A 46-year-old male nurse is being questioned by police probing the poisoning of patients at Stepping Hill Hospital.
His arrest came as it emerged that the death of an 82-year-old man on New Year's Eve is also now being investigated as his medicines were contaminated.
Glasgow-born Bill Dickson, who lived in Cheadle Hulme, Cheshire, is the fourth death being examined by police after the contamination of saline drips in June and July last year.
Police are now investigating whether the poisoning he suffered at Stepping Hill, in Stockport, Cheshire, during the summer was a factor in his death.
The nurse was arrested at his home in Stockport yesterday after it was found that medical records had been tampered with on Monday.
The arrested nurse worked on the same wards at the same time the patients were poisoned last summer.
The man arrested yesterday was held after another nurse noticed the medical records of a patient on Ward A3 had been altered overnight between January 2 and 3. The patient who had his records altered was given medicine he should not have received but was not harmed.
Police believe 20 patients to have been deliberately poisoned.
Of those poisoned, the four fatal cases, along with Mr Dickson, are Tracey Arden, Arnold Lancaster and Derek Weaver who died shortly after their hypoglycaemic episodes.
Previously, nurse Rebecca Leighton was arrested on suspicion of murder but released without charge six weeks later.
Assistant Chief Constable Terry Sweeney of Greater Manchester Police said: "we will not and cannot rule out making further arrests in the coming days or weeks."
This suspect is one of the 650 people already spoken to by police during their complex and long-running investigation at the hospital since the saline drip contamination were discovered.