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Professor Philip Nolan has explained 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 a “one in 10 chance” of having contracted the disease.
The chairman of the Irish Epidemiological Modeling Advisory Group of the National Public Health Emergency Team said on Twitter that there are two reasons why tests may not detect Covid-19.
“First, at any given time, around 30% of infected people are in the latency period, three to five days after being infected, where the virus is multiplying within their cells and is not yet detectable. .
“Second, even when they start shedding the virus, a sensitivity of 70% to 90% means that the test will miss the virus in 10% to 30% of cases.
“An ‘undetected’ test does not prove that you do not have the virus; it does not show that you are ‘Covid free’,” he warned.
Pro 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 a one in ten chance of being infected.
“If you have symptoms highly suggestive of Covid-19 and your tests show ‘not detected’, or your doctor has other reasons to suspect that you really may have SARS-CoV-2, they will repeat the test.
“It is vital that the tests have proper clinical supervision and interpretation,” said Professor Nolan.
The test may not detect cases for two reasons. First, at any given time, about 30% of infected people are in the latency period, the 3-5 days after infection, where the virus is multiplying within 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 that a test is negative when there is no virus) is 99.7%, then three out of every 1,000 tests are a false positive.
His explanation follows a revelation made yesterday by a Donegal doctor that close contacts who receive a negative Covid-19 test do not receive the message that they must restrict their movements for 14 days.
Speaking about RTÉ’s Brendan O’Connor, Lifford-based Dr. Martin Coyne said that people are interpreting a negative test as “okay” when it “doesn’t mean that.”
He said they have seen cases where a patient tested negative the first time, before being tested a second time and testing positive, adding that people should get tested and then a second test seven days later.
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