Understanding Your Transplant Compatibility Tests

Section Editors: Christopher McCudden Ph.D., DABCC, FADLM, FCACB and Kamran Mirza MD PhD
July 28, 2026

Transplant compatibility tests are blood tests used to match a donor and a recipient before an organ or stem cell transplant, with the goal of lowering the chance that the recipient’s immune system will reject the transplant. The immune system recognizes tissue from another person as foreign, mainly through markers called human leukocyte antigens (HLA). The main tests are HLA typing (also called tissue typing), the panel reactive antibody (PRA) test, donor-specific antibody (DSA) testing, and the crossmatch. These tests are done while a person waits for a transplant, at the time a donor is identified, and after the transplant to watch for signs of rejection.

This article explains what each of these tests measures and what the results mean, so you can understand the terms on a transplant report.

These tests describe the immune match between a donor and a recipient, and they are interpreted by the transplant team rather than as a simple normal or abnormal result. Some results are categories, such as your HLA type or a positive or negative crossmatch, and some are numbers, such as a percentage or an antibody level. Importantly, these results can change over time as the immune system is exposed to foreign tissue through pregnancy, a transfusion, or a previous transplant, which is why the antibody tests are repeated while you wait for a transplant and after you receive one.

What are transplant compatibility tests?

Transplant compatibility tests measure how well a donor and recipient match, and whether the recipient’s immune system is likely to attack the donated organ. The body uses markers called human leukocyte antigens (HLA), found on the surface of most cells, to tell its own tissue from foreign tissue. Everyone inherits a set of HLA markers, half from each parent. When a transplanted organ carries HLA markers that differ from the recipient’s, the recipient’s immune system may recognize it as foreign and make antibodies against it, which can damage the organ in a process called rejection. Compatibility testing measures the HLA markers of the donor and recipient and checks whether the recipient already has antibodies against them. The ABO blood type must also be compatible for most organ transplants, in the same way it must be for a blood transfusion.

How are the tests performed?

Transplant compatibility tests are performed on blood samples from the recipient and, when available, the donor. HLA typing is done by analyzing DNA to determine a person’s exact HLA markers. Antibodies against HLA are measured using specialized laboratory tests that can identify which specific HLA markers a person has antibodies against. The crossmatch, the final compatibility check, can be done either by physically mixing the recipient’s blood with the donor’s cells or by comparing the recipient’s known antibodies against the donor’s HLA type on paper, which is called a virtual crossmatch. Because a person’s antibodies can change over time, the antibody tests are repeated regularly while a person waits for a transplant.

The individual tests

A transplant report may include some or all of the tests below.

HLA typing (tissue typing)

HLA typing, also called tissue typing, identifies the specific set of human leukocyte antigen markers a person carries. Both the recipient and the donor are typed, and the results are compared to see how closely they match. A closer match generally lowers the risk of rejection, although modern medications that suppress the immune system allow many transplants to succeed even without a perfect match. HLA matching is especially important in stem cell (bone marrow) transplants and in kidney transplants. Your HLA type is inherited and does not change during your life.

Panel reactive antibodies (PRA) and calculated PRA (cPRA)

The panel reactive antibody (PRA) test estimates how sensitized a person is, meaning how likely their immune system is to react against a random donor. It is now usually reported as a calculated PRA (cPRA), a percentage from 0 to 100 that represents the proportion of potential donors a person is expected to be incompatible with, based on the HLA antibodies they carry. A cPRA of 0 percent means a person has no detectable HLA antibodies and can match most donors, while a cPRA approaching 100 percent means a person is highly sensitized and only a small number of donors will be compatible. A higher cPRA usually means a longer wait for a suitable organ, and it is taken into account in how organs are allocated.

Donor-specific antibodies (DSA)

Donor-specific antibodies (DSA) are antibodies in the recipient’s blood that target the HLA markers of a specific donor. They are measured using a test that reports the strength of each antibody as a numerical level, which each laboratory interprets against its own thresholds rather than a single universal cut-off. Before a transplant, the presence of strong donor-specific antibodies signals a high risk of rejection and may mean a particular donor is not suitable. After a transplant, new donor-specific antibodies that develop over time (called de novo DSA) can be an early sign of antibody-mediated rejection, and they are monitored during follow-up.

The crossmatch

The crossmatch is the final check of compatibility between a specific donor and recipient. In a physical crossmatch, the recipient’s blood is combined with the donor’s cells in the laboratory to see whether the recipient’s antibodies react against them. A positive crossmatch means a reaction occurred and usually indicates the donor is not compatible, while a negative crossmatch supports going ahead with the transplant. Increasingly, a virtual crossmatch is used, in which the recipient’s known antibodies are compared against the donor’s HLA type without physically mixing the samples. A virtual crossmatch can be done quickly, which helps organs reach recipients sooner, and it is particularly useful for highly sensitized patients.

What makes finding a compatible donor harder?

Some people are more difficult to match because their immune system has already been exposed to foreign HLA markers and has made antibodies against them, a state called sensitization. The most common causes of sensitization are a previous pregnancy, a previous blood transfusion, and a previous transplant. Sensitized people have a higher cPRA and a smaller pool of compatible donors, which can mean a longer wait. Several strategies can help. Desensitization uses treatments to lower harmful antibodies so that more donors become acceptable. Paired exchange programs, used mainly for kidney transplants, match recipients who have a willing but incompatible living donor with other similar pairs, so that each recipient can receive a compatible kidney. Your transplant team will explain which options apply to your situation.

Compatibility testing after the transplant

Compatibility testing does not end once the transplant is complete. The transplant team continues to monitor for donor-specific antibodies, because new antibodies can develop and signal that the immune system is beginning to attack the organ. If rejection is suspected, based on antibody results, blood tests of organ function, or symptoms, a biopsy of the transplanted organ may be done to confirm whether rejection is occurring and what type it is. For kidney transplants, these findings are described using a standardized system, as explained in our article on understanding your kidney transplant biopsy report. The antibody results and the biopsy are interpreted together by the transplant team to guide treatment.

Questions to ask your doctor

What is my HLA type, and how well does it match my donor?
What is my cPRA, and what does it mean for my expected wait time?
Do I have any donor-specific antibodies, and how strong are they?
Was my crossmatch negative, and was it a physical or a virtual crossmatch?
Am I considered sensitized, and if so, what caused it?
Would desensitization or a paired exchange program help me find a compatible donor?
How often will my antibody levels be checked while I wait?
After my transplant, how will I be monitored for rejection?
If donor-specific antibodies are found after my transplant, what happens next?
How do my compatibility results affect the choice of a donor for me?