Professor Philip Nolan has outlined why tests for Covid-19 can sometimes fail to detect cases of the virus.
He also said that a close contact of an infected person has "about a one in 10 chance" of having contracted the disease.
The chair of the National Public Health Emergency Team Irish Epidemiological Modelling Advisory Group said on Twitter that there are two reasons why tests may not detect Covid-19.
"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’," he warned.
Prof Nolan added: "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," said Prof Nolan.
The test can fail to detect cases for two reasons. First, at any given time about 30% of infected people are in the latent period, the 3-5 days after they have been infected, where the virus is multiplying inside their cells and is not yet detectable. 6/10— Professor Philip Nolan (@President_MU) September 26, 2020
He also explained that if the specificity (probability of a test being negative where there is no virus) is 99.7% then three in every 1,000 tests is a false positive.
His explanation follows a revelation yesterday by a Donegal GP that close contacts who receive a negative Covid-19 test are not getting the message that they need to restrict their movements for 14 days.
Speaking on RTÉ's Brendan O'Connor, Dr Martin Coyne, who is based in Lifford, said that people are interpreting a negative test as them being "fine", when "it doesn't mean that".
He said they have seen cases where one patient has tested negative the first time around, before being tested a second time and coming back positive, adding that people should get a test done, and then a second test seven days later.