Health officials are trying to trace 140 patients who have not yet responded to a major recall alert linked to concerns about the work of a consultant neurologist.
Earlier this month, Belfast Trust ordered the recall of 2,500 patients following a probe into Dr Michael Watt amid concerns about potential misdiagnosis.
The move followed a year-long examination by the Royal College of Physicians of patient notes relating to the work of Dr Watt.
Stormont's Department of Health said 678 people were seen as part of the recall up to May 30, with a further 1,650 appointments scheduled.
They said approximately 140 of Dr Watt's patients had not responded to the recall.
Officials said efforts would be made to reach the patients, including contact by phone and through GPs if required.
In addition, they said a small number of recalled patients had declined appointments or not shown up to booked appointments.
The department statement said: "The full picture on the extent of changed or updated diagnoses and treatments will only be ascertained at the conclusion of both the recall process and any follow-up appointments."
The Belfast Trust said patients had until June 15 to secure an appointment as part of the recall process.
Patients treated by Dr Watt in two private-sector clinics in Northern Ireland are also being reviewed.
An independent inquiry has already been launched into the events around the recall.
The Department of Health-commissioned inquiry into the Belfast Trust's handling of the controversy will be chaired by Brett Lockhart QC.
A panel led by Mr Lockhart will review the process initiated by the trust in December 2016 in relation to the concerns raised about the clinical practice of Dr Watt.
It will assess whether any complaints or concerns relating to Dr Watt prior to that date should have provided grounds for "earlier intervention".
Stormont politicians were briefed on the recall on Thursday.
The patients recalled by the trust were being treated for a wide range of neurological conditions, including epilepsy and MS, and are aged 14 and up.
Restrictions were placed on aspects of Dr Watt's practice after the issues were flagged to the trust. He has not been seeing patients since June 2017.
The department has also asked the Regulation Quality and Improvement Authority (RQIA) to examine the records of all patients seen by Dr Watt over the last decade.
The RQIA is also undertaking a review of governance of outpatient services in the Belfast Trust with a particular focus on neurology services.
The review will be rolled out across all five health and social care trusts in Northern Ireland.