Doctors say that if Jonah Lomu's kidney fails it will fall well short of its life expectancy of 20 years.
The former All Black winger has been told his transplanted kidney has a slim chance of survival after he was admitted to Auckland City Hospital with renal problems on September 24.
He is being treated with daily dialysis, while tests are being carried out to see whether his kidney can be revived.
Auckland District Health Board transplant unit clinical director Stephen Munn said seven years was a short lifespan for a kidney donated by a live person.
"The average life expectancy of a live donor kidney transplant should be around 20 years. That of course means that 50 per cent of them will fail before 20 years, but you wouldn't expect them to fail quite this early. But people can be unlucky."
Lomu was diagnosed with a debilitating kidney disorder in 1995, a year after his All Black debut, and was donated a new kidney in 2004.
Professor Munn said some kidneys transplanted more than 40 years ago in New Zealand continued to function: "We have people who were transplanted right back in the 1970s who still have kidneys going now."
He said there were "literally a hundred different reasons" why Lomu's transplanted kidney might have failed, including infection, rejection, and mechanical problems within the new organ.
Reviving the kidney was always the first choice for doctors. But if Lomu's kidney could not be saved, he would be placed back on a waiting list for a new transplant.
New Zealand's deceased donor waiting list has about 600 people on it, with an average wait time of three years. The rarer the person's blood type, the longer the wait.
But like Lomu's first donated kidney, many transplanted organs came from friends or family.
Auckland renal specialist John Rosman said: "The waiting list is a rough estimate because there are people who wait only six months for a kidney.
"But if they have a more difficult blood type then they have to wait for a kidney for, maybe, eight years.
In order to work around that, 50 per cent of all the [transplanted] kidneys in New Zealand are from living donors."
People who had a second transplant faced new complications as the body had to re-adjust to another foreign organ. They must also repeat the extensive health and psychological checks to qualify for a donation.
If Lomu loses his kidney, the All Black hero is unlikely to be short of offers for a new one. His wife, Nadene, immediately offered one of her own, if compatible, and numerous Herald readers responded to yesterday's story by pledging theirs to the Tongan giant.