A transplant crossmatch test estimates whether a recipient’s antibodies are likely to attack cells from a specific donor. A negative crossmatch generally supports immunologic compatibility, while a positive crossmatch suggests that donor-reactive antibodies are present and may raise the risk of hyperacute or antibody-mediated rejection. The result is important, but it is not interpreted as a simple pass-or-fail number in every transplant program.
Crossmatching may be performed physically by mixing recipient serum with donor lymphocytes or virtually by comparing the recipient’s anti-HLA antibody profile with the donor’s HLA type. Laboratories may use complement-dependent cytotoxicity, flow cytometry, or solid-phase antibody data. The type of crossmatch, whether T cells or B cells react, antibody strength, treatment history, and organ being transplanted all affect meaning. A positive result can delay or redirect transplantation, prompt additional testing, lead to paired exchange or desensitization, or require an individualized risk plan.
- A negative crossmatch means no clinically significant donor-reactive antibody was detected by that method, but it cannot guarantee freedom from rejection.
- A positive T-cell cytotoxic crossmatch is usually a major concern because it may indicate strong class I donor-specific antibodies.
- A positive flow crossmatch can detect weaker antibodies than a cytotoxic crossmatch and requires correlation with single-antigen bead testing.
- A virtual crossmatch compares donor HLA typing with the recipient’s current antibody profile and may allow faster organ allocation.
- Recent transfusion, pregnancy, transplant, infection, antibody-removing treatment, or immune therapy can change or interfere with results.
Table of Contents
- What a Transplant Crossmatch Tests
- CDC, Flow Cytometry, and Virtual Crossmatch
- How the Test Is Performed
- What a Positive Crossmatch Means
- Negative, Borderline, and Indeterminate Results
- How Crossmatch Use Differs by Transplant Type
- False Results, Technical Limits, and Interference
- Next Steps After a Crossmatch Result
What a Transplant Crossmatch Tests
A crossmatch asks whether antibodies in the intended recipient bind to cells or HLA molecules from a particular donor. The recipient’s serum is the antibody-containing part of blood. The donor contributes cells, usually lymphocytes, or an HLA profile used for a virtual comparison.
HLA proteins help the immune system distinguish self from foreign tissue. They vary greatly among people. A transplant recipient may develop antibodies against HLA types that the recipient does not have after pregnancy, blood transfusion, a prior transplant, or sometimes without an obvious sensitizing event. If a potential donor expresses the targeted HLA, those antibodies are called donor-specific antibodies, or DSA.
A strong preexisting DSA can bind the graft’s blood vessels immediately after transplantation. It may activate complement, injure endothelial cells, cause clotting, and lead to hyperacute rejection. Modern crossmatch and antibody testing have made classic hyperacute rejection uncommon, but antibody-mediated injury remains an important risk.
Crossmatching is not the same as HLA matching. Matching measures how similar the donor and recipient HLA types are. Crossmatching tests whether the recipient already has antibodies that react with that specific donor. A poorly HLA-matched donor can still have a negative crossmatch if the recipient has not formed antibodies to those mismatches. A well-matched donor could be incompatible if the recipient has antibody to one of the donor’s HLA antigens.
The crossmatch also differs from a general HLA antibody test. Antibody screening identifies the specific HLA targets in the recipient’s serum. The crossmatch translates that information into compatibility with one donor.
A result must be tied to the method and sample date. “Crossmatch negative” is incomplete without knowing whether it was a complement-dependent cytotoxicity test, flow cytometric test, virtual assessment, or a combination. Each method has different sensitivity and limitations.
CDC, Flow Cytometry, and Virtual Crossmatch
Three approaches are commonly used in solid organ transplantation. Some centers use more than one method, while others rely primarily on virtual crossmatching for selected recipients and donors.
Complement-dependent cytotoxicity crossmatch
The complement-dependent cytotoxicity crossmatch, often called CDC or CDCXM, mixes recipient serum with donor lymphocytes and adds complement. If recipient antibodies bind the donor cells strongly enough and activate complement, the cells die. A dye identifies dead cells under a microscope or automated reader.
CDC detects relatively strong, complement-fixing antibodies. A positive T-cell CDC crossmatch is usually considered high risk because T cells express HLA class I but little class II. The finding often points to strong class I DSA. A positive B-cell CDC result can reflect class I or class II antibodies because B cells express both.
Laboratories may perform an antihuman globulin-enhanced CDC assay to increase sensitivity. They may also treat serum with dithiothreitol to reduce IgM effects and help distinguish potentially less relevant IgM reactivity from IgG antibodies.
Flow cytometric crossmatch
Flow cytometry crossmatch, or FCXM, also mixes recipient serum with donor lymphocytes. Fluorescent secondary antibodies detect recipient immunoglobulin bound to donor T cells and B cells. A flow cytometer measures the signal shift relative to controls.
