How does a donor search for a transplant work?

Families often wonder how they can find a donor. The patient does not have to find one. NMDP, which runs the US registry, says the transplant team works with it to look for the best donor or cord blood unit. In the UK, Anthony Nolan, a stem cell charity, describes its own search team doing the same job.

The search starts with a test of the patient’s HLA (human leukocyte antigens). These are proteins, or markers, found on most cells in the body. The immune system uses them to tell which cells belong in the body and which don’t. NMDP says HLA typing starts with a blood draw or a swab from the inside of the cheek.

Timing matters. NMDP’s guidance for doctors covers patients who may need a donor transplant. It calls HLA typing at the time of diagnosis a critical first step in finding a related or unrelated donor. It adds that early referral to a transplant center matters, because for many conditions the window for a transplant can be narrow.

HLA is inherited: half comes from the mother and half from the father. That is why the team often tests brothers and sisters first. NMDP says each brother or sister with the same two parents has a 25% chance, or 1 in 4, of being a full match. Aunts, uncles and cousins are rarely a match, so NMDP says they are unlikely to be tested.

A full match in the family is not the usual case. NMDP says about 75% of patients who need a transplant do not have a match in their family. For them, the team turns to registries of unrelated volunteers and to donated umbilical cord blood.

Relatives may still be able to help. Parents and their children are always a half match for each other, called haploidentical, and brothers and sisters have a 1 in 2 chance of being one. NMDP notes that not all hospitals offer half-matched transplants.

When a volunteer joins a registry, their HLA type is tested and added to the registry, NMDP explains. That lets the registry run a first search, called a preliminary search, when the transplant center asks. It produces a list of volunteers and cord blood units who may match. Anthony Nolan says it sends a shortlist of possible donors to the transplant center within 24 hours.

Registries share their lists. The World Marrow Donor Association (WMDA) lists volunteer donors and cord blood units from 99 organizations in 57 countries. Its search service handles more than 60,000 patient searches a year. NMDP says a search of its registry covers US and global registries.

A search keeps running. NMDP says doctors get daily updates and are told if a newly added donor or cord blood unit matches their patient. The cells can travel far. Anthony Nolan says that after a donor gives cells, it brings them from anywhere in the world within 72 hours.

A name on a list is not yet a donor. Once doctors decide the patient needs an unrelated donor, the formal search begins. The registry asks the most promising volunteers for a new blood sample. This step, called confirmatory typing, confirms the HLA match and screens for infections. The volunteer also fills out a health history form, and the registry checks that they are available, DKMS, a large donor registry, explains.

The volunteer’s side of this step matters. NMDP says it first contacts the volunteer to confirm they still want to donate. It may then ask for another cheek swab or a blood sample. Joining and donating are voluntary, and a volunteer can change their mind at any point. NMDP asks anyone who does to say so right away, because delays could put the patient’s life at risk.

Next, the team chooses among the matches. HLA comes first, but other things count too. Guidelines published in 2025 by NMDP and CIBMTR, a transplant research network, say donors aged 30 or younger should be prioritized. Younger donors are linked with better survival. They also recommend testing the patient for antibodies against HLA before the formal search. Donors who are the target of strong antibodies should not be used.

If the patient’s doctor picks a volunteer, NMDP holds an information session about the donation and its risks and side effects. The chosen donor then has a full medical exam to make sure they are fit to donate, Anthony Nolan explains. The cells are collected, and a trained courier brings them to the transplant center, NMDP says, sometimes from another country.

NMDP says testing a possible donor can take as little as 1 to 2 weeks, from the request until the lab has the sample and the HLA results. Sometimes it takes longer. From the start of a search to transplant day usually takes about 3 months.

Some searches are faster and some take longer. NMDP’s patient fact sheet says finding the right match can take as little as a few weeks and sometimes many months. Cord blood can be quicker, sometimes just a few weeks, because the units have already been collected and stored.

To save time, the 2025 NMDP/CIBMTR guidelines say searches for all donor types should run at the same time, not one after another. They describe tools that use the patient’s HLA and ancestry to predict early whether a fully matched unrelated donor is likely, so teams can weigh other donors sooner.

Because HLA is inherited, a patient is most likely to match a donor of a similar ethnic background, NMDP explains. Some groups have more complex tissue types, which makes a close match harder to find. NMDP also says donor registries are not yet diverse enough to ensure every patient finds a fully matched donor.

The gap shows in the numbers. A 2024 article from NMDP reported the chance of finding a fully matched (8 of 8) donor on the NMDP Registry. It was 29% for Black and African American patients, 48% for Hispanic patients and 79% for non-Hispanic white patients.

Recruitment alone cannot close that gap. The same article describes a 2021 NMDP analysis. Even if every eligible Black or African American adult aged 18 to 35 joined and was available, the chance of a full match would rise only from 29% to about 64%.

Other kinds of donors now widen the options. When there is no full match, care teams may consider a half-matched relative, a partly matched (mismatched) unrelated donor or cord blood. The 2025 NMDP/CIBMTR guidelines say that with a medicine called post-transplant cyclophosphamide, half-matched and mismatched unrelated donors have outcomes similar to each other. They add that mismatched donors expand the donor pool, especially for patients from racial and ethnic minority groups.

Every unrelated donor in a search is a volunteer who joined a registry. Tissue types vary so much that NMDP says it cannot predict how likely any one member is to be asked to donate.

Joining means being ready to respond if called. In the US, NMDP says people can join between ages 18 and 35 and can stay on the registry until age 61. Other countries have their own registries and rules.

A 2025 paper from WMDA says a more diverse donor pool reduces unsuccessful searches. WMDA supports registries in recruiting under-represented groups, especially younger male donors.

The Jada Bascom Foundation raises awareness and points people to the official registry for their country. It does not run donor searches, add anyone to a registry, test tissue types or send swab kits.