Victorino Chua, 46, was initially arrested Thursday on suspicion of tampering with medical records at Stepping Hill Hospital in Stockport, northwest England.
But Greater Manchester Police said late Saturday he had now been arrested on suspicion of three murders and 18 instances of causing grievous bodily harm between June 1 and July 15 last year.
“We have taken the decision to arrest this man on suspicion of murder in close consultation with (prosecutors),” said Assistant Chief Constable Terry Sweeney.
“It is important that no one jumps to any conclusions because of these steps or second guesses where our investigations will take us.”
The nurse is being questioned over the contamination of saline drips, which caused patients’ blood sugar to fall to unusually low levels.
Three patients, all on acute wards for the seriously ill, died shortly after such episodes.
Officers are also investigating the death of an 82-year-old man on New Year’s Eve, several months after he was allegedly poisoned at the hospital.
Police believe 21 patients were deliberately poisoned. They have questioned some 650 people in their investigation into the hospital deaths.