Kidney matching

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Here is a comprehensive overview of kidney (renal) transplant matching, drawn from Brenner and Rector's The Kidney and Fischer's Mastery of Surgery:

Kidney Transplant Matching

Successful kidney transplantation depends on careful immunological and clinical matching between donor and recipient. The key components are:

1. ABO Blood Group Compatibility

ABO compatibility was historically the first and most absolute criterion. Transplanting an ABO-incompatible kidney causes immediate antibody-mediated hyperacute rejection (HAR) - the graft becomes infarcted within hours. The standard rule follows blood transfusion compatibility:

Blood group O: universal donor
Blood group AB: universal recipient

ABO-incompatible transplantation is now possible but requires desensitization (plasmapheresis, IVIG, rituximab) to reduce anti-A/B antibody titers before transplant. It is practiced more widely in Japan and some European centers, with outcomes similar to compatible transplants when carefully managed. - Brenner and Rector's The Kidney

2. HLA (Human Leukocyte Antigen) Matching

HLA antigens are the major histocompatibility complex (MHC) molecules. They are the primary target of immune rejection. Typing is done for:

Locus	Class
HLA-A	Class I
HLA-B	Class I
HLA-C	Class I
HLA-DR	Class II
HLA-DQ	Class II
HLA-DP	Class II

Traditionally, 6-antigen matching (at A, B, DR) was the gold standard. More mismatches = higher risk of rejection and sensitization. The relationship between HLA mismatch number and reduced graft survival is well established, though modern immunosuppression has narrowed (but not eliminated) this gap.

Key clinical implications:

Better HLA matching at a first transplant reduces sensitization and improves outcomes for subsequent transplants - especially relevant in pediatric patients who will likely need multiple grafts over their lifetime.
De novo donor-specific antibodies (DSAs) against HLA-DQ are among the most common and damaging, appearing in ~35% of children in the first years post-transplant. - Brenner and Rector's The Kidney

3. Panel Reactive Antibody (PRA) / Calculated PRA (cPRA)

The PRA is a measure of how broadly sensitized a recipient is against HLA antigens in the general donor population, expressed as a percentage. It is determined using solid-phase immunofluorescent assays (single-antigen bead technology).

High cPRA (e.g., >80%) = antibodies against a large proportion of potential donors = longer waiting time, harder to find a compatible kidney.
Causes of sensitization: prior transplants, blood transfusions, pregnancy.
High cPRA patients may need desensitization protocols (IVIG + plasmapheresis ± rituximab) to proceed with transplantation. - Fischer's Mastery of Surgery

4. Donor-Specific Antibodies (DSAs)

DSAs are preformed antibodies in the recipient directed specifically against the donor's HLA antigens. Their presence increases the risk of:

Hyperacute rejection (if high-titer and complement-fixing) - contraindication to transplant
Acute antibody-mediated rejection (AMR)
Chronic AMR and progressive graft loss
Complement-binding DSAs (C1q-binding) confer the highest risk of graft loss.

DSA levels are quantified by single-antigen bead assays. Low-level DSA may be manageable with desensitization; high-titer DSA is generally a contraindication. - Brenner and Rector's The Kidney

5. Crossmatch Testing

The crossmatch is the final compatibility check immediately before transplant - a "surrogate transplant" in vitro where donor lymphocytes are exposed to recipient serum.

a) Complement-Dependent Cytotoxicity (CDC) Crossmatch

The original method (introduced 1969)
Detects high-titer DSA capable of activating complement
Positive = contraindication (hyperacute rejection risk)
AHG (anti-human globulin)-enhanced CDC increases sensitivity for moderate-titer antibody

b) Flow Cytometry Crossmatch (FCXM)

More sensitive - detects lower-level and non-complement-fixing antibodies
A strongly positive FCXM is associated with higher rates of both acute and chronic AMR
Weakly positive FCXM: clinical significance is less clear-cut and center-dependent

c) Virtual Crossmatch

Uses HLA typing and DSA data from solid-phase assays to predict crossmatch result computationally - without needing live donor cells
Increasingly used, especially for deceased donor transplants where time is limited
Highly accurate when DSA data is complete; physical crossmatch still performed for high-risk/sensitized recipients

6. Other Factors in Allocation

Beyond immunologic matching, allocation algorithms weigh:

Factor	Relevance
Cold ischemia time	Longer time worsens outcomes; higher donor age amplifies this
Donor age	Older donors (especially >60) = higher risk of delayed graft function
Recipient waiting time	Ensures equitable access
Recipient sensitization (cPRA)	Prioritized access for highly sensitized patients
Geographic proximity	Minimizes cold ischemia time
Pediatric priority	Children receive priority for well-matched kidneys

Summary Table

Matching Parameter	Test Used	Clinical Impact
ABO blood group	Serology	Hyperacute rejection if incompatible
HLA typing	Molecular/serologic typing	Mismatch → rejection risk, sensitization
cPRA	Solid-phase bead assay	Measures breadth of sensitization
DSA	Single-antigen bead assay	Predicts rejection type and severity
CDC crossmatch	Cytotoxicity assay	Absolute contraindication if positive
Flow cytometry crossmatch	FCXM	More sensitive; guides risk stratification
Virtual crossmatch	Computational	Fast; used in deceased donor setting

Sources: Brenner and Rector's The Kidney (2-Volume Set); Fischer's Mastery of Surgery, 8th ed; Tietz Textbook of Laboratory Medicine, 7th ed.

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