DSA. Methods Sera with antibodies against HLA-B*13:01 and/or B*13:02 falling within a MFI ranging from 3286 to 11793, as determined by LABScreen Single Antigen beads (SAB) (One Lambda), were used in FCXM against cells expressing HLA-B*13:01, B*13:02 (sera n = 8, surrogate cells n = 5). Each serum sample was obtained from a unique donor. FCXM with non-B13 HLA class I DSA of a similar MFI were also conducted for comparison to B13 FCXM data. A linear regression analysis was conducted to determine correlation between MFI and MCS for both B13 and non-B13 DSA. Results Sera containing non-B13 DSA had a significant correlation between MCS and MFI values for both T (p 5000 producing positive reactions in only 66.7% of cases, compared to non-B13 DSA producing all positive FCXM reactions for DSA MFI > 5000. Additionally, of the five B13 sera tested on more than one cell, only two sera (#4 & #8) gave the same result (Fig. 1). Sera #1, 5 and 7 each resulted both negative and positive reactions. Conclusions High incidences of discrepancies in B13 FCXM indicate that the unpredictable nature of antigen availability at the cell surface occurs extensively in HLA-B13. One factor in these weak antigen-antibody interactions may be a lower salt bridge formation deduced from decreased protein-ligand interaction between B13 and KIR3DL1, which is unlike the other Bw4 + alleles. Another possibility, is that affinity level of anti-B13 reactions may be susceptible to certain eplets accessible only in naive antigen conformation. For certain specificities, including B13, it will be vital to evaluate FCXM reactions to avoid unnecessarily eliminating potential donors.