FCXM is more sensitive than standard CDC and can detect antibodies that bind cells without causing immediate complement-mediated cell death. This sensitivity can reveal clinically relevant lower-strength DSA, but it also creates more borderline or unexplained positives. Results may be reported as median channel shift, mean fluorescence shift, ratio, or simply positive/negative based on the laboratory’s validated cutoff.
T-cell positivity generally suggests class I reactivity. B-cell positivity may result from class I or class II antibodies, non-HLA antibodies, autoantibodies, or technical factors. B-cell assays are often more difficult to interpret because B cells have higher HLA expression and more surface molecules that can bind nonspecifically.
Virtual crossmatch
A virtual crossmatch, or VXM, does not physically mix serum with donor cells. The laboratory compares two independent data sets: the donor’s HLA type and the recipient’s anti-HLA antibody specificities. If the recipient has no antibody against a donor HLA antigen, the virtual crossmatch is predicted negative. If a donor antigen matches a listed unacceptable specificity, it is predicted positive.
VXM can be completed quickly, which matters for deceased-donor organs when every hour of cold ischemia can affect outcomes. It also avoids poor donor-cell viability and can be more specific when high-resolution HLA typing and current antibody data are available. More detail about donor typing appears in the HLA typing test guide.
Virtual crossmatch accuracy depends on complete donor HLA typing, recent recipient serum, reliable antibody assignment, and expert understanding of assay artifacts. It is not merely an automated lookup.
How the Test Is Performed
The recipient provides one or more blood samples before transplantation. Transplant programs often collect serum periodically while a person is on the waiting list and after any sensitizing event. Samples may be stored so the laboratory can test current and historical sera against a potential donor.
For a physical crossmatch, donor lymphocytes come from blood, lymph nodes, spleen, or another suitable tissue. The laboratory separates T cells and B cells or identifies them during flow analysis. Recipient serum is incubated with donor cells, and controls establish background and positive reactivity.
The laboratory may test:
- Current serum, representing the recipient’s latest antibody status
- Peak or historical serum, representing the strongest known sensitization
- Untreated serum
- Serum treated to reduce IgM or interference
- T cells and B cells separately
- Multiple dilutions to assess very strong antibody or a prozone effect
For a virtual crossmatch, the laboratory reviews the recipient’s single-antigen bead results, prior antibodies, calculated panel-reactive antibody value, sensitizing history, and donor HLA type. Some programs also consider allele-level differences, epitopes or eplets, complement-binding tests, and antibody trends.
No fasting is required. The most important preparation is accurate history. The transplant team should know about recent transfusions, pregnancies, miscarriages, prior grafts, infections, vaccinations, immunoglobulin infusions, desensitization, plasma exchange, monoclonal antibodies, and other immune treatments.
Timing matters because antibodies can rise days to weeks after a sensitizing exposure. A serum sample drawn before a recent transfusion may not represent current risk. Programs establish rules for how recent a sample must be, often requiring a newer specimen after a sensitizing event.
Turnaround may range from under an hour for an urgent physical assay to several hours, depending on cell preparation and testing. A virtual assessment can be rapid once complete data are available. The final report may include T-cell and B-cell results, method, strength, controls, interpretation, and recommended correlation with DSA testing.
What a Positive Crossmatch Means
A positive crossmatch means recipient serum reacted with donor cells or that the virtual comparison identified donor HLA targeted by recipient antibodies. It does not carry one universal risk level.
| Pattern | Possible interpretation | Typical concern |
|---|---|---|
| T-cell CDC positive | Strong class I donor-reactive antibody | High risk of hyperacute or early antibody-mediated rejection |
| B-cell CDC positive only | Class II or class I antibody, IgM, or nonspecific reactivity | Variable; requires DSA correlation |
| T-cell flow positive, CDC negative | Lower-strength class I antibody or interference | Increased risk in some settings, not always a contraindication |
| B-cell flow positive only | Class II DSA, low-level class I DSA, autoantibody, or technical effect | Highly context dependent |
| Virtual positive | Recipient antibody specificity matches donor HLA | Depends on antibody strength, history, locus, and transplant plan |
A positive result often leads to confirmation that the antibody is truly donor specific. The team checks the donor’s HLA typing, recipient bead pattern, historical antibodies, and physical crossmatch. Antibody strength is often described using mean fluorescence intensity, but MFI is a semiquantitative laboratory signal rather than a direct concentration or universal toxicity threshold.
