As she wobbles on shaky legs, drained of energy by the dialysis, her mouth parched, she dare not drink for fear of shortening the interval until she needs the treatment again.

When she can no longer bear the thirst, she sometimes sucks an ice cube when she gets home.

But Ms Sun, who weighs only 39kg, never loses hope that one day she will get a new kidney and will no longer have to endure the painful five-hour process of having toxins removed from her blood every other day.

That she can still hold such optimism is amazing, given the shocking treatment the former waitress has experienced at the hands of the mainland's medical system.

Like many young rural girls hoping to work in the city and help lift their families out of poverty, the Hebei native moved to Beijing in 2001 when she was just 18. She found a waitress job in a state-run guesthouse and was happy she could help her illiterate shepherd father.

But her hopes for a better life were dashed nine months later when she was diagnosed with uraemia, a condition in which the kidneys fail, allowing urea and other toxins to build up in the bloodstream.

A Hebei doctor told her parents uraemia was a 'disease for the wealthy', meaning the poor would not be able to afford treatment, and that it would be best if they made their daughter as comfortable as possible and await her inevitable death - the fate that befalls 90 per cent of the mainland's uraemia patients.

Her parents would not give up. They sold everything, borrowed money and raised funds from local township enterprises with the help of a local official.

An elderly doctor agreed to operate at a discount and Ms Sun finally received one of her mother's kidneys at Beijing's Chaoyang Hospital in June 2002. But her body rejected the kidney five days later and it had to be removed.

A second transplant, from an unknown donor, followed in November that year, and again the kidney failed.

'When Wenjuan lay on the bed at that time, she was so tiny and she almost died,' her mother, Qiao Xianhua , said.

The ordeal was only just beginning, however. When the family ran out of money, Ms Sun was expelled from hospital in February 2003, even as her parents pleaded with hospital officials - a scene common across the mainland.

Then, Ms Sun's employer at the time decided to terminate her medical insurance in August 2003. Just as the family was reaching the end of its tether, help came from an unexpected quarter.

A week before the insurance coverage was to due end, South China Morning Post readers donated 200,000 yuan, enough to cover her dialysis fees of about 7,000 yuan a month, another transplant and drugs.

After reviewing Ms Sun's case, PLA Hospital 301 agreed in September 2003 to perform a transplant if a suitable kidney could be found. Since there was no vacancy for dialysis at the hospital at that time, Ms Sun went to nearby Zhongxing Hospital to have dialysis while she waited.

Everything seemed to be looking up, but she was about to discover she had fallen victim to a medical system that puts lives at risk, even for those with money to pay the bills.

In June 2004, the director of the Zhongxing Hospital's dialysis ward sent her for a hepatitis B test at a nearby hospital specialising in infectious diseases. The results showed she had hepatitis B, but the doctors did not tell her.

Medical records showed Ms Sun had not been infected with any type of hepatitis before starting dialysis at Zhongxing Hospital.

It was only in January 2005, when a new doctor saw the medical records and sent her for another hepatitis test, that she was told she had contracted the virus.

It was devastating news because the chances of a hepatitis carrier receiving a kidney transplant were extremely low, if not nil. The patient's liver, weakened by hepatitis, would be unable to deal with the drugs given to stop rejection of the new organ.

After that, Zhongxing Hospital tried many ways to persuade her to leave. It eventually succeeded when Ms Sun, full of pent-up anger over the infection and the attitude of the doctors, lost her temper and threw a glass on the floor.

Shards injured a nearby cleaner and the hospital jumped on the incident as justification to throw Ms Sun out.

Months later, during negotiations for compensation from the hospital for the infection, officials tried to deflect blame by raising such possibilities as Ms Sun having been infected when eating at a restaurant or from mosquito bites. Foreign medical experts generally believe it is impossible to contract hepatitis B in such ways, although some mainland medical textbooks do not rule it out.

When Ms Sun, with the help of Facilitator - an NGO that helps migrant workers - filed a lawsuit against the hospital, her family realised that the large amount of time and money required to seek justice was out of their reach.

Even gaining access to medical records for a lawsuit was extremely difficult, and Ms Sun managed to obtain only part of her file after much effort. Moreover, there was no way to ensure hospitals did not modify the records.

Theoretically, patients could apply to have their medical records sealed, but this was extremely difficult to do in practice. For a poor family like that of Ms Sun, finding a lawyer was also very difficult, and few lawyers had the knowledge to read and understand medical records.

The bad news did not end there. When Ms Sun started dialysis at another hospital in June 2005, a blood test showed she had also contracted hepatitis C, which further reduced her already slim hope of a transplant.

In her fight for justice, Ms Sun has won the sympathy of a mainland doctor and a Hong Kong nurse, who have examined her medical records and are advising on her case.

They have identified serious malpractices at Zhongxing Hospital, where the records suggest Ms Sun contracted hepatitis B - patients with and without hepatitis use the same dialysis machines, which presents a high risk of cross-infection.

The hospital, testifying to the district medical association, admitted it had hepatitis B and hepatitis C patients on dialysis, but claimed they did not share machines.

But the medical experts helping Ms Sun's case found that she had had dialysis on many machines before and after she tested positive for hepatitis B, showing that the hospital had never followed government instructions to separate hepatitis patients from others.

'The more I went through the medical records, the sadder I felt,' said Chong Pui-yu, the Hong Kong nurse who volunteered to help and who has prepared documents for a district medical malpractice commission, which will decide if the hospital was responsible for Ms Sun contracting hepatitis B and C.

A lawsuit was also filed in a Beijing court last year, but the court is awaiting the conclusion of the medical malpractice commission.

Now Ms Sun is at the end of her tether again. Though she does not know it yet, the donations from Post readers will run out this month.

Despite her ordeal, Ms Sun is grateful for the past four years of precious time with her family. And no matter how weary she feels after dialysis, she cooks for her mother every night when she returns from work, happy to show in any small way her gratitude for the love and care - and longer life - she has received from her family.

Asked how she manages to do it when she is so weak, she says: 'It does not matter, I will just do the cooking and washing slowly, bit by bit.'

Notwithstanding her stoic optimism, Ms Sun's experience is a powerful and none-too-rare example of a system in which the only goal for hospitals is to maximise profits; one where regulators fail to detect and punish errors, and where ethics and patients' rights are neglected.