But she is happy. Because for the first time in her life she understands what mysterious condition has been causing her to suffer everything from muscle weakness to stomach problems for as long as she can remember.

Three years ago, doctors finally discovered she had mitochondrial disease. ''When you know there is a reason for the things that are happening to you, you find a way to deal with them and you can move on,'' she said.

Mitochondria are tiny parts inside every cell that are vital to our survival; they allow our bodies to convert food into energy as well as helping cells stay healthy.

When they stop working, they can cause anything from mild deafness to seizures or muscle fatigue, said the head of neurology and director of neurogenetics at Royal North Shore Hospital, Carolyn Sue.

''Mitochondrial disease can basically masquerade as almost anything,'' she said. ''Our mitochondria can also stop working properly for a number of different reasons, from mutations in its DNA, which is separate from the rest of our DNA, or from other genetic disorders that can impact on its function.''

Because of this, Ms Egan's experience of countless tests and painful muscle biopsies is common. But new research undertaken by Professor Sue, and published on Tuesday in the journal Neurology, could change that.

She has discovered a simple blood test can be used to diagnose the disease in about 70 per cent of cases. ''This is a huge step forward,'' she said.

Currently, tests used in most hospitals will only identify about 10 per cent of cases. Even hugely expensive genome sequencing, not available outside of research studies, will only catch between a quarter and half of all patients.

''Looking for a gene mutation is like looking for a needle in a haystack,'' Professor Sue said. ''In mitochondrial disease it's … like looking for a needle in a haystack that keeps moving.''

Many people with mild symptoms are never diagnosed. The Australian Mitochondrial Disease Foundation estimates up to 100,000 Australians could be affected. Scientists have yet to create a cure, but some symptoms can be treated.

Professor Sue conducted blood tests examining the levels of a special substance, known as ''serum fibroblast growth factor 21'', in the blood of 140 people with and without mitochondrial disease.

She found the serum blood test was nearly 45 times more effective than any previously used blood test.

She hopes that within a year she will have a program to help doctors diagnose the disease.

''The lucky ones are the ones that get to the clinic,'' she said. ''The rest don't even know they are sitting on time bombs.''