Health chiefs have admitted that as many as one-in-five deep-throat swab tests for the Covid-19 virus may deliver "false negatives".
The figure could be even higher in circumstances where the tests are not taken from swab sites sufficiently deep in the nasal cavity.
The revelation came after doctors at a number of Irish hospitals became concerned when such swab tests either failed to confirm Covid-19 in patients displaying classic viral symptoms, or registered different results to blood tests or so-called lung-washing bronchial tests, both of which are known to be highly accurate.
"Broadly speaking, the sensitivity of testing in the upper respiratory tract seems to range from 70pc to 80pc," a Health Service Executive (HSE) spokesperson said.
In one test case, an Irish patient who was ill underwent two different deep-throat swab tests in hospital - both of which showed negative for Covid-19.
However, the individual also underwent bloodwork which then showed positive for antibodies to the virus.
Chest scans also showed an escalating lung problem.
The person later fell seriously ill with Covid-19 before successfully recovering.
In another Irish patient, an individual who had been treated for Covid-19 underwent swab tests to confirm they were now clear of the virus and capable of being moved out of a high dependency ward.
Successive swabs tested clear, but the patient subsequently had a temperature spike and the decision was made to test their lungs via a "washing" or mucus sample.
That test showed the patient still had a relatively high viral "load" in their lungs.
It had been assumed that the problems with such deep-throat swab tests had been due to how the test was conducted - with samples either collected incorrectly or not from deep enough in the throat.
"No single test is 100pc," the HSE spokesperson added. "Like many tests, the Covid-19 test has a false negative rate.
The majority of Covid-19 tests around the world use a technology called PCR, which detects pieces of the coronavirus in mucus samples.
"There are a lot of things that impact whether or not the test actually picks up the virus."
"All tests have limitations; this is the same with the test for coronavirus. The test will find the virus in most people who have symptoms."
Imperial College London expert Dr Graham Cooke said inaccurate results were nothing new in relation to swab tests and analysis errors, adding swab tests were an important indicator of infection rates.
Meanwhile, an additional 12 cases of the virus were reported by the Department of Health yesterday, bringing the total number of cases of Covid 19 so far to 25,881.
No further deaths were reported yesterday.