The transplant coordinators usually refer to the list like a pool - everyone is in the pool. The candidates on the list are assigned points, notably, a 1/365 of the points for each day a candidate has been on the list; a fraction of the point for sensitized patients (PRA>20%), and high points for most patients with PRA=80% or higher as they are hard to match. At the time of matching, additional points are assigned on how well the donor matches the potential recipient. For more information, refer to the OPTN policies. Transplant Centers use two different approaches to manage transplant list:
Approach 1: The transplant center requires all patients send their monthly blood sample (PRA kit) in, allowing them to run crossmatches for all patients.
When the kidney offer come, the following steps are followed:
1 First see if anybody (no matter where they are on the list) matches perfectly (6 out of 6) to a kidney offer. If yes, the kidney offer goes to that patient. If no, proceed to step 2.
2 Run crossmatches against the kidney offer for the top 20%-30% of the list candidates who have the highest points (aka have been on the list the longest time or have high PRA). See how many of those 20%-30% people matched and how well, ie assign the match quality points for each candidate that matched.
3 If none from the above of candidates matched, see if anybody at all on the list matches.
4 If nobody got a kidney offer from step 3, send kidney to regional. Literature indicates that 80% of the kidneys stay transplanted locally, so Step 4. is used only in 20% of the cases.
With this approach, the top 20%-30% of patients who were on the waiting list the longest get priority over the rest of the list. It is still possible to get a kidney offer for the lower 70%-80% of patients if the top 20%-30% did not match or get a perfect match.
Approach 2: Only top 20%-30% who waited the longest are required to send their PRA kits in for crossmatch and considered against the potential donor.
Patients who waited the longest and the transplant center professionals think there is a chance for those people to get transplanted within this year, are being asked to send their monthly PRA kits in. All other patients on the list have no chances to get a transplant since no monthly blood sample is required from them. Only after several years on the list they go high enough on the list to get a call from coordinator that would ask them to start sending their monthly PRA kits in. This approach is mostly used in centers with thousands of candidates on the waiting list such as at USCF, University of Maryland and others.
Regardless of the approach, the waiting time is the dominating factor. The 20%-30% top of the list who waited the longest are the contenders and most likely the recipients of the new kidney. The years on dialysis are considered as waiting time if start date of dialysis was prior the listing date at the transplant center.
O recipient can only receive the kidneys from O donor;
A recipient can receive kidney from O or A;
B recipient can receive kidney from O or B;
AB is a universal recipient and can receive from anybody.
Blood type O is hard to match because it only can receive from O, so there is really a separate queue for patients with blood type O at each transplant center. In addition, it is estimated that O recipients received only about 85% of all O donor kidneys. This happens because either they did not match blood type O candidates on the list or others with blood type A,B,AB had a better match. There are a lot of patients with O blood type competing for the O type organs, it is the most common blood type. Some transplant centers, such as the University of Kansas Hospital have most of their patients of blood type O; the Miami Transplant Institute that has high attrition rate or O blood type patients and thus 5 year wait for O blood type, while only 3 year wait for blood type A.
Blood type B is hard to match because its a rare blood type. While B can receive from O and B, the pool of kidney offers for B recipients is limited by just small percentage of people with B blood type donors, that are only 11% of population; plus the small percentage of O donors that spilled out from O queue because were not compatible to O. To circumvent this, B should also list in high volume centers.
Blood type A is easier to match because in addition of fairly large percentage of potential donors with blood type A, some of the O will also match A.
Blood type AB is the easiest. University of Maryland Transplant Center reports to transplant their AB patients in 3 months. Even if an AB on the bottom of the list at the transplant center, he/she will be transplanted because he could be the only one who matched the donor offer.