How does Kidney Paired Donation work?

People interested in participating in Kidney Paired Donation (whether donors or recipients, incompatible or compatible) are entered into a Kidney Paired Donation database. The information entered into the database includes their blood type, HLA antigen typing, and other basic medical information.

Sophisticated computer software designed to match up donors and recipients within a Kidney Paired Donation registry is run on a regular basis, usually anywhere from daily, to once per week to once per month, depending on the number of donors and recipients entered into the registry since the prior run. The software then identifies donor and recipient pairs that may be good matches for each other.

Based on the matches generated by the software, a living donor transplant chain is formed, consisting of numerous donors and recipients that are compatible, as determined by the registry. These donors and recipients are usually involved at different transplant centers, often across the country. Therefore, the matched donor and recipient may be on opposite sides of the country. In most cases, the matched donors and recipients are not asked to travel to another transplant center. Rather, the respective surgeries are performed at a transplant center close to the homes of the matched donor and recipient, and the donor's kidney is shipped to the recipient's transplant center. Kidneys are shipped via commercial airlines, similar to the process in which deceased donor kidneys are shipped.

KPD Programs in the U.S.

The largest KPD program is the National Kidney Registry (NKR), which was founded by the father of one of our young transplant candidates at NewYork-Presbyterian/Weill Cornell. Since 2008, we have performed over 200 kidney paired donation transplants by working with the National Kidney Registry. Visit to view the current number of transplants that have been facilitated by the NKR.

An additional program is the Kidney Paired Donation Pilot Program of the United Network for Organ Sharing (UNOS), whose goal is to develop a national matching program. 

What are important factors in finding a match?

There are several factors that influence the length of time it will take to find a match within a Kidney Paired Donation registry.

For some transplant candidates, a match is found within a week or two of entering the registry, while for others, it may take a couple of years to find a good match.

It is important to remember that transplant candidates will also be on the deceased donor waiting list during this time, and a kidney could also become available via the deceased donor list. The factors that influence the length of time to find a suitable match are listed below.

Antibody Levels of the Transplant Candidate. Transplant candidates develop antibodies through prior exposure to human cells through blood transfusions, pregnancies or from previous transplanted organs. It often takes longer for sensitized recipients to find a compatible donor if the donor has genetic makeup (antigens) that the recipient has antibodies against.

Although these transplant candidates may find a match within a KPD program, it is possible that the candidate might have some antibody reactivity against the matched donor. In cases such as this, transplant candidates may need additional medications and treatments (called desensitization) designed to reduce antibody levels to a point where transplantation is possible. 

Blood Type. Similar to getting a blood transfusion, kidney transplantation requires that the donor and recipient have compatible blood types. The chart below shows which blood types are compatible.

Chart that details which blood types are compatible

Because 'O' donors are the universal donor and can donate to any blood type, there tend to be fewer 'O' donors available for 'O' recipients, who can only receive organs from type 'O' donors. This is an area where entry of compatible donor/recipient pairs into Kidney Paired Donation registries may be beneficial since this could free up some 'O' donors for 'O' recipients. To illustrate this, consider the scenario below.

A 40-year-old man is blood type 'AB', which means he can get a kidney from anyone. His wife is going to be his donor, and she has type 'O' blood. If this compatible pair enters a KPD registry, his wife's kidney could be donated to someone in desperate need of a transplant from someone with type 'O' blood, while the man can receive a kidney from a donor of any blood type, since a person with blood type 'AB' can receive an organ from a donor of any blood type.

Number of People in the Registry. The larger the pool of donors and recipients in the database, the greater the chance of identifying a match pair. In addition, the chance of finding matched pairs also has the potential to increase significantly when blood type and crossmatch compatible pairs are entered into KPD programs.

For these reasons, national exchange programs have the potential to generate the most transplant opportunities.

A match has been found – what happens next?

Once a potential match is found through a kidney paired donation registry, the crossmatch process is initiated between the matched donor and recipient.

 The crossmatch is a test where the donor and recipient blood are mixed together to see if the recipient's immune system has any reactivity against the donor. Reactivity signals that the recipient has antibodies against the donor, which are caused by previous exposure to human genetic material through blood transfusions, prior transplantation, and/or pregnancy.

 The presence of antibodies can increase the risk of acute rejection and graft loss after transplantation. However, this depends on the strength of the reaction.

