Junior doctors treating Heather Bills the night of her catastrophic deterioration were "inattentionally blind" to the "gorilla" killing her, a coroner's inquest has heard.

Bills, 64, died at Middlemore Hospital on January 2, 2013, six weeks after she suffered burns to 35 per cent of her body in a fire at her Orakei home in Auckland.

It was later discovered that she suffered a suspicious and massive hypoglycaemia-related cerebral injury.

Police suspected three medical workers of administering a fatal dose of insulin to Bills on the night of December 26-27, 2012.

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Bills had articulated suicidal thoughts and offered nurses cash to help her die.

Dr Ross Boswell, a chemical pathologist and head of Middlemore Hospital's laboratory, said the "junior" doctors treating Bills that night were suffering from "inattentional blindness".

He said they failed to diagnose Bills' low blood sugar earlier because they were focusing on respiratory problems and had no reason to suspect it was an issue after a "normal" blood glucose reading.

A nurse, who is a police suspect and has interim name suppression, said she conducted a blood sugar test with a glucometer which showed a 6.4 mmol/L reading.

However, when investigators searched the past readings on the glucometers no evidence of the nurse's reading was found.

Boswell described the insulin as a "gorilla" in Bills' system hidden among several other red flags.

"A gorilla walking through a group of basketballers is a lot more conspicuous than one more red result amongst 13 or 14 more red results," he explained.

He said Bills' hypoglycaemic event was also consistent with her receiving exogenous insulin from an external source.

"It is also possible that a massive dose of soluble insulin was administered into an intravenous (IV) drip or solution or by intramuscular injection," his brief of evidence reads.

"On the balances of probabilities I believe she was injected. Is it proven beyond all doubt? I don't think it can be ... If I'd see the insulin injected I could be [certain]. The evidence we've got is all circumstantial and not clear-cut," he told the inquest.

He said it was far less likely it was administered through an through an IV drip.

The homicide case has been unable to proceed to criminal trial because a tumour was later discovered in Bills' body, Boswell said.

He said a pathologist's evidence wouldn't hold up under cross-examination when asked if the tumour could have produced the insulin.

"It is possible but it is very unlikely," Boswell said.

Heather Bills' death is the focus of an inquest before chief coroner Judge Deborah Marshall in the Auckland District Court. Photo / Michael Craig
Heather Bills' death is the focus of an inquest before chief coroner Judge Deborah Marshall in the Auckland District Court. Photo / Michael Craig

Acting chief executive officer of the Counties Manukau District Health Board Gloria Johnson said the DHB has investigated why there was a delay in an accurate diagnoses.

At the time of Bills' death Johnson was the DHB's chief medical officer.

Key findings from the investigation found there were several reasons for Bills' low blood sugar level not being detected earlier.

There was no history of hypoglycaemia or insulin treatment, the nurse had reported a normal blood glucose reading, there was a history of recurrent respiratory distress, and a laboratory procedure for telephoning extreme results was not reliably and confusing.

As a result of the investigation, the lab's software was modified so that immediately life-threatening values are now highlighted differently.

The DHB has not been willing to reinvestigate Bills' death, despite the wishes of her daughter Michelle Maher, due to the police investigation and inquest.

The inquest before chief coroner Judge Deborah Marshall continues on Friday.

WHERE TO GET HELP:

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