A 46-year-old male nurse has been arrested by police investigating the poisoning of patients at Stepping Hill Hospital in Stockport.
The 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 during last summer.
His is the fourth death being investigated by police working on the inquiry.
The nurse, who was arrested at his home in Stockport, was held on suspicion of unlawfully administering or causing a person to take a noxious substance, or poison, after it was found that medical records had been tampered with on Monday.
Sources close to the investigation stressed that the arrest forms part of a separate investigation to the one looking at the contamination of saline drips in June and July last year, and as yet there is no link between the two.
Assistant Chief Constable Terry Sweeney, of Greater Manchester Police, 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.
"I cannot stress enough just how complex an investigation this is and it is one that we continue to commit significant resources to.
"I know relatives of the confirmed victims affected and the public in general continue to seek answers and we are working as quickly as we can to provide them without jeopardising the quality or integrity of the investigation.
"For obvious reasons, I cannot go into detail about the ongoing work we are doing but would like to take the opportunity to thank the hospital and staff for their continued and unreserved co-operation."
Police have yet to establish a causal link between the deaths of the four patients and the contamination of their saline drips.
The man arrested was held after another nurse noticed the medical records of a patient on Ward A3 had been altered overnight between January 2 and 3.
All four of the deaths took place on wards A1, A2 or A3 - acute care wards caring for seriously ill patients.
The patient who had his records altered was given medicine he should not have received but was not harmed. He was carefully monitored and subsequently discharged.
Stockport NHS Foundation Trust, which runs the hospital, said it was "appalled".
Chief executive Chris Burke said: "We are aware that Greater Manchester Police has arrested a 46-year-old man on suspicion of tampering with prescription charts.
"Because of highly experienced doctors and nurses immediately spotting the changes on prescription charts, no patient has been harmed.
"The police were called as soon as the prescription chart changes were discovered via our routine good practice.
"Thanks to the skill and experience of our staff it was spotted and reported immediately, and most importantly we ensured no patient suffered any ill-effects.
"This does not reflect on the good care provided by dedicated and committed staff and the effective working practices in our hospital. The safety of our patients remains our highest priority.
"As no charge has been made, it is important to note that police are continuing with their inquiries. We continue to support the police in this ongoing investigation."
As part of the wider operation, detectives are investigating the deliberate contamination of products at the hospital after patients' blood sugar dropped on two wards between June 1 and July 15 last year.
Police said 20 patients are believed to have been deliberately poisoned.
The cases of 21 patients who suffered "hypoglycaemic episodes" - blood sugar falling too low - are not being treated as suspicious.
Of those poisoned, four people have died. Tracey Arden, Arnold Lancaster and Derek Weaver died shortly after their hypoglycaemic episodes. A fourth patient, an 82-year-old man, died on New Year's Eve.
The suspect held today was one of the 650 people already spoken to by police during their complex and long-running investigation at the hospital since the saline drip contaminations were discovered.
While treating the latest arrest as separate to the original investigation, sources said detectives will "follow the evidence" wherever it leads.
Previously, 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.