For a kidney transplant to be successful, it is better if the tissues of the new kidney are fairly similar to the recipient’s original kidney. If the new kidney is a very poor match, the recipient’s immune system (natural defence system) is more likely to attack and reject it. (Click here for a description of the rejection process.)
Before a suitable kidney can be looked for, a number of tests must be performed. The most important of these are to find out the person’s blood group and tissue type. The results will then be checked against the results of similar tests carried out either on an available kidney, or on a relative who is considering donating one of their kidneys.
The blood group is an inherited characteristic of red blood cells. It stays the same throughout life. There are four main blood groups. These groups are called A, B, AB and O. In white Europeans group O is the most common, followed by group A. In Asians, groups B and AB are the commonest.
The blood group depends on whether or not there are certain substances called antigens (types of protein) in the body. Two different antigens – called A and B – determine the blood group. If these antigens are present, they will be on the outer surface of all cells, not just on blood cells. If there is only antigen A, the blood group is A. If there is only antigen B, the blood group is B. If there are both antigen A and antigen B, the blood group is AB. If there are neither of these antigens, the blood group is O.
The function of the blood group antigens is to act as a ‘friendly face’ for the cells – so that the rest of the body can recognise the cells as their own, and leave them alone. A person’s immune system will attack any cells that have a foreign antigen. This means that someone can only be given a transplant kidney if the recipient’s and donor’s blood groups are matched as follows:
|Group O||Group O|
|Group A||Group A or group O|
|Group B||Group B or group O|
|Group AB||Any group (O, A, B, or AB)|
In Britain, the current national practice is to match the blood group exactly between the donor and recipient when cadaveric kidneys are allocated. An exception is made for some people of blood group B. There are more people of blood group B waiting for kidneys than there are donors, so a few kidneys from blood group O donors are offered to blood group B patients each year, to try and even out the chances of getting a kidney.
The principle of matching for tissue type is similar to that for matching for blood group. Again, the recipient and the donor kidney or potential donor are matched using a blood test.
The tissue type is an inherited set of characteristics (antigens) on the surface of most cells. Each individual has one tissue type throughout life (just as there is only one blood group), but the tissue type is made up of many different characteristics, six of which are important in transplantation.
The tissue type characteristic that are important in transplantation are called A, B and DR (A and B tissue types have nothing to do with A and B blood groups). Everyone has two of each (one from each parent) – making six in all. Just to make it more complicated, there are 20 or more different versions of each A, B and DR characteristic. This means that there are millions of different possible tissue types. So, for example, a tissue type could be A3/A7, B5/B9, DR3/DR21.
As there are so many possible tissue types, matching tissue types is a little more complicated than matching blood groups. Fortunately, a kidney transplant can work if there are differences between the tissue type of the donor and recipient, so the match does not have to be perfect. However, the more of these that are the same for the patient and the donor kidney or potential donor, the better the chances are that the transplant kidney will work. To make it more complicated, getting a match for DR is more important than getting matches at A and B.
So, for example, a transplant might be offered in the following situation:
As the A3, B9 and DR3 characteristics are the same in this example. It would be called a ‘3 out of 6 match’. Given the large number of tissue type possibilities, it is very unusual to get an exact match (called a ‘6 out of 6 match’ or ‘full-house match’) between a recipient and donor. Most units will offer a transplant if there is a ‘3 out of 6’ match or better.
The better the match, the more likely it is that the body will accept the kidney ‘as its own’ and not try to reject it. Unfortunately, it cannot be guaranteed that even a ‘6 out of 6’ match will not be rejected. This is because the blood group and tissue type are not the only cell surface characteristics that are important. However, these other important characteristics have not all been identified.
The waiting list works on the basis of finding the ‘right’ person for the ‘right’ kidney when one becomes available. It does not work on a ‘first-come-first-served’ basis.
In order to achieve the best survival for transplanted kidneys, there is a national kidney sharing scheme. Therefore, if some kidneys are obtained in London, they may be sent as far away as Aberdeen and Cardiff if someone in one of those centres has the right person for that kidney.
The matching scheme is run by United Kingdom Transplant (UKT), which is based in Bristol. Everyone in the country waiting for a transplant is listed on their computer, with their tissue types. There are a set of rules which govern the allocation of kidneys, and these are reviewed regularly by a group of transplant specialists. Major changes were made to the allocation system in 2006, and it has been ‘tweaked’ several times since then, with a continual system of monitoring to try and get the fairest allocation of kidneys across the whole of the UK.
The national scheme is complicated, and the following description does oversimplify it. When a kidney becomes available, the UK Transplant computer sees who on the list had the most points. Most points are awarded for waiting time and tissue type match. Further points are also awarded to younger adults, to people with difficult to match tissue types, or who have two copies of the same tissue type (as they are harder to match). The way the points are constructed is that a kidney with a perfect tissue type match may go to anyone on the list, however long they have been waiting. So someone could get a transplant the day after going on the list. For less than perfect tissue type matches, waiting time is more important, and once someone has been on the waiting list for more than 5 years they get enough waiting time points to ‘beat’ other patients who may have a slightly better tissue type match with the donor.
The national allocation scheme is for heart beating deceased donor kidneys. For non-heart beating kidneys, it is more important to do the transplant as soon as possible after removing the organs from the donor, so these are usually used locally. Each transplant unit is allowed to have its own local allocation policy. It is a national standard that this policy is in writing, and anyone should be able to see a copy of their local policy.
Adapted from ‘Kidney Failure Explained’ by Janet Wild and Andrew Stein, published by Class Publishing.
NKF Controlled Document No. 258, How are kidneys allocated?, written 18 May 2001. Last reviewed 20 March 2012.
The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.