In order to perform the crossmatch, the matched donor and recipient will each receive a kit in the mail containing blood tubes that need to be drawn and then shipped to a specified tissue typing laboratory.

 All donors and recipients involved will be notified within a few days of the results of the crossmatch. If the crossmatch results show compatibility, the transplant will then be scheduled as long as all donors and recipients are medically cleared for transplantation. 

Are there donor agreements?

Donors who choose to enter a KPD registry are not obligated to sign an agreement. However, potential donors must sign a consent form indicating their willingness to participate in a KPD program.

 Due to the nature of KPD programs, the medical information of potential donors is entered into the registry and thus may be shared with other transplant centers that are participants in that particular KPD program, so potential donors must agree to have their medical information shared with the KPD program. Only medical information needed to match a potential donor with a recipient is entered into the registry. 

One risk of KPD programs is that donors back out of the program after the person they originally wanted to donate to receives a transplant.

 At times, this is due to the donor being a bridge donor whose personal situation changes (such as getting a new job that prevents them from taking the required time off to donate, or developing a medical issue of their own).

 Due to the way the KPD registries work, one donor backing out can cause numerous transplant chains to fall apart. This is why donors are asked to make a firm decision to participate so that logistics can be confirmed and there will not be a last-minute change of mind.

 If a donor were to back-out at the last minute, this would "break the chain", which could impact the ability to transplant many patients who have already been scheduled to receive a transplant.

What is the donor's responsibility?

While a potential donor is participating in a registry and waiting to be matched with a compatible recipient, he or she will be contacted periodically by the Transplant Coordinator.

 It is extremely important to notify the Transplant Coordinator in between these points of contact if you develop any medical conditions, if your personal situation has changed, or if there are any other issues that could impact your ability to serve as a donor.

What happens on the day of surgery?

Due to the fact that donor and recipient pairs matched through the registry may be on opposite sides of the country, all donors in the transplant chain may not go to the operating room at the same time to donate their kidney.

Often donors go to the operating room first thing in the morning and are followed by the recipients later that same day. Other times, particularly when the donor is on the West Coast and the Recipient on the East Coast, the donor may go to the operating room in the evening, the kidney is sent on a "red-eye" flight, and the recipient's surgery occurs early the next morning. 

In other cases, a donor may be asked to donate their kidney a few weeks to months after their intended incompatible recipient is transplanted with their matched KPD donor kidney. In this situation, the donor is referred to as being a bridge donor because they will continue a transplant chain at some point in the future.

Alternatively, if a bridge donor does not have a good match within the registry and/or has personal obligations that prevent them from waiting to donate, the bridge donor's kidney may be given to a patient waiting on the list for a deceased donor kidney. This takes a patient off the waiting list, allowing more transplants to occur.

Can matched donors and recipients meet?

It is up to the individual participants as to whether or not they meet their donor/recipient.

 Regardless of their individual preferences, all participants in the transplant chain are kept anonymous at least until after the transplant has taken place. If participants then wish to meet each other, the Transplant Coordinators at the various Transplant Centers will work together to arrange this. 

Who is eligible to participate in Kidney Paired Donation?

Any potential donors and transplant candidates who have received medical clearance to donate are eligible to participate in a KPD program.

Both donors and recipients undergo standard donor or transplant evaluation prior to entering the database. Although most pairs participate because the donor and recipient are incompatible due to blood type or presence of a positive crossmatch, compatible pairs looking to improve their genetic and/or age match are encouraged to participate as well.

Most exciting about these types of transplants are the amazing possibilities that can occur if blood type and crossmatch compatible patients routinely enter the registry, as described below. 

Imagine a world where everyone who needs a kidney and has a potential living donor would selflessly enter a national registry so that living donation could be optimized.

Sure, patients who have an incompatible donor (blood type or antibody incompatibility) have a good reason to enter a registry program. But what about these other scenarios?

- A 75 year-old gentleman needs a kidney and his compatible 25 year-old granddaughter wants to donate. Perhaps if this pair entered a registry, the elderly gentleman could get a kidney from a 60 year-old donor and his granddaughter's kidney could be given to someone who is 32 years old.
- A 40 year-old man is blood type AB, which means he can get a kidney from anyone. His wife is going to be his donor, and she is an O blood type. If this pair enters the registry, his wife's kidney could go to someone in desperate need of an O kidney since patients with O blood type can only get a transplant from someone with O blood type.

The possibilities are amazing and endless if compatible pairs would routinely participate in kidney paired donation programs!