A mother-of-two diagnosed with two extremely rare autoimmune disorders has had funding for her life-saving treatment cut.

A Perth woman suffering from a unique combination of extremely rare autoimmune disorders faces an uncertain Christmas with federal funding for her life-saving treatment being cut.

Mother-of-two and research scientist May Ali is the only person in the world to be diagnosed with both necrotising autoimmune myositis (NAM) and atypical haemolytic uremic syndrome (aHUS).

By the time that conclusion was reached, the 43-year-old had become severely disabled, losing the ability to sit, walk and feed herself, and sustained organ damage before being given Eculizumab, which she needs every fortnight.

But last week, Ms Ali was advised her doctor's application to continue the Pharmaceutical Benefits Scheme-subsidised treatment had been rejected - and she can't afford the $500,000 a year cost herself.

After a public outcry over recent days, including a change.org petition to Health Minister Sussan Ley that has so far been signed by more than 31,000 people, Ms Ali was granted one last treatment on Thursday.

Beyond that, her future is uncertain.

With her next treatment due on Christmas Eve, Ms Ali, her husband and two young daughters have put their festive travel plans on ice.

She and her doctors now have two weeks to convince federal authorities to keep up her treatment.

Ms Ali said it had been suggested it could be resumed as soon as any adverse symptoms emerged, but "that's not the case".

"They have specific criteria that constitutes an abnormal response and they can require for some damage to be done before they act," she told AAP.

"In some people that might be safe enough. My body is already heavily compromised.

"It's a gamble. Last time (I didn't have the drug), I had multiple seizures and sustained brain damage which thankfully reversed. What if it doesn't reverse this time?"

Ms Ali said her kidneys were only functioning at 50 per cent and any less would require dialysis.

Research showed 30 to 40 per cent of patients taken off Eculizumab relapse, she said.

"My specialists have already said 'do not take her off this drug'.

"And then this committee - and none of them have met me or examined me, and none of them have experienced someone who has NAM and aHUS - they have decided that their opinion and their criteria trumps my doctors' informed opinion."