It is nearly one year since staff at Stepping Hill Hospital in Stockport, Greater Manchester, raised the alarm after they found contaminated vials of saline and noticed "unexplained" low blood sugar levels in patients.
Police later found the saline drips of 22 people had been deliberately contaminated between June 1 and July 15 last year.
They are reviewing and analysing each of the hundreds of interactions all those patients had with staff or visitors during their time at Stepping Hill.
Experts have conducted a series of specialist medical tests including comprehensive examinations of drugs present in the body before and after death, as well as signs of natural disease.
The tampering affected patients on acute care wards. The deaths of Tracey Arden 44, Arnold Lancaster, 71, and Derek Weaver, 83, are being investigated as murders while those of Bill Dickson, 82, Linda McDonagh, 60, John Beeley, 73, and Beryl Hope, 70, are being treated as cases of grievous bodily harm.
Detective Superintendent Simon Barraclough said: "We are making good progress. Ultimately our aim is to ensure justice for all victims"
A male nurse arrested in connection with poisoning deaths is on bail pending further inquiries.