Linda McDonagh, 60, is suspected of being the fifth person to have suffered a hypoglycaemic episode at Stepping Hill hospital between June and July last year after her saline drip was contaminated.
McDonagh, from Offerton in Stockport, died on 14 January, police have confirmed.
"The coroner has been made aware and further tests are ongoing," said a Greater Manchester Police spokesman.
Tracey Arden, 44, Arnold Lancaster, 71, and Derek Weaver, 83, all died after suffering similar hypoglycaemic episodes. All the deaths took place on the same three wards at Stepping Hill.
Police are looking into whether the death of William Dickinson, 82, on New Year's Eve was due to the alleged poisoning that took place while he was at the hospital during the summer.
Victorino Chua, a 46-year-old male nurse, has been released on bail on charges of murder, causing grievous bodily harm and tampering with medical records.