An IT glitch led to nearly 100 people who were positive for the coronavirus being told they were negative, it has emerged.
The blunder happened in recent days after those affected were contacted by text about their test results following analyses at a German laboratory.
They were first told their test was negative, but later informed it was positive.
Dr Cillian de Gascun, head of the National Virus Laboratory, said the results came back from Germany as "invalids" and the computer, as it was configured, read this as a negative.
He said in the ordinary course of events the Irish lab would have looked for a follow-up sample from the patients, but because they had given a swab around two weeks previously, the likelihood is they had cleared the infection and a new swab would be negative.
Instead the UCD lab collaborated and re-analysed them.
The fact the swabs would have been taken around 14 days previously meant the patients would have likely recovered during self-isolation at that point, although some could have deteriorated, said chief medical officer Dr Tony Holohan.
The glitch is the latest problem to hit the testing system here, which will be central to the decision to relax some of the restrictive measures imposed until May 5.
Over the past week, 20,468 tests were carried out in Irish laboratories, of which 4,233 were positive. Around one in five had the virus.
The plan is to widen the net, and expand the criteria for testing outside the current priority groups, in order to capture more people who have the virus.
However, Dr Holohan said yesterday that he has "no confidence" yet that the testing regime, as of now, would be able to cope with this expansion.
It can still take four to five days for a test result to be returned to one of the priority groups in the community, although it is faster if the test is carried out on a hospital patient.
The testing and tracing system must be available in real time if there is to be any easing of restrictions here, otherwise a rise in cases would not be detected.
Leading infectious disease consultant Dr Sam McConkey believes that contact tracing of people who are first-degree contacts of a person who has tested positive should be done in less than six hours.
They should go into rapid, voluntary self-isolation with the use of GPS mobile phone locations, Bluetooth links and QR codes that record, with the person's consent, where they have been in the last 14 days.
The HSE has said the testing system will be ramped up over the next week with the use of more domestic laboratories, but there are also uncertainties about the future supply of reagent for laboratory analysis.
It is also unclear if the German laboratory will continue to be able to provide a back-up service which would allow thousands of tests to be processed in a day.
Dr De Gascun said: "Having come through a challenging few weeks, we have significantly strengthened testing capacity and will continue to do so over the coming week, to put us in a very strong position to identify and suppress the virus."
The Health Protection Surveillance Centre is also to develop a strategy to conduct a study which will identify the proportion of the population who have had Covid-19.
This will include people who have had the virus but were unaware of it because they had little or no symptoms.
A blood test may detect antibodies.