The deaths of five babies are among 11 under investigation at the Northern Health Trust, the Assembly has been told.
In an urgent written statement to MLAs, Health Minister Edwin Poots said issues had been identified in radiology, obstetrics and gynaecology, and accident and emergency.
Mr Poots said 20 instances in the period 2008 to 2013 had been brought to his attention by Trust last week. Of the cases, 11 involved a death. Five of the deaths were of babies who were either still born or died shortly after birth.
The Trust is also completing a review of about 35,000 X rays taken at the Causeway Hospital between 2011 and 2012.
Nine patients have been recalled. That review is now being extended across the entire health trust, involving 48,000 X rays.
The probe will examine whether the Trust correctly followed up on issues identified in chest X-rays - in some cases a radiographer’s request for a scan to be carried out was not acted on and, as a consequence, serious conditions were not diagnosed in time.
Mr Poots has stressed that the exercise was precautionary and there were no concerns about the accuracy of the X-ray reporting.
In obstetrics and gynaecology, concerns were identified about how serious incidents were managed.
The minister said: “It is not clear that these were avoidable deaths but it is clear that the Trust’s response should have been better.”
Health officials have previously raised concern about under-reporting of so-called Serious Adverse Incidents (SAIs) within the Northern Trust.
Of the 20 instances highlighted by the minister, eight were not originally reported as SAIs. In many of those cases health care failures only emerged after family members complained or took legal action against the trust.
The performance of the trust has been under intense scrutiny in recent years and in 2012 Mr Poots ordered a so-called ‘turnaround team’ from England to start working alongside management to raise standards.
Mr Poots said the Northern Trust had acted to address the failings identified in the 20 cases.
“I want to assure the Assembly that the trust has now taken prompt and appropriate action such as initiating fuller investigations and making sure all affected patients and families are given all appropriate information and support,” he said.
“I have asked the trust to confirm as soon as possible that all such action has been completed to ensure that these individual cases have all been reported appropriately, properly investigated and that learning from those instances is effected within the trust and more widely within the HSC (Health and Social Care) as appropriate.
“There can never be room for complacency. Safety will always be the component of quality that needs to be guarded and continually improved and consistent and timely reporting is fundamental to that. The price of quality is eternal vigilance.”