Organ donation: Does an opt-out system increase transplants?
The NHS has released figures revealing that 457 people died in England last year while waiting for an organ transplant.
Doctors' union the British Medical Association (BMA) and politicians alike have called for an opt-out system to be adopted in England, where people's consent to donate their organs after their death is presumed unless they have explicitly said otherwise.
This system is currently in force in Wales and in a number of other countries throughout Europe.
In a recent Parliamentary debate on organ donation, Labour MP Dan Jarvis said: "England must now move to an opt-out system. The evidence is clear - hundreds of people a year are paying a price of us not doing so."
But there is a lack of evidence to support this claim.
In Wales, where an opt-out system was introduced in December 2015, there has actually been a small dip in the number of deceased donors, from 64 in 2015-16 to 61 in 2016-17. This resulted in a drop in organ transplants from 214 to 187 respectively.
This is not to say the opt-out scheme is having a negative effect - some fluctuation is to be expected - but so far, despite the claims, we don't have any evidence that it is having a positive effect.
The BMA says it believes that over time such a scheme would lead to an increase in organ donation.
The Welsh government is in the process of evaluating the scheme and plans to publish a report by the end of this year.
One concern raised by Dr Margaret McCartney, a GP, in a paper for the British Medical Journal is that the Wales model of organ donation creates a group of non-donors who did not exist before.
In the English system there are two groups of people: those who have opted in and registered their wish to be a donor, and those who have done nothing whose families will be asked to decide.
In Wales there are now effectively three groups of people: those who have opted in and so registered their wish to be a donor; those who have done nothing for whom it is assumed they are happy to donate their organs, but it is still ultimately for their family to decide; and a third group who have opted out and so expressly registered their wish not to be a donor.
As it currently stands, 6% of the Welsh population has opted out of organ donation. This is a group of people who in an opt-in system were still potential donors, depending on their families' wishes - they may not have ended up donating organs, but we just don't know.
There is correlation between countries having opt-out schemes and having a higher number of organ donors.
But the countries which have the most donors per head combined the introduction of their opt-out schemes with other changes, like better infrastructure, more funding for transplant programmes and more staff working to identify and build relationships with potential donors before their death.
Spain is often touted as an opt-out scheme success story.
So-called "presumed consent" legislation was passed in 1979 but donor rates only began to go up 10 years later when a new national transplant organisation was founded which co-ordinates the whole donation and transplantation process.
The legislation is also not strictly enforced since families are always consulted and have the final say.
However, opt-out schemes don't always translate to increased organ donor rates. In Sweden, for example, such a scheme has been in force since 1996 and it remains one of the lowest-ranked countries for organ donation in Europe. Luxembourg and Bulgaria also have opt-out systems and low rates of organ donation.
In France and Brazil, variations on a "presumed consent" system actually led to a decline in the rate of organ donation.
Another difficulty in assessing whether opt-in or opt-out schemes are driving different countries' donation rates is that these schemes take different forms across the globe.
In both Spain and Wales, families of potential organ donors are always given the chance to refuse. But this is not universal - Austria and Singapore both have "hard opt-out" systems where those who have not opted out are presumed to have consented to organ donation regardless of their families' wishes.
And there are other differences, for example in Israel a priority incentive scheme means those who have agreed to donate their own or a deceased family member's organs are given priority on transplant lists should they themselves need an organ in the future.
In "hard opt-out" systems there were increases in the organ donor rate of up to 25%.