A Virtual Crossmatch is a crossmatch that involves a determination of the presence or absence of donor HLA specific antibodies (DSA) in a patient by comparing the patients’ HLA antibody specificity profile to the HLA type of the proposed donor without carrying out a ‘wet’ crossmatch such as a Complement Dependent Cytotoxic (CDC) or flowcytometric crossmatch.
Patel and Terasaki demonstrated in 1969 that hyperacute rejection can result from allograft injury caused by preformed donor specific anti-HLA antibodies (DSA) and since then a pre transplant crossmatch became a mandatory requirement for renal transplantation with a positive cytotoxic HLA crossmatch a contraindication to transplant. This pre transplant crossmatch almost completely eliminated hyperacute antibody mediated rejection. The single most important technological advance which has allowed this wet crossmatch to be potentially replaced with a Virtual Crossmatch is the development of solid phase assays such as Luminex for HLA antibody screening and identification. Use of Luminex assays has revolutionised HLA antibody investigation. One of the main advantages is the sensitivity and specificity of the results obtained. When using single antigen bead assays, for each antibody detected, a Mean Fluorescence Intensity (MFI) value is obtained which provides a measure of the strength of the antibody. Studies have found good correlation (>85%) between the Virtual Crossmatch as predicted based on MFI values obtained in Luminex and flowcytometric crossmatch results, with MFI figures of between 6000 and 10000 shown to correlate with positive flowcytometric crossmatch. There is however some inter laboratory variation which makes it important for each laboratory to establish the MFI values that correlate with positive crossmatch in their hands. The correlation indicated in published literature between a Virtual Crossmatch and CDC results has not been as strong.
The concordance between a Virtual and flowcytometric crossmatch is not 100% with cases of flowcytometric crossmatch Pos., Virtual Crossmatch Neg. and some cases of Virtual Crossmatch Pos, flow crossmatch Neg. The cases of flowcytometric crossmatch Pos. Virtual Crossmatch Neg. are possible explained by incomplete HLA typing of the donor, HLA-DP and DQA typing for instance may be required or it may be explained by the use of a solid phase assay with incomplete allele coverage on the panel. The cases of Virtual Crossmatch Pos., flow crossmatch Neg. are possibly explained by denatured epitopes in the solid phase giving false positive results.
The current British Transplant Society/British Society for Histocompatibility and Immunogenetics (BTS/BSHI 2010) guidelines permit the use of Virtual Crossmatching for renal transplantation in place of the pre-transplant wet crossmatch provided the laboratory has validated procedures in place. When a Virtual Crossmatch is used, an early post transplant wet crossmatch is required.
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