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What is a Perfect Match for a Living Donor Kidney Transplant?

Dr. Raja Kandaswamy, Surgical Director, Kidney Transplant Program, University of Minnesota

When you hear about kidney patients receiving a transplant using a kidney from a living donor, you may hear that a donor was a “perfect match” for the recipient. But what does it mean to be a perfect match, exactly?

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.

The first step in kidney matching is matching blood type. 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. For more on blood type matching, see Does Blood Type Matter for Kidney Transplant?

Once blood type compatibility is determined, the next step is tissue typing, which is also called HLA (human leukocyte antigen) typing or antigen typing. After that comes crossmatching, 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.

What is a Perfect Donor–Recipient Match?

Traditionally, tissue typing for kidney donor–recipient matching was measured using antigen compatibility between six specific antigens in the cells of the body, with a match score from zero out of six (the worst match, indicating a high risk of rejection) to six out of six (a “perfect” match, which usually occurs only between certain siblings).

Antigen matching has been the standard for determining donor–recipient compatibility for many years, but now there is an improved, more granular method that is based on eplets.

Similar to how atoms are made of many smaller components, antigens are made of many smaller components, 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 may have a lower risk of rejection and as a result, the kidney could potentially last longer.

Using eplet mismatch technology, a perfect match is defined as a “zero eplet mismatch,” which means that all the eplets between the donor and recipient match. This is considered an ideal scenario in terms of donor–recipient matching, as it minimizes the chances that the recipient’s immune system will see the transplanted kidney as foreign and mount an immune response against it.

Achieving a zero eplet mismatch can be challenging, as the genetic diversity of HLA molecules means that finding a perfect match can be rare. In practice, transplant centers aim for the best possible eplet match between donors and recipients to optimize the chances of a successful kidney transplant and minimize the risk of rejection.

To harness the power of the eplet mismatch method to find the best matches between kidney donors and recipients, the National Kidney Registry launched the Kidney for Life initiative. The Kidney for Life initiative now has 22 participating transplant centers in the U.S. and expects to facilitate 90 “perfect match” (zero eplet mismatch) transplants in 2023 as well as 303 low eplet mismatch transplants.

For more information, visit www.kidneyforlife.org.