More than 165,000 patients across the country are on waiting lists for ultrasounds, CT scans and MRIs.
This is in addition to the more than 700,000 people who are on inpatient and outpatient waiting lists, which are published separately on a monthly basis.
New figures for diagnostic waiting lists are not published regularly, but were obtained by Fianna Fáil health spokesman Stephen Donnelly.
They show that almost 131,000 patients are waiting for scans under the HSE’s national integrated medical imaging system (NIMIS) of electronically stored images. A further 34,000 patients are waiting on a non-NIMIS list.
Over 25,000 patients waiting for diagnostic tests have been on the list for more than 12 months.
NIMIS is a long-running project to provide electronic radiology systems in 35 hospitals, thereby moving away from the films traditionally used in this area.
“These figures show yet more extensive waiting lists within the healthcare system. And they don’t even represent the totality of those waiting for diagnostic scans,” said Mr Donnelly.
The HSE in 2016 started a pilot project to collect national radiology waiting list data in collaboration with hospital groups and the National Treatment Purchase Fund. However, the system still does not capture a number of other scans for which Mr Donnelly sought information, including cholangiopancreatography, electroencephalograms, angiograms and bone scans.
The Dublin Midlands Hospital Group accounts for the largest single share of the waiting list, with almost 42,000 patients awaiting tests, followed by the Royal College of Surgeons in Ireland Hospital Group with almost 27,000 patients waiting.
An individual hospital breakdown is provided only for the non-NIMIS data; this shows the Mater Hospital in Dublin has 2,530 patients waiting for a CT scan, 4,378 waiting for an MRI, and 2,734 for an ultrasound.
At St Vincent’s Hospital 3,634 patients were waiting for a CT scan, 4,095 for an MRI, and 2,980 for an ultrasound.
In contrast, just 36 patients at the National Maternity Hospital were waiting for scans, and there were no waiting lists at Temple Street children's hospital.
Mr Donnelly said it was essential that urgent efforts were made to reduce waiting times, in particular with 25,000-plus patients waiting over a year.
The HSE, which provided the data to Mr Donnelly, said it was still being validated at hospital level.