The transplant response depends on the organ, urgency, available alternatives, and center experience. A strong positive T-cell CDC crossmatch may stop a kidney transplant because another donor or paired exchange could provide a safer option. In a life-threatening heart or lung situation, the team may accept more immunologic risk if no alternative exists. Liver grafts are relatively resistant to some antibody effects, but strong DSA and positive crossmatch can still matter.
A positive result does not always mean the transplant is impossible. Options may include:
- Selecting a different donor
- Entering a kidney paired donation program
- Removing the donor HLA specificity from acceptable options
- Repeating testing with a fresh sample or alternate method
- Desensitization with plasma exchange, immunoglobulin, B-cell or plasma-cell therapies, and other center-specific protocols
- Proceeding with intensified induction and monitoring in carefully selected cases
- Deferring transplantation until antibody strength falls
These strategies carry tradeoffs. Desensitization can increase infection, bleeding, medication toxicity, cost, and rejection risk. A transplant across DSA or a positive crossmatch should involve explicit counseling about expected outcomes and alternatives.
A donor-specific antibody test helps identify which donor target may explain the crossmatch and whether it persists after transplantation.
Negative, Borderline, and Indeterminate Results
A negative crossmatch means the assay did not detect reactivity above its validated threshold. It substantially lowers concern for preformed donor-reactive antibody detectable by that method, but it does not eliminate all rejection risk.
A recipient can experience rejection despite a negative crossmatch because:
- Antibody levels were below detection at the time of testing.
- A rapid memory response produced antibody after transplantation.
- The antibody targets a donor allele not included or correctly assigned in the virtual analysis.
- Non-HLA antibodies contribute to injury.
- T-cell-mediated rejection occurs independently of preformed antibody.
- New DSA develops after transplantation.
- The serum sample was outdated or collected before a sensitizing event.
A borderline result sits near the laboratory cutoff. Small changes in background fluorescence, cell quality, or analysis can move it between positive and negative. The laboratory may repeat the assay, test another serum sample, use pronase-treated or untreated cells, perform dilution studies, or compare with single-antigen bead findings.
An indeterminate test means the laboratory could not provide a reliable positive or negative conclusion. Causes include poor donor cell viability, inadequate cell number, excessive nonspecific binding, failed controls, or medication interference. An indeterminate result should not be treated as negative.
Discordance is common enough to require expert review. A virtual crossmatch may be positive while the physical assay is negative if the DSA is weak, the donor cells express low levels of that HLA, or the bead assay overestimates reactivity. The reverse can occur when the physical assay detects non-HLA reactivity, an antibody to an untyped donor allele, or interference not visible in the virtual analysis.
Historical antibodies remain relevant even when current testing becomes negative. Antibody can fall below detection but memory B cells may persist. Many programs continue to list a well-documented historical specificity as unacceptable or treat it as elevated risk.
How Crossmatch Use Differs by Transplant Type
Crossmatch policy varies because organs differ in urgency, susceptibility to antibodies, storage time, and availability of alternatives.
Kidney transplantation: Crossmatch and DSA assessment are central because kidney transplantation is often elective enough to consider another donor, paired exchange, or waiting. A positive CDC crossmatch is generally a major barrier. Selected flow-positive or DSA-positive transplants may proceed under center-specific protocols.
Heart transplantation: Allocation urgency and limited donor availability can require rapid virtual assessment. Preformed DSA and positive crossmatch increase antibody-mediated rejection risk, but decisions balance immunologic risk against the danger of remaining on mechanical support or the waiting list.
Lung transplantation: Programs use virtual or physical crossmatch based on sensitization and logistics. Avoiding donor HLA targeted by strong antibody may reduce early graft dysfunction and rejection, but prolonged waiting can also be dangerous.
Liver transplantation: The liver can absorb or modulate some circulating antibody, so a positive crossmatch is not always an absolute contraindication. Strong class II DSA, persistent antibody, or combined organ transplantation can carry added risk. Center policy and the full clinical situation guide decisions.
Pancreas and intestinal transplantation: These organs may be vulnerable to antibody-mediated injury, and compatibility testing is important. Practices vary with simultaneous kidney transplantation and urgency.
Hematopoietic stem cell transplantation: The terminology and direction of risk differ. Recipient anti-donor HLA antibodies can cause graft failure, especially with mismatched or haploidentical donors. Cell-based crossmatching may be used in selected settings, but donor-specific antibody testing is often the central assessment. HLA matching has greater importance because donor immune cells can also attack recipient tissues, causing graft-versus-host disease. See the HLA matching test for the distinction.
A crossmatch result should therefore never be interpreted without naming the transplant type and the center’s protocol.
False Results, Technical Limits, and Interference
Cell-based crossmatches can be affected by donor-cell quality, nonspecific antibodies, therapeutic drugs, and laboratory technique. Flow cytometry is particularly sensitive to interference.
Potential causes of false-positive or difficult results include:
- Autoantibodies that bind the recipient’s own and donor lymphocytes
- IgM antibodies with uncertain clinical relevance
- Therapeutic monoclonal antibodies that bind lymphocyte surface proteins
- Intravenous immunoglobulin containing broadly reactive antibodies
- Nonspecific B-cell binding through Fc receptors
- Dead or damaged donor cells
- Pronase treatment that alters cell-surface proteins
- Inadequate negative controls or unusually high background
Daratumumab, an anti-CD38 therapy, can cause false-positive flow crossmatches because lymphocytes express CD38. Rituximab and other B-cell-directed therapies can affect B-cell crossmatching. Laboratories need medication history so they can use blocking reagents, alternate cells, adsorption, or other methods.
False-negative risk also exists. Very high antibody levels can cause a prozone or hook effect in solid-phase assays, producing a lower signal than expected. Serum dilution or EDTA treatment may reveal the hidden reactivity. Antibody against denatured HLA on beads may appear strong in the solid-phase test but not recognize native HLA on donor cells.
Virtual crossmatch has data-quality limits. Incomplete donor typing at HLA-DP, DQ alpha chains, or allele level can miss relevant targets. Antibodies may recognize shared epitopes across several antigens, making specificity assignment difficult. Different bead lots and laboratories can produce different MFI patterns.
These issues explain why MFI cutoffs are not interchangeable among centers. A value of 2,000 in one laboratory cannot automatically be interpreted like 2,000 in another. Trends generated with the same assay and laboratory are usually more useful than isolated comparisons across systems.
Next Steps After a Crossmatch Result
Ask for the full histocompatibility interpretation rather than only “positive” or “negative.” The report or transplant team should clarify:
- Which method was used?
- Were T cells, B cells, or both positive?
- Was current serum tested, historical serum, or both?
- Which donor-specific antibodies were identified?
- How strong and reproducible is the reaction?
- Could medication, autoantibody, or sample quality explain it?
- Does the result prohibit transplantation at this center?
- What alternatives or risk-reduction strategies are available?
For a negative result, the team confirms that the serum is recent and donor HLA typing is complete. The patient should report any new transfusion, pregnancy, transplant, or other sensitizing event because repeat antibody testing may be needed.
For a positive result, the laboratory may repeat the physical crossmatch, test serum dilutions, review antibody epitopes, or obtain higher-resolution donor typing. The clinical team then weighs waiting-list risk, donor quality, organ urgency, antibody characteristics, and the expected success of desensitization.
For living-donor transplantation, an unexpected positive result can lead to a more deliberate review rather than an immediate final decision. The team may obtain a new serum sample, confirm donor HLA typing, repeat testing with another method, and compare current findings with historical antibody records. Depending on the organ and the center, options may include kidney paired donation, choosing another donor, or a desensitization plan. Each option has tradeoffs: waiting for a better match may reduce immunologic risk, while delaying transplantation can carry its own medical risks.
The age of the serum sample also deserves attention. Antibody profiles can change after transfusion, pregnancy, prior transplantation, infection, or changes in immune therapy. A result based on an older specimen may therefore need confirmation with a recent sample, especially in a highly sensitized patient or after a new immune exposure.
After transplantation, pretransplant findings remain part of the risk record. Clinicians may monitor DSA, organ function, and biopsy findings more closely. A rise in creatinine, reduced urine output, shortness of breath, declining heart function, abnormal liver tests, or other graft dysfunction requires prompt evaluation. Antibody results alone do not diagnose rejection; organ function and pathology are often needed.
A crossmatch is best viewed as a focused compatibility assessment, not a guarantee. Its value comes from combining cell-based findings, HLA antibody specificity, donor typing, clinical history, and the realities of the transplant being considered.
References
- Virtual crossmatching: Principles, practices, and the path forward-An American Society for Histocompatibility and Immunogenetics/College of American Pathologists collaborative 2025 (Review)
- Development of the Crossmatch Test in Kidney Transplantation Up to the Virtual Level 2025 (Review)
- From hero to zero: A single center retrospective review of the utility of routine physical crossmatching 2025
- Selective Elimination and Rationalization of Cell-based Crossmatches in the Era of Virtual Crossmatching 2024
- Crossmatch assays in transplantation: Physical or virtual?: A review 2023 (Review)
- Principles of Virtual Crossmatch Testing for Kidney Transplantation 2022 (Review)
Disclaimer
Crossmatch interpretation is transplant-specific and must be performed by the transplant and histocompatibility teams. Do not assume that a positive result always prevents transplantation or that a negative result removes all rejection risk. Urgent symptoms after transplant require immediate contact with the transplant center.
