Blood types O and B are the hardest to match for kidney transplants. Both face the longest wait times, averaging around 6 years for a deceased donor kidney, compared to about 4 years for type A and just over 2 years for type AB. The reason comes down to basic compatibility rules and supply-demand math that puts these two blood types at a significant disadvantage.
Why Blood Types O and B Wait the Longest
The core issue is who can donate to whom. Kidney transplants follow strict blood type compatibility rules set by the national organ sharing network (UNOS). A type O recipient can only receive a kidney from another type O donor. That’s it. Types A, B, and AB kidneys are all incompatible. Meanwhile, type O donors are universal: their kidneys can go to recipients of any blood type. This creates a brutal imbalance. Type O kidneys get pulled in every direction, while type O patients can only draw from one pool.
Type B faces a similar bottleneck. A type B recipient can only accept kidneys from type B or type O donors. Since type B is one of the less common blood types and type O kidneys are in high demand across all groups, the available supply shrinks fast. Type AB recipients, by contrast, can accept a kidney from any blood type, which is why they have the shortest wait times by a wide margin.
National data backs this up clearly. Among patients placed on the waiting list between 2017 and 2019, only 42.5% of type O patients and 44.7% of type B patients had received a transplant within three years. For type A, that figure was 58.4%, and for type AB, it was 72.5%.
Blood Type Is Only Part of the Equation
Blood type compatibility is the first hurdle, but it’s not the only one. Your immune system also produces antibodies against specific proteins (called HLA markers) found on donor cells. If you’ve had previous transplants, blood transfusions, or pregnancies, your body may have developed antibodies against a wide range of these markers. Transplant centers measure this with a score called PRA, or panel reactive antibodies, which estimates the percentage of donors your immune system would reject.
Most patients have a low PRA score and can match with many donors. But highly sensitized patients, those with PRA scores of 98% or 99%, are compatible with only 1% to 2% of potential donors. When a high PRA score combines with type O or type B blood, the pool of suitable donors becomes extremely small. These patients often face the longest and most uncertain waits.
Policy Changes Aimed at Type B Patients
Recognizing the disadvantage faced by type B candidates, the national kidney allocation system introduced a specific policy in 2014. Certain type A kidneys, specifically a subtype called A2, produce weaker immune reactions and can sometimes be safely transplanted into type B recipients. This policy allows transplant programs to expand the donor pool for type B patients who meet specific criteria.
In practice, adoption has been slow. A survey of U.S. transplant programs found that many struggled to implement the required protocols. About 32% of programs cited difficulty establishing the right testing thresholds, 21% had trouble developing informed consent procedures, and 18% found it hard to determine which patients qualified. Federal health agencies have continued pushing for broader adoption, but the impact so far has been uneven across the country.
Options for Hard-to-Match Patients
If you have a willing living donor whose blood type doesn’t match yours, a kidney paired exchange program may help. In these programs, your incompatible donor gives a kidney to a different recipient whose donor is incompatible with them but compatible with you. Essentially, two or more pairs swap donors so everyone gets a match. These exchanges are particularly valuable for type O and type B recipients, who are most likely to have incompatible living donors. Simulation studies suggest that expanding the donor pool in these programs by even a modest amount, such as converting 15% of non-O donors to universal compatibility, could increase transplant rates by 26% and cut average wait times by over two months.
Another option is ABO-incompatible transplantation, where you receive a kidney from a donor whose blood type would normally be rejected. This requires an intensive preparation process before surgery to lower the antibodies in your blood that would attack the mismatched organ. Patients typically undergo multiple blood-filtering sessions over several weeks to bring antibody levels down to a safe threshold. The preparation is demanding, and about one in three patients in high-antibody cases experience a form of rejection afterward that requires additional treatment. Despite these challenges, long-term survival rates for patients who receive blood-type-incompatible kidneys from living donors are comparable to those who receive standard compatible transplants.
What This Means in Practice
If you’re type O or type B and need a kidney transplant, the most important thing to understand is that your wait for a deceased donor kidney will likely be significantly longer than average. Getting on the waiting list as early as possible matters, since time on the list factors into allocation priority. Pursuing a living donor, even one who isn’t a blood type match, opens up paired exchange options that can bypass the wait entirely. Ask your transplant center whether they participate in kidney paired exchange programs and, if you’re type B, whether they use the A2 subtype policy that could expand your donor options.
Type AB is the easiest blood type for kidney transplant. Type O is arguably the hardest because of the single-source restriction, with type B running neck and neck due to similar supply constraints and the added challenge of being a less common blood type overall.