Donor-specific Antibodies (DSAs) are risk factors for rejection and graft loss in solid organ transplantation, especially associated with all HLA (Human Leukocyte Antigen) loci and class II in case of pre- and post-transplantation, respectively. It is recognized that the reactivity of HLA antigens varies according to the difference in HLA allele in the same HLA serotype. Therefore, epitope analysis becomes the useful method for evaluation of the HLA antibody specificity. Thus, HLA allele matching is more useful than HLA serotype matching, because it is possible that the same epitope in different HLA allele of patients and donors possessing the same HLA serotype is not shared. According to analysis of the association with epitope mismatch and rejection or graft loss reported by Wiebe et al, more numbers of HLA epitope mismatch causes worse outcome. The reason is attributed to the higher opportunity of de novo DSA production.
When the focus is on HLA antibodies detection, LCT (Lymphocyte Cytotoxicity Test), FCM (Flowcytometry), and Luminex method are widely used. The characteristics of these three methods are different. The LCT and FCM methods are based on the reaction of panel lymphocyte mixed with patient's serum but they are considered unsuitable for the identification of HLA antibodies specificities as this requires various kinds of panel lymphocytes. The Luminex method is based on solid phase assay, which uses the polystyrene bead coated with purified HLA antigen prepared by two different methods. One of them is the PRA (panel reactive antibody) method that utilizes HLA antigen extracted from a lymphocyte, and the other one is the SAB (single antigen bead) method, which utilizes HLA antigen prepared through gene-recombination technology. Interpretation of HLA antibodies specificity is difficult because of dissimilarity of beads made in two methods.
Two ways are considered for crossmatch test. One way is called the "direct crossmatch", in which test is made to determine whether donor lymphocytes in added patient serum are living or dying, similar to the LCT or FCM method. Another way is called the "virtual crossmatch", in which test is made to determine whether HLA type of donor and HLA antibodies specificity of the patient are same or different. The "virtual crossmatch" is more useful when not using donor lymphocytes and high sensitivity, but this is not so close to reaction in the body. The most ideal strategy is to conduct the "virtual crossmatch" test first because of high sensitivity, followed by the secondary "direct crossmatch" test if the result in the first test is positive, for more closer reaction of the body. Recently, ICFA method has been shown to be a more sensitive "direct crossmatch" test. We must choose the best way for crossmatch according to the situation.