A 46-year-old male nurse has been arrested by police investigating the poisoning of patients at Stepping Hill hospital in Stockport.
A fourth patient died at the hospital on New Year's Eve. The unnamed 82-year-old victim was believed to have had his saline drip poisoned.
The suspect, who has not been identified, was arrested at his home in Stockport and held on suspicion of unlawfully administering a noxious substance, or poison, after it was discovered that medical records had been tampered with.
Sources close to the investigation stressed that the latest arrest was not related to enquiries into the contamination of saline drips which led to the deaths of three patients at Stepping Hill in summer 2011. No link between the two investigations has been made.
It is believed that up to 20 people were deliberately poisoned at Stepping Hill hospital between 1 June and 15 July 2011.
"This is, and always has been, a search for the truth," said Assistant Chief Constable Terry Sweeney.
"It is due to the diligence of staff at the hospital that we were made aware about the potential tampering of medical records. Everyone should be reassured that, like the hospital, we initiated a swift response and have consequently arrested a member of staff in relation to this [latest] matter.
"I must stress that at this time he is only being spoken to in relation to these [latest] matters. We have previously said we will not and cannot rule out making further arrests in the coming days or weeks
"I know relatives of the confirmed victims and the public in general continue to seek answers and we are working as quickly as we can to provide them without jeopardising the quality or integrity of the investigation."