Virtual cross match (vXM)
In a vXM, profiles of reactivity from stored recipient samples, plus remote HLA typing of a deceased donor, are used to predict reactivity in advance of donor cells becoming available. Enables transplant to go ahead with certainty and shorter cold ischaemic times. Only suitable for low-risk recipients on the vXM list.
- Potential recipient checked against vXM list.
- Decision to proceed with vXM alone must be confirmed with on call H&I scientist.
Prospective cross match
For all other recipients, a full CDC crossmatch* must be performed using donor lymphocytes with recipient serum. Recipient serum usually must be fresh; occasionally a recent stored sample may be useful but later confirmation will then usually be required.
- Send spleen and lymph nodes to SNBTS H&I staff for an urgent lymphocytotoxic (CDC) crossmatch. Please ensure that a completed SNBTS histocompatibility /platelet immuno-haematology form is sent with the tissue.
- Results will be reported to the renal recipient coordinator. In addition a direct conversation should be had between consultant surgeon on call and H&I consultant.
About crossmatching and methods to detect anti-donor antibodies (DSA)
T cell (CDC) crossmatch
CDC = complement-dependent cytoxicity. Fresh donor lymphocytes (from blood or lymph nodes) are mixed with recipient serum in the presence of complement, to identify pre-formed cytotoxic antibodies to the donor. This is the classic crossmatching technique for organ transplantion. A positive result usually precludes transplantation because of the risk of early severe antibody-mediated, ‘hyperacute’ rejection.
Other donor-specific antibodies (DSA)
DSA detected by these methods correlate with risk of rejection and medium to long term outcomes, but to varying degrees depending on target and titre. They are implicated in antibody-mediated rejection, and used to select suitable donors and risk of rejection, and therefore intensity of immunosuppression.
- Flow cytometry – ‘B cell crossmatch’ – fresh donor B cells are mixed with recipient serum and analysed by FACS to detect recipient antibody binding. More sensitive than a CDC, and no test of cell lysis. Not associated with hyperacute rejection.
- Solid phase assay (e.g. Luminex) – recipient’s serum is tested against a panel of purified HLA molecules to detect number and precise targets of any anti-HLA antibodies. This assay is conducted without knowledge of potential donors, in advance of transplantation, or for monitoring post-transplant. Having uniformly low titres of DSA to donor antigens without a history of previous exposure to foreign HLA antigens makes a positive CDC crossmatch very unlikely – this is the basis of the Virtual Crossmatch (vXM).
Interpretation of the significance of results of tests for DSA, and reconciliation with crossmatch results, often requires expert input.