A male nurse arrested by detectives investigating the Stepping Hill Hospital poisonings was named by sources today as Victorino Chua.
Mr Chua, 46, was arrested at his Stockport home yesterday by officers from Greater Manchester Police.
He is being questioned on suspicion of unlawfully administering or causing a person to take a noxious substance, or poison, after it was found that medical records were tampered with on Monday and a patient given incorrect medicine.
The patient was monitored and subsequently discharged.
Today, a spokesman for Unison, the public sector trade union, confirmed Mr Chua is a member and said they might issue a statement later.
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.
Mr Chua worked on the same wards at the time the patients were poisoned last summer.
The suspect was held after another nurse noticed the medical records of a patient on Ward A3 were altered overnight between January 2 and 3.
The patient whose records were altered was given medicine he should not have received but was not harmed.
Assistant Chief Constable Terry Sweeney said: "This is, and always has been, a search for the truth.
"It is due to the diligence of staff at the hospital that we were made aware about the potential tampering of medical records and 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 matter.
"I must stress that at this time he is only being spoken to in relation to these matters and, as we have previously said, we will not and cannot rule out making further arrests in the coming days or weeks."
He stressed that it was a "complex" investigation and police were committing "significant" resources to it.
Meanwhile, the family of the latest patient to allegedly die as a result of poisoning 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, well respected by his colleagues.
"A loving husband, father and grandfather, he will be sorely missed by his family."
Mr Dickson's is the fourth death being examined by officers investigating the contamination of saline drips in June and July last year, causing patients' blood sugar to fall in "hypoglycaemic episodes".
The four fatal cases, along with Mr Dickson, are Tracey Arden, Arnold Lancaster and Derek Weaver who died shortly after their hypoglycaemic episodes.
All the deaths took place on wards A1, A2 or A3 - acute care wards caring for seriously ill patients.
Police said 20 patients are believed to have been deliberately poisoned.
The cases of 21 patients who suffered hypoglycaemic episodes are not being treated as suspicious.
Police have yet to establish a causal link between the deaths of the four patients and the contamination of their saline drips.
The Nursing and Midwifery Council said: "The NMC has been made aware of the arrest of a nurse at Stepping Hill Hospital in Stockport.
"We are liaising with the Greater Manchester Police force and Stockport NHS Foundation Trust to determine the need for fitness to practise proceedings to be commenced.
"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."
Mr 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.
Charges that Miss Leighton tampered with saline solution with intent to endanger life were dropped by the Crown Prosecution Service in September.
According to the Daily Telegraph, she has released a statement through her solicitor stressing that anyone arrested in connection with a crime "should be treated as innocent until proven guilty".
"While Rebecca wishes to have closure both for herself and also for the families involved in the tragedy at Stepping Hill, it is important that any police investigation or press coverage of such an investigation is fair and orderly so that justice can be done," the statement said.