![]() Most people who are infected with coronavirus remain contagious for 4–8 days, with most transmission occurring two days before to five days after developing symptoms. Infection versus infectiousnessĪlthough PCR tests are widely regarded as the most accurate way to diagnose COVID-19, one problem with them is that fragments of viral RNA can linger for weeks after infectious virus has been cleared – meaning you may test positive even though you no longer pose a risk to other people. To reduce this risk, some countries ask anyone who tests test positive on a LFT to do a confirmatory PCR test. This still means that a small number of people who receive a positive test result will be unnecessarily told to self-isolate (the precise number depends on the prevalence of the virus in the community at any given time). Can dogs be trained to sniff out COVID-19?.Can we stop wearing masks after being vaccinated?. ![]() ![]() Is it safe to mix and match COVID-19 vaccines?.Conversely, if an LFT (or any diagnostic test) