More than 1,000 people may have been wrongly diagnosed with heart problems because test results were misread.
The technician no longer works for the trust
An inquiry has begun at Fairfield Hospital, in Bury, Greater Manchester, after errors were found in heart ultrasound results.
A cardiac technician, who no longer works at the hospital, said patients were suffering from heart complaints when in fact readings were normal.
The records of patients seen by him will now be re-examined.
Associate director for medicine, Dr Mike Finnegan, said the technician was qualified and experienced at his job.
Letters have been sent to 1,053 patients seen by him between 23 May and 7 December telling them errors were found in echocardiogram examinations.
Dr Ruth Jameson, acting medical director of Pennine Acute Trust, said the technician was supplied by an agency and was not working for the trust anymore.
"We are looking through all those patients' notes and that review is now well under way," she said.
"Some of them have already been invited back for review clinics.
"For most patients an echocardiogram forms part of a broader clinical assessment so we would hope that any potential problems would have been picked up by other means too."
The review of patients' notes is expected to take several weeks to complete.
"It is possible that patients have got medication that they don't need but that's what we can't be sure of at the moment and that's the reason for reviewing the notes," Dr Jameson added.
Any patients with questions regarding the accuracy of results should call a hotline set up by the trust on 0161 656 1999.