Police now believe 16 people were poisoned by the contamination of saline solution at Stepping Hill Hospital.
Following the release of nurse Rebecca Leighton last month, Greater Manchester Police said they were looking at around 42 potential victims who may have been harmed by the contaminated solution.
Detectives and medical experts have been reviewing those cases and have now concluded of the 42 in total, 16 people were poisoned, 16 other cases have been eliminated from the inquiry and 10 others are still being investigated.
Amongst the 16 confirmed poisoned are the cases of Tracey Arden, 44, Arnold Lancaster, 71, and Alfred Derek Weaver, 83, who all died at Stepping Hill in Stockport.
All three were unlawfully administered insulin but it has not yet been established whether that was a significant contributing factor to their deaths, police say.
Nurse Miss Leighton, 27, spent more than six weeks in custody but was dramatically freed on September 2 after proceedings against her were discontinued due to insufficient evidence.
Charges that she tampered with saline solution with intent to endanger life were dropped by the Crown Prosecution Service (CPS).
Miss Leighton has since been cleared to return to work by the Nursing and Midwifery Council subject to conditions, despite hearing claims that she had admitted to the theft of opiate-based drugs.
The alarm was first raised by hospital staff on July 12 when a higher than normal number of patients were reported to have "unexplained" low blood sugar levels amid fears that saline solution had been contaminated with insulin.
Heightened security measures remain in place at Stepping Hill and will continue for the foreseeable future.
No-one is allowed to administer intravenous drips alone and all keys to medicine cabinets have to be signed for.
Detectives have drafted in a Cracker-style criminal profiler to help identify the mystery poisoner.
Greater Manchester Police have updated the list of potential victims from 16 to 17 people who are believed to have been poisoned.
Three of these were Tracey Arden, Arnold Lancaster and Alfred Weaver, who later died.
Police said there is still no official cause of death and it is not known whether the administration of insulin contributed to their deaths.
In total, 16 people have been eliminated from the inquiry. The cases of nine others are still being reviewed, police said.
A police spokesman said the figure of 16 reported earlier today was correct at the time but a meeting in relation to "victim statuses" which took place this afternoon has led to an update in the figure.