A stroke victim accidentally drank floor cleaner which was in a water jug left at her hospital bedside by a member of staff, an inquest has heard.
Joan Blaber died on September 23 last year while being treated at the Royal Sussex County Hospital in Brighton.
The widow had been admitted with a minor stroke on August 22 but her condition worsened after ingesting the Flash fluid on September 17.
Her family saw a "housekeeper" take a green water jug to her bedside after lunch while they were visiting. They had recently poured her a drink so did not touch it or notice its contents before they left, jurors heard on Monday afternoon.
It was later found to contain a "fair amount" of cleaning fluid, senior coroner Veronica Hamilton-Deeley said.
At around 10pm that night some of the liquid was poured into a beaker when medication was administered, Detective Inspector Julie Wakeford, who led the police investigation, told the hearing at Jury's Inn near Brighton Station.
Jurors had heard earlier that mystery still surrounds the death. The coroner said it was "probably no exaggeration" to say "no-one can say what happened" despite a "rigorous police investigation" in which 100 people were interviewed.
There was an eight-day delay in reporting the incident to the authorities, Ms Wakeford said.
By the time forensics teams examined the Baily ward the beaker could not be found and its contents had been discarded. The jug was seized but tests could not determine whether the fluid was neat or diluted, she added.
Giving evidence, she said officers had not identified anyone who should face a criminal prosecution, adding: "We have not identified any reason why it would be malicious but again, because we don't know how it has happened, we cannot completely rule that out.
"We couldn't find any reason why anybody would maliciously cause her any harm."
CCTV due to be viewed during the inquest shows three cleaners carrying a five-litre box of Flash into the ward at around midday on the day Mrs Blaber swallowed the substance, and later emerge with it, Ms Wakeford told the hearing. But she said police did not believe this was relevant to the incident.
Mrs Blaber was also given the wrong water jug that day, the inquest heard.
Her family noticed because it was a different colour to her normal container. Like most patients, she was given a clear jug with a blue lid. Dementia patients were given green jugs and those needing encouragement to drink were given a red one.
The retired shopkeeper did not have dementia but was an "extremely vulnerable woman" who was "completely dependent" on the hospital for care, Mrs Hamilton-Deeley told jurors.
She had a history of cancer and "significant and enduring medical conditions" including arthritis and chronic leg ulcers, the inquest heard.
Born in Tidworth on the Wiltshire/Hampshire border, and living in Lewes, East Sussex, she had been housebound since the beginning of the year after becoming very frail, and had been admitted to hospital three times in the months leading up to her death.
Philip Thompson, a stroke consultant at the hospital, said her health worsened after she drank around 15ml of the industrial fluid.
Mrs Hamilton-Deeley told the jury it was "incredibly worrying" no-one knows what happened, adding: "It could happen again."
She asked the jurors to "winkle out" what happened and described their task in the inquest as "hugely complex" and "difficult", adding: "We have to look at everything."
Brighton and Sussex University Hospitals NHS Trust immediately stopped using Flash after Mrs Blaber died. The coroner previously expressed concern when the trust reintroduced the substance in March following complaints over cleanliness.
In May, the Care Quality Commission (CQC) told the trust it must do more to protect patients from potentially hazardous cleaning materials and there was "no legitimate reason to decant cleaning products into other containers".
Health inspectors visited several of the hospital's wards unannounced in October and expressed concern that cleaning trolleys with hazardous substances were found unattended, a report said.
But staff had good knowledge of rules for keeping cleaning products after being retrained recently, the watchdog added.
Mrs Blaber's son Gary and his wife Sandra, who live in Lewes, and her sister Rosemary Bird, of Eastbourne, attended the hearing and described her as someone who was "very house proud and loved her home" with a good sense of humour despite her heath difficulties.
A post-mortem found she died of respiratory failure with pneumonia being a contributing factor after ingesting cleaning fluid.
Mrs Hamilton-Deeley said it had been "erroneously reported" the liquid was bleach but it was not.
She added: "We are bound to call it by its name. It is Flash. There are other cleaning fluids. This Flash is not particularly noxious."
The trust previously said a "comprehensive review" of potentially hazardous substances took place immediately after Mrs Blaber's death with improvements made to staff training and the way substances were stored and used.
The inquest continues on Wednesday and could last eight days.
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