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The NKF Helpline is available Monday to Thursday 08:30am - 5:00pm Friday 08.30am – 12.30pm on 0800 169 09 36 or email [email protected].

Key Points

• Kidneys are allocated according to a points score for each potential recipient, this uses waiting time, tissue type matching and other factors
• If you have an unusual tissue type or blood group there may be a long wait for a kidney
• Do not imagine the kidney allocation system is against you – it tries to be fair
• You continue to accumulate points even if you are suspended on the register

Finding a suitable kidney

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 not a good enough match, the recipient’s immune system (natural defence system) will attack and reject it.

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.

Matching the blood group

The 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:

Recipient	Donor
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 initially match the blood group exactly between the donor and recipient when cadaveric kidneys are allocated.

Matching the tissue type

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 characteristics 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:

Donor:	A3/A11	B1/B9	DR3/DR8
Recipient:	A3/A7	B5/B9	DR3/DR21

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. As immunosuppression has got better, most units will offer a transplant if there is a mismatch of greater than‘3 out of 6’.

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 National Matching Scheme

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 the kidney.

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 the kidney.

The matching scheme is run by NHS Blood and Transplant (NHSBT) which is based in Bristol. Everyone in the country waiting for a transplant is listed on their computer register 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 2019 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 kidney offering scheme is complicated and the following description is a simplified version. The kidney transplant team caring for you can provide more detailed and specific information applicable to you. All deceased donor kidneys are allocated via an evidence-based computer algorithm (except some donors aged 70yrs or older). The computer first tries to identify matches for patients who are extremely difficult to match and/or those waiting for more than 7 years (called Tier A recipients). Within Tier A the computer prioritises based on waiting time and difficulty in matching. All other eligible patients (called Tier B) are prioritised according to a number of variables including waiting time, donor-recipient risk index combinations and tissue type match. For each variable, points are calculated for each wait-listed patient e.g. 1 point for each day on waiting-list. The donor kidney is matched to the patient with the highest number of points. If there is no match within Tier A, then the two kidneys from one deceased donor will be matched to the two highest scoring recipients in Tier B.

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Last reviewed March 2023
Next review March 2026

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.

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