How are organs chosen for transplant?

Two men have died after worm-infected kidneys were transplanted from an alcoholic donor.

The case has raised questions about the checks in place during transplantation and why other hospitals seemingly rejected the organs.

So how are organs chosen for transplant?

More than 20 million people are on the UK's organ donor register, but only a small number will eventually become organ donors.

People have to die in the correct circumstances, usually in intensive care or A&E, to ensure the organ is preserved for donation to be even considered.

If donation is a possibility then two questions dominate: "Can the organs be used safely in anybody?" and "Who are they allocated to?"

First there is an assessment of the quality of the organs. Transplant teams use their medical experience to decide whether an organ is functioning well at the time of death or if it is likely to improve after transplantation.

Being an alcoholic does not mean an organ fails this test. However, it is more likely to prevent a liver being donated than other organs such as the kidney.

Meanwhile a team of 250 specialist nurses based at hospitals across the UK assess the suitability of the donor.

There is a physical assessment of the donor as well as other tests, such as those for hepatitis and HIV. Their GP is contacted for their medical history, and the family is contacted in an attempt to fill in any blanks.

Information is collected on the donor's behavioural, social and travel history - and their tissue type and blood group is recorded.

The nurses do not assess suitability themselves. That is done by the transplant centres.

According to the NHS, only rarely are organs completely ruled out for donation. But scenarios where they are include:

• a donor aged over 85

• where a cancer that has spread in the past three years

• a severe or untreated infection such as TB or West Nile disease

• suspected or confirmed cases of the human form of "mad cow disease" - Creutzfeldt-Jakob disease

However, the available information can be scant. Healthy people may never visit a GP and then die suddenly, while others may hide personal details from their family.

Who gets it?

If any organs are suitable for donation, they are made available through the NHS Blood and Transplant Electronic Offering System.

The exact details depend on the type of organ.

Kidneys are allocated to individual patients on the waiting list using a complex computer algorithm.

Others such as livers are offered to transplant centres rather than being designated for a specific patient.

It is then up to the transplant team to assess the risks and benefits of an offered organ for their patient.

An organ that would be rejected for one patient may be suitable for another.

Doctors may decide to reject a particular organ if they think there could be a better tissue match.

Another consideration is that the odds of heart problems rapidly increase for dialysis patients over the age of 50. Doctors with an older patient on their books would be more likely to accept a marginal organ that could have been rejected by other teams.

When an organ comes from a patient with encephalitis/meningitis, as the worm-infected kidneys did, then this is a major alarm bell for doctors as it is a sign of an undetected infection.

It shifts the balance of risk and benefit - but it does not rule out a transplant.

Transplants are successfully carried out from such donors, even when the cause of their illness is unknown.

In the 10 years to March 2013, there have been 159 transplants from 52 such donors. Ninety-four of those transplants were kidneys.

Risk assessment

If a transplant centre decides against using the offered organ then it is offered to the next patient on the computer-generated list.

After five rejections - which might be for donor or organ quality reasons, or because the time when an organ can be used is running out - a "fast-track offer" kicks in and the organ is made available to all 25 transplant centres in the UK.

This is to try and ensure that organs do not go to waste.

Units that express interest are ranked by patient need and the destination is chosen.

Prof Derek Manas, vice-president of the the British Transplantation Society and a surgeon at the Freeman Hospital in Newcastle, said it came down to assessing risk.

"Some surgeons are very risk-averse as they only want the best organs and guaranteed outcome, while others take a risk for a specific patient if the chance of survival is otherwise short.

"We are the most regulated and monitored country, we are audited all the time. It's not like we're not allowed to do maverick things."