Challenge March-2019

Offering Donated Organs for Transplant                           

The Business Context
The Example
The Objective

The Business Context:

The development of techniques for organ transplant means that now one person’s death may lead to a lifesaving operation for another. This challenge considers one of the many crucial decision-making processes involved in this miracle of modern medicine. It is based on a (very) simplified description of one step in one of the processes used by the NHS Blood and Transplant (NHSBT) service in the UK.

Once an organ becomes available for donation, how is it decided who will be the recipient? The number of people wanting transplants considerably exceeds the number of organs available, so in general the recipient must be selected from a set of potential candidates for the organ. Strict guidelines are published by NHSBT to ensure this process is fair and equitable. A register is held of all patients awaiting transplants and using these guidelines a prioritized list of compatible candidates can be drawn up. In this context, a ‘candidate’ may be a named individual or may be one of the transplant centres around the country. In the latter case, the centre is responsible for selecting the individual patient most suited to receive the donated organ.

Once the candidate list is created, an organ can be ‘offered’ to the candidate at the top of the list. If the offer is accepted, the organ is delivered to the appropriate transplant centre where a patient can have surgery to implant the organ into their body. However, frequently the initial offer of the organ is declined. There are many reasons this may happen, for example:

  • The surgeon responsible for the patient may determine that match between organ and potential recipient is inadequate
  • The potential recipient may be too ill to undergo surgery at the time the organ becomes available
  • There may be operational challenges at the transplant centre meaning surgery cannot take place in a timely manner

If the offer is declined, then the organ will be offered to the next candidate on the list, again this may be declined. Typically, an organ is offered two or three times before it will be accepted. A fall-back policy exists in the event that none of the candidates on the list can accept the offer.

Now we have a very high-level outline of how the recipient of a donated organ is selected from the possible candidates, let us move to a more concrete but still very simplified description of the offer process which will form the basis of the challenge.

The Example:

We will consider only heart and lung transplants, and the problem is probably best understood in terms of an example scenario. Let us suppose a young man dies in a motor accident and his heart and lungs become available for donation. The register of patients is consulted to find potential recipients. This will result in three distinct lists.

  • An ordered list of named high priority patients requiring a heart transplant
  • An ordered list of named high priority patients requiring a lung transplant
  • An ordered list of transplant centres (TCs) responsible for patients requiring either heart transplants or lung transplants (or both)

To be concrete, let us suppose that the lists are as follows:

High Priority Heart Candidates High Priority Lung Candidates Transplant Centres Candidates
Adam at TC North Diane at TC West TC South
Barbara at TC South Edgar at TC South TC West
Charles at TC West Fiona at TC North TC North

An important element in the offering process is time. Offers can never be accepted immediately and are rarely declined immediately. Surgeons have to be contacted and they need time to consider any offer. Since donated organs are only viable for a limited period of time once an offer is made, a response is expected within about an hour.

In our scenario, the offering process starts out by TC North being contacted offering the Heart specifically for Adam, and TC West being contacted offering the Lungs specifically for Diane. Ideally, both offers are accepted, and the organs are retrieved and sent to the TCs.

Things get more complicated if offers are declined. When a donor’s heart and lungs are both available and if no one on either of the high priority lists is able to accept the offers for them, NHSBT policy is that the two organs are now offered as a ‘ heart-lung block’ to the TCs in turn. The two independent offer processes for a heart and a pair of lungs are replaced by one combined process for the block. Since the organs are initially offered separately and the responses are not instantaneous, this means there must be a synchronization phase where the first organ to be declined by all the candidates on its high priority list ‘waits’ to see if the other organ is accepted by a candidate on the other high priority list, or if it also is declined by all.

When offered a heart-lung block a TC may accept the whole block, just the heart or just the lungs. Any declined organs are then offered to the next TC on the list.

To give a few of the scenarios which might play out:

  • The first two offers of the heart are declined, but it is accepted for Charles, while the offer of the lungs is accepted for Diane.
  • The first offer of the heart is declined, but it is accepted for Barbara, no one on the high priority lung list accepts the offer of the lungs. The lungs are offered to the TCs in turn and finally accepted by TC North.
  • No offers are accepted on the high priority lists, so the organs are offered as a heart-lung block to TC South. TC South accepts the heart but declines the lungs which are then offered to TC West, which accepts the lungs.

The last scenario is the critical one, as two independent asynchronous processes merge into a single process. This changes the decision-making process from simply picking the next option on a list to something more complex.

The Objective:

The objective of the challenge is to devise a decision system which determines what to do next, following a response to an offer of an organ or organs. The response can be a full accept (‘I accept all the offered organs’), a partial accept (e.g. ‘I accept the heart but decline the lungs’) or a decline (‘I decline all the offered organs’).

The possible next steps are:

  • Report the process is complete if the response is an accept and the organ in the response is the only one left which had not already been accepted
  • Invoke the fall-back policy for an organ if the response is a decline and now all candidates have been offered and have declined the organ
  • Take no action at this point (this occurs when one organ has been declined by all candidates on its high priority list, but the other organ has not)
  • Offer the organ(s) in the response to the next candidate on the (appropriate) list
  • Offer the other organ to the one in the response to the next candidate on the appropriate list for this other organ (this occurs for example when the lungs are ‘waiting’ to see if the heart is accepted by a candidate on the high priority list, and the current offer response is an accept for the heart, at which point the lungs can be offered to the first TC on the TC list)
  • Offer a heart-lung block to the first TC on the TC list because all candidates on both high priority lists have declined the individual organ offers
  • Offer the remaining unallocated organ following a partial accept of a heart-lung block to the next TC on the TC list

The next step obviously depends on the candidate lists and the organs being offered as well as the offer response, but the sting in the tail is that it also depends on the current state of the system. Specifically:

  • Which offers have previously been declined (so – among other things – the next candidate on a list can be determined)
  • Which offers have previously been accepted (so – among other things – you don’t try to offer a heart-lung block after someone else has already accepted the heart)
  • Which offers have been made, for which a response has not yet been received (so – among other things – you know whether to move from single organ offers to a heart-lung block offering)

Hence, we are dealing with a stateful system. With this in mind, to be deemed ‘satisfactory’ a solution should make it clear where the current state is held in the solution and how it is both accessed and updated. One hopes that solutions to the challenge based around both stateful and stateless decision engines will be submitted, with state held internally or externally as appropriate.

Typically, the system will be called multiple times before all the organs from a single donor are accepted. It would be nice therefore if solutions could also outline how this recurrent activation of the system might take place, including the initial offering of the organ(s), perhaps with BPMN or CMMN diagrams as deemed appropriate.

Send your solutions to


P.S. This challenge is offered by Dr. Bob Moore