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.

Twenty males were arrested on suspicion of violent disorder or affray after the fight in Price Street, Birmingham, at 16:15 GMT on Saturday.
Police said eight Villa supporters and seven Leicester fans were held after trying to run away from officers.
Five more were arrested after being found hiding in a nearby car park.
West Midlands Police said the fans were aged between 16 and 21 and had been bailed until dates in February and March.
Insp Richard Vickers said: "These few individuals who choose to display their 'support' through anti-social or criminal activity are dealt with strongly and we will always push for Football Banning Orders against such offenders."
The match ended in a 1-1 draw.
A 61-year-old woman arrested by police investigating the murder of a man at a County Antrim nursing home has been released unconditionally.
[NEXT_CONCEPT]
Football fans hurled bricks and bottles at each other in a "violent clash" before Aston Villa played Leicester City.