The HSE has launched an investigation into a major flaw of its IT system, which may lead to thousands of patients having to undergo their medical tests again.

According to a report in the Irish Times, the problem relates to images of patients' tests, including X-rays, ultrasounds and MRIs, which are held at the HSE's National Integrated Medical Imaging System (NIMIS). This system stores diagnostic images electronically.

It was found that when images were archived at the NIMIS, the ‘less than' symbol (‘<') was not visible when used in a report. This would mean that the patient's results could look worse than they actually were.

It is understood that at least 25,000 patients' tests could be affected.

One doctor has claimed that patients may have been treated unnecessarily as a result. However, the HSE's national clinical advisor for acute hospitals, Dr Colm Henry, has said that this problem is ‘unlikely' to have led to patients being treated inappropriately.

"The quality of radiology is not affected. In addition, clinical decisions on a patient are rarely based nowadays on a written report or single X-ray alone. For important decisions, doctors looking at reports will usually verify by other means, usually by looking at the scans themselves," he told the Irish Times.

The error was discovered by a radiologist on July 28. All hospitals who use the NIMIS system have been told about the issue and are being asked to examine affected records.

Reports sent electronically to GP practices are not affected.

The HSE has convened a serious incident management team to assess the potential risk to patients.