Kidney Compatibility: Finding the Best Match

When you need a kidney transplant, finding the best-matched living kidney donor can mean a more successful transplant. The better the match between the donor and the recipient, the longer the transplanted kidney is likely to last.

What is a Kidney Donor Match?

A “match” between a living kidney donor and a potential transplant recipient refers to the biological compatibility between the two people. Compatibility is determined by looking at blood type, tissue type and crossmatching.

Blood Type Compatibility for Kidney Donation

There are four basic blood types—O, A, B and AB—and the blood types of the donor and recipient must be compatible in order for a transplant to be successful. The compatible blood types for kidney donation follow the same rules as blood type compatibility for blood transfusions:

Donor’s with blood type:Are blood type compatible with recipients with blood type:
OO, A, B or AB
AA or AB
BB or AB
ABAB

Tissue Typing for Kidney Donation

Once blood type compatibility is determined, the next step is tissue typing, which is also called HLA (human leukocyte antigen) typing or antigen typing. There are more than 100 antigens in the cells of the body, but six have been identified as the most important in organ transplantation. These are the A, B and DR antigens.

Traditionally, tissue typing for kidney donor-recipient matching was measured using antigen compatibility between these six antigens, with a match score from zero out of six (the worst match, indicating a high probability that the donated kidney would be rejected by the recipient) to six out of six (a perfect match, which usually occurs only between certain siblings).

Crossmatching

Crossmatching is a test designed to identify whether the intended recipient is “sensitized,” which means they have antibodies that will attack foreign tissue—the donated kidney—potentially causing the body to reject the transplant.

With the crossmatch test, blood samples from the potential donor and intended recipient are mixed to see how the recipient’s antibodies react with those of the potential donor. If the recipient’s antibodies attack the donor’s, the test is positive, which means the two have failed the crossmatch test and are not compatible. If the recipient’s antibodies do not react to the donor’s, the test is negative and the donor-recipient pair has passed the crossmatch test.

How Does the NKR Find the Best Donor-Recipient Match for Kidney Transplants?

Antigen matching has been the standard for determining donor-recipient compatibility for many years, but now there is a better, more accurate way that is based on eplets. Similar to how atoms are made of many smaller components, we now know that antigens are made of many smaller components. These are called eplets. Assessing the eplet match between a donor and recipient—called eplet mismatch analysis—has proven to be a more precise measurement of a donor-recipient match compared to antigen matching.

The closer the match (i.e., the lower the mismatch) between the eplets of the potential donor and the intended recipient, the better the kidney match. With a low eplet mismatch, the transplant has a higher chance of being successful and the kidney is likely to last longer.

Kidney Compatibility Chart: Eplet Matching vs HLA Matching

As shown in Table 1 below, one kidney patient in need of a transplant has two potential living donors. Both Donor 1 and Donor 2 are a three out of six antigen mismatch, so based on traditional HLA matching they are equally good matches for the recipient.

HLA-AHLA-AHLA-BHLA-BHLA-DRHLA-DR
RecipientA2A24B8B18DR1DR18
Donor 1A68A24B42B18DR1DR17
Donor 2A11A24B44B18DR1DR4
Table 1

When you look at the same two donors at the eplet level, the picture changes. As shown in Table 2 below, which shows how many mismatched eplets within each antigen each donor has with the recipient, Donor 1 has fewer mismatched eplets to the recipient and would therefore be a more compatible donor for the recipient than Donor 2.

HLA-AHLA-AHLA-BHLA-BHLA-DRHLA-DRTotal Mismatched Eplets
Donor 1110500521
Donor 221028001746
Table 2

Why Compatible Pairs Can Benefit from Using the NKR

Paired exchange—when a donor donates their kidney to another recipient in exchange for a compatible, or better-matched, kidney for their loved one—is a type of kidney donation that was originally created to overcome cases of donor-recipient incompatibility, when the donor could not give directly to the intended recipient.

However, compatible pairs—donor-recipient pairs where the donor can donate directly to the patient—can also greatly benefit from entering paired exchange.

Through paired exchange and the Kidney for Life initiative—which utilizes the latest generation in DNA sequencing technology to assess the histologic match between patients and donors—compatible pairs can find excellent matches, and the recipient may be able to safely lower the dosage of immunosuppression drugs they take to prevent the rejection of the donated kidney.