Seamus McCollum, 56, died at Maine private nursing home in Randalstown in September of last year.
However, the death was not treated as murder until April of this year, following the results of an autopsy.
Mr McCollum had cerebral palsy and had lived at the care home for nine years.
Police said the woman was being questioned at Antrim PSNI station.
A 61-year-old woman arrested by police investigating the murder of a man at a County Antrim nursing home has been released unconditionally.