One of Ireland's top experts has explained why tests sometimes fail to detect coronavirus.
Professor Philip Nolan has warned close contacts must continue to isolate even if they return a negative result.
He said that this was due to tests sometimes not returning a positive result, which can happen in the early stages of the virus as it begins spread throughout the cells in the body.
And as someone starts to recover from the bug, tests can still fail to identify a positive case up to 30% of the time.
Taking to Twitter this morning, the chair of NPHET's Epidemiological Modelling Advisory Group said: "First, at any given time, about 30% of infected people are in the latent period, the three to five days after they have been infected, where the virus is multiplying inside their cells and is not yet detectable.
"Second, even when they start to shed the virus, a sensitivity of 70% to 90% means the test will fail to detect the virus in somewhere between 10% and 30% of cases.
"A 'not detected' test does not prove you do not have the virus; it does not show you are 'Covid-free.'
"This is why close contacts of a confirmed case are tested twice, and why we must restrict our movements even if the first test is negative.
"A close contact has about a one in 10 chance of being infected.
"If you have symptoms strongly suggestive of Covid-19 and your tests shows 'not detected', or your doctor has other reasons to suspect you may really have SARS-CoV-2, they will repeat the test.
"It is vital that testing has proper clinical oversight and interpretation."