Organ Allocation - pediagenosis
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Tuesday, April 16, 2019

Organ Allocation

Organ Allocation
There are many more people on the transplant waiting list than there are organs available. To manage this shortage access to the waiting list is restricted to those meeting strict eligibility rules. Once on the waiting list allocation follows pre-defined rules to ensure fairness.

Eligibility for transplantation
Criteria vary from organ to organ, and country to country. In addition, different considerations may be necessary for patients needing a second transplant after the first has failed, particularly since for most organs the results for second and subsequent trans- plants are inferior to first transplants. For kidney, pancreas and liver there must be an expectation that the recipient will survive 5 years after the operation. UK listing criteria are given below.
Organ Allocation, Eligibility for transplantation, Kidney transplantation, Pancreas transplantation, Liver transplantation, Heart transplantation, Lung transplantation, Principles in organ allocation, Allocation in practice, Kidney, Pancreas for islets or whole organ,

Kidney transplantation
Already on, or estimated to be within 6 months of starting dialysis (e.g. using a reciprocal creatinine graph). Re-transplantation is permitted providing it is surgically feasible and the patient is fit; the main limiting factor is sensitisation against HLA antigens.

Pancreas transplantation
1.     Combined (simultaneous) pancreas and kidney (SPK) transplantation: GFR ≤ 20 ml/min or on dialysis and type 1 diabetes (or type 2 if BMI <30 kg/m2).
2. Pancreas or islet transplantation alone (PTA/ITA): life-threatening hypoglycaemic unawareness.
3.    Pancreas after kidney transplantation (PAK): severe diabetic complications and satisfactory function of prior renal transplant, since function is affected by increased doses of nephrotoxic immunosuppression.

Liver transplantation
There is no bar on re-transplantation, but since results of retransplants are so much poorer, the patient should be otherwise in good health. Individual criteria exist for subgroups, such as hepatocellular tumours or acute liver failure (see Chapter 33).

Heart transplantation
Patients are accepted according to internationally agreed criteria. Many patients are now supported by mechanical devices, and are regarded as stable on the waiting list. They only receive priority if they develop complications such as drive-line infections. Re-trans- plants can be done with reasonably good outcomes, but not in the first 3 months after the initial procedure.

Lung transplantation
Most patients are now listed for bilateral lung transplants. The only group regularly receiving single lungs are those with fibrotic disease, where the shrunken chest cavity cannot easily accept a pair of lungs.
Re-transplants are done with increasing frequency, although still amount to only 5–6% of activity.

Principles in organ allocation
Organ allocation is an exercise in distributive justice, how to fairly divide up a limited resource. There are several criteria that may be used for organ allocation.
Equity (fairness): everyone should have equal access to organs. Such a scheme would allocate organs first to those who have been waiting longest, and to young and old alike.
Utility: organs should be allocated to achieve the greatest number of life-years following transplantation, independent of other factors. For example, since outcomes of kidney transplantation are poorer in those already on dialysis and in the elderly, these two groups would be excluded in a utilitarian allocation scheme, in direct contrast to the egalitarian approach.
Greatest need: the organ goes to the person whose medical condition demands it the most.
Greatest benefit: organs are allocated to achieve the greatest benefit, in terms of life-years gained, compared with remaining on the waiting list. Such allocation acknowledges that organs are different, with young donor organs having a better anticipated longevity than older organs. Thus an old donor kidney may be best allocated to an older recipient, who has a high mortality on dialysis and for whom an old kidney would increase their survival significantly. A young recipient has a better survival on dialysis so there is less gain from having an old kidney, which would last only a short time period.

Allocation in practice
In reality, current allocation schemes involve a mixture of the above principles. Organs are allocated to ABO-identical recipients, with the exception of group A organs, which may go to AB recipients, and occasional group O organs, which may go to group B (or A or AB) recipients in special circumstances (e.g. medical urgency or HLA sensitisation).
Organs are transplanted to avoid pre-existing donor-specific HLA antibodies (a positive cross-match), with the exception of the liver, which can be transplanted into a recipient who possesses antibodies to the donor’s MHC class 1 antigens.

Kidneys are allocated primarily to HLA-matched recipients, prioritising sensitised patients over non-sensitised, children over
adults. Thereafter allocation is according to a complex formula that assigns points for:
·       HLA mismatch, aiming to optimise matching
·      time on the waiting list, prioritising long waiters
·  sensitisation (HLA antibodies) and matchibility (unusual HLA type), giving priority to patients who are hardest to find a compatible transplant
·    HLA-B and -DR homozygous recipients, correcting an imbalance that prioritising according to HLA mismatch creates
·     age difference, aiming to minimise age difference between donor and recipient.
In addition children (under 18) get priority over adults.

Pancreas for islets or whole organ
An algorithm assigns points for:
·       HLA mismatch, aiming to optimise matching
·       HLA sensitisation and matchibility
·    waiting time, giving additional priority to an islet recipient awaiting a second graft and a pancreas recipient on dialysis
·      distance of donor to recipient centre, to minimise ischaemic time.

Livers are allocated within seven zones in the UK corresponding to each liver transplant unit. Priority is given to the sickest patient (UKELD score, see Chapter 33) of a compatible size – big livers don’t fit small abdomens.
A ‘super-urgent’ scheme exists for anyone with acute liver failure with an expected of survival of less than 3 days; a third of these patients die while waiting and outcomes are poorer than for chronic liver disease.

Like livers, hearts and lungs are allocated within zones corresponding to each of the six transplant centres. Matching is done by blood group and size of donor, which needs to be within 10% of that of the recipient. Female hearts placed in male recipients do measurably less well, and this combination is avoided.
There is also an urgent scheme for hearts, which accounts for nearly half of all transplants performed. The results are at least as good as those for ‘elective’ patients. These recipients have the most to gain from transplantation.

Size is of great importance in lung allocation–large lungs do not fit into small recipients. If small lungs are placed in a large chest they become over-inflated. Allocation is done as for hearts and livers, on a local basis, but there is no urgent system. Individual centres identify the sickest patients on their waiting list. A lung that cannot be used locally is offered nationally around the other centres.

Intestinal donors are offered as a priority to the four intestinal transplant centres (two adult, two child). For most intestinal transplants size is the critical factor, with only the smaller donors (below 50 kg) being suitable.

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