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.