What are the criteria and steps to determine kidney transplant compatibility, including ABO blood type matching, HLA typing, cross‑match results, panel‑reactive antibody levels, and management of donor‑specific antibodies?

Kidney Transplant Compatibility Assessment

Kidney transplant compatibility requires ABO blood type compatibility, close HLA matching (particularly HLA-A, -B, and -DR), and a negative crossmatch for clinically relevant IgG antibodies to prevent hyperacute and accelerated rejection.

ABO Blood Type Matching

• The recipient's blood group must be compatible with the donor's blood group 1.
• ABO-incompatible transplantation should be considered experimental and requires specialized desensitization protocols 1.
• The exception is A2 donors into O recipients, as A2 blood type has lower antigen expression and may behave like type O 1.

HLA Typing and Matching

• Donors and recipients should be matched as closely as possible for HLA-A, -B, and -DR antigens 1.
• HLA-A, -B, and -DR antigens should be determined using standard serological and/or molecular (low to intermediate resolution) procedures 1.
• Molecular genotyping of HLA antigens should be strongly considered for African American transplant candidates 1.
• HLA-DQ matching is increasingly recognized as critical, as HLA-DQ mismatches are strongly associated with de novo donor-specific antibody formation, antibody-mediated rejection, and graft loss 1.

Panel Reactive Antibody (PRA) Testing

• Patient sera must be tested regularly (monthly, bimonthly, or quarterly) for antibodies to HLA antigens, as antibody levels fluctuate over time 1.
• Methods should be sensitive enough to identify clinically relevant IgG antibodies to HLA antigens 1.
• For highly sensitized patients (PRA ≥50%), avoid blood transfusions to reduce PRA levels; if transfusion is necessary, use leukocyte-poor blood 1.
• Newer assays using ELISA and Flow Bead technology with specific HLA antigens are more predictive of post-transplant rejection and graft loss than membrane-dependent assays 1.

Crossmatch Testing Algorithm

Sera Selection for Crossmatching

• When performing donor-specific crossmatches, use historically reactive (immune memory) sera, current sera, and immediately pre-transplant sera 1.
• Historical sera should not be limited to 3-6 months, as this may miss immune memory from 12-18 months prior, resulting in false-negative crossmatches in sensitized patients 1.

Crossmatch Methodology

• Anti-human globulin (AHG)-enhanced CDC or flow cytometry crossmatching should be used, as these are more sensitive than standard NIH-CDC assays 1.
• Both IgM and IgG antibodies can cause positive crossmatches, but only IgG antibodies are clinically relevant contraindications to transplantation 1.
• IgM reactivity should be eliminated by heat or chemical treatment to avoid false-positive results 1.

Interpreting Crossmatch Results

Critical Decision Points:

1. A positive T-cell or B-cell crossmatch with IgG antibodies to HLA antigens is an absolute contraindication to transplantation 1.

2. A positive crossmatch using sera without HLA antibodies (directed against non-HLA antigens) is NOT a contraindication to transplantation 1.

3. Positive B-cell crossmatches due to IgG antibodies are associated with poorer outcomes and should be treated as contraindications 1.

4. For flow cytometry-positive but AHG-CDC-negative results, each transplant center must use their own clinical data to determine acceptability, though the clinical relevance remains uncertain 1.

Management of Donor-Specific Antibodies (DSA)

Pre-Transplant DSA Assessment

• Sera should first be tested for the presence of IgG antibodies to HLA antigens using sensitive methods (ELISA, Flow Bead) before crossmatching 1.
• Knowing whether antibodies are HLA-specific is essential for correctly interpreting crossmatch results 1.

Acceptable DSA Levels

• Patients with DSA mean fluorescence intensity (MFI) >100,000 and flow crossmatch >200 median channel shifts are at significantly higher risk for antibody-mediated rejection 2.
• For successful transplantation after desensitization, DSA levels should ideally remain <100,000 MFI and decrease to approximately 10,000 MFI post-transplant 2.

Desensitization Strategies

• For highly sensitized patients where compatible donors cannot be found, desensitization may be performed using plasmapheresis or immunoadsorption, supplemented with IVIG and/or anti-CD20 antibody 3.
• Newer therapies such as imlifidase may offer alternatives 3.
• Kidney paired exchange programs should be preferred over desensitization when possible 3, 4.

Common Pitfalls to Avoid

• Do not limit historical PRA sera testing to only 3-6 months, as this misses immune memory and can result in false-negative crossmatches in sensitized patients 1.
• Do not proceed with transplantation based on a negative NIH-CDC crossmatch alone in sensitized patients; use AHG-enhanced or flow cytometry methods 1.
• Do not dismiss positive B-cell crossmatches as irrelevant; when due to IgG antibodies, they are associated with significantly poorer outcomes 1.
• Do not ignore HLA-DQ mismatches, as they are the predominant target for de novo DSA formation and are strongly associated with rejection and graft loss 1.
• Ensure correct HLA typing of donors, as typing errors can lead to catastrophic outcomes including hyperacute rejection 5.