Victorino Chua, 46, was arrested on Thursday by police investigating the poisoning of patients at the hospital in Stockport, Cheshire. Detectives have obtained a warrant of further extension that is due to expire today, Greater Manchester Police said.
Chua was arrested at his Stockport home 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.
It emerged also that police are investigating a fourth death, that of Glasgow-born William Dickson, a former journalist who died on Hogmanay. It is the latest to be examined by officers investigating the contamination of saline drips in June and July.
Police stressed that Chua continued to be questioned only in relation to reports from hospital staff on Tuesday that medical forms had been altered overnight and a patient was given additional medicine. The patient was monitored and subsequently discharged.