A mother accused of injecting her son with faecal matter was only brought to trial because of "wild speculation", her lawyer says, after the crown prosecutor admitted there was not enough evidence to prove the charges.
"In 23 years as a crown prosecutor I've never in any trial...(said) that there is a reasonable doubt in relation to this matter," barrister Lou Lungo told the NSW District Court in his closing submissions on Friday.
"I can't be any more forthright than that."
The concession means the Blue Mountains mother, now 39, will be acquitted more than six years after being accused of administering faecal matter to her young son while he was in Sydney's Westmead Hospital.
Paediatrician and infectious diseases specialist Professor David Isaacs sounded the alarm that something might have been injected into the boy's blood stream, after a blood sample on September 27, 2014 tested positive for e.coli and one other bowel organism.
It came after a number of sterile blood cultures and urine cultures performed following his admission to hospital on September 2.
Prof Isaacs said it was unusual that the organisms would grow despite the son receiving antibiotics, and other explanations for the presence of e.coli did not make sense.
Giving evidence to the court, Prof Isaacs admitted that there were other scientific explanations for the sample result, including contamination of the sample.
But he said that in the context of the abnormal result and the boy's previous admissions to hospital, he thought that the mother had factitious disorder, previously called Munchhausen syndrome.
Prof Isaacs had previously missed a case of factitious disorder and the child had died.
An expert infectious diseases doctor, Dr Bernie Hudson, later told the court that contamination of blood cultures can occur and there was not evidence to rule it out.
"The accused is here today only ... because of the speculation of one person, namely Dr Isaacs," said defence barrister Pauline David in her closing submissions on Friday.
She said it was "somewhat wild speculation" as there were commonly known, obvious and available explanations for a positive isolated blood culture.
A number of doctors raised the possibility of innocent explanations for the blood culture result, Ms David said.
But those possibilities were "quite stunningly and persistently ignored by other health professionals at Westmead and by (Family & Community Services) and also by the police," Ms David said.
Mr Lungo told Judge Justin Smith that Prof Isaacs' opinion may have been understandably clouded due to the "tragic loss of a child" earlier in his career.
The court had earlier heard that nurses heard the boy ask his mother "why are you poisoning me?" and "what have you done to my cannula this time?".
He later told police his mother had not poisoned him and that when he was sick he said "random stuff ... my brain goes all weird".
The woman's four children were removed from her care following the accusation.
Ms David will continue her closing address on Friday.
