A male nurse arrested over the Stepping Hill Hospital poisonings has been named by sources as Victorino Chua.
Mr Chua, 46, was arrested at his Stockport home yesterday by officers from Greater Manchester Police.
Chua is being questioned after it was found that medical records were tampered with on Monday and a patient given incorrect medicine.
Sources close to the investigation stressed the nurse's arrest formed part of a separate probe into four deaths of poisoned patients at the hospital, and as yet there was no link between the two.
Chua worked on the same wards at the time the patients were poisoned last summer.
Assistant Chief Constable Terry Sweeney stressed: "we will not and cannot rule out making further arrests in the coming days or weeks."
Meanwhile, the family of the latest patient to be poisoned and die at Stepping Hill Hospital have paid tribute to him.
Glasgow-born William Dickson, 82, of Cheadle Hulme, Cheshire, died on New Year's Eve and detectives are investigating whether the poisoning he suffered at the hospital in Stockport, Cheshire, during the summer was a factor in his death.
Today, the pensioner's family issued a statement saying: "Bill was a very kind and generous man with a great sense of humour.
"He was a hardworking and dedicated journalist. He will be sorely missed by his family."
Dickson is the fourth death being examined by police after the contamination of saline drips caused patients' blood sugar to fall in "hypoglycaemic episodes".
The four fatal cases, along with Dickson, are Tracey Arden, Arnold Lancaster and Derek Weaver. Police said 20 patients are believed to have been deliberately poisoned.
The Nursing and Midwifery Council said: "The NMC is the only organisation with the authority to protect the public by suspending a nurse's registration while they are under investigation. If this is necessary and in the public interest we will take action as soon as possible."
Chua is one of 650 people already spoken to by police during their complex and long-running investigation at the hospital since the saline drip contaminations were discovered.
Nurse Rebecca Leighton was arrested on suspicion of murder but released without charge six weeks later.