Pre-transplant compatibility testing: Transition from CDCXM to flow cytometry in India.

Over 60 % of kidney transplant candidates are non-sensitised while remaining 40 % are sensitised because of previous exposure to human alloantigens during previous transplants, blood transfusions, and pregnancy in women. Pre-transplant compatibility testing is mandatory prior to renal transplantation for detecting the presence of donor-specific antibodies (DSAs), which are associated with early hyperacute/acute and later chronic rejections. Initially, complement-dependent cytotoxicity crossmatch (CDCXM) was used as a traditional method for detecting preformed DSAs. However, its limited sensitivity fails to detect low-level antibodies, and all non-complement-binding antibodies lead to early graft rejection despite a negative CDC-XM result. Flow cytometry crossmatch (FCXM), introduced in 1983, addresses these limitations by offering superior sensitivity in detecting DSAs. FCXM uses indirect immunofluorescence to differentiate T cell (expressing class I HLA) and B cell (expressing class I/II HLAs) reactivities providing a more precise assessment of DSAs, significantly decreasing the risk of rejection. Studies demonstrate that positive FCXM results, even with negative CDC-XM, correlate with higher rejection rates and reduced graft survival. Various modifications, including pronase treatment and especially streamlined Halifax and Halifaster protocols, have improved the test's specificity and speed. Despite its advantages, FCXM is also susceptible to potential false-positive and false-negative results because of non-HLA antibodies, cross-reactivity with therapeutic agents, and other technical factors. Nonetheless, FCXM remains a gold standard in modern transplantation immunology, enhancing safety of graft outcomes through better identification of immunologically significant DSAs. This review outlines the transition from CDCXM to FCXM in the Indian context, emphasizing its impact on pre-transplant testing and clinical decision-making.