A nurse is being questioned on suspicion of three counts of murder by police investigating the poisoning of patients at Stepping Hill Hospital.
Victorino Chua, 46, is also being held on suspicion of 18 counts of causing grievous bodily harm with intent.
Police have until 10:10 GMT on Monday to question him over the tampering of records at the Stockport hospital.
Greater Manchester Police said it was important that “no-one jumps to any conclusions”.
Officers said they were questioning him in relation to the deliberate contamination of products at the hospital between 1 June and 15 July.
Mr Chua was arrested on Thursday as it emerged a fourth death at the hospital was being investigated.
He was initially held over reports medical forms were altered and a patient given extra medication.
Assistant Chief Constable Terry Sweeney, of Greater Manchester Police, said: “Firstly and most importantly I must stress that we have not established the degree to which deliberate contamination of products may have contributed to the death of four patients.
“We have taken the decision to arrest this man on suspicion of murder in close consultation with the CPS [Crown Prosecution Service] and it is important that no-one jumps to any conclusions because of these steps or second guesses where our investigations will take us.
“This specific inquiry, and the investigation as a whole, is a difficult and complex piece of work that requires very detailed forensic and medical analysis, which is ongoing.
“We know that the victims and their families want answers. We are acutely aware of this and are working very closely with them as well as hospital staff and the local community to minimise the upset caused and to keep them fully updated of any and all developments.”
There are now 21 people detectives believe have been affected by contaminated saline, including Tracey Arden, Arnold Lancaster and Derek Weaver, who died in the summer.
Bill Dickson, 82, had his saline poisoned over the summer, and later died on New Year’s Eve. He is currently being treated as one of the victims of GBH with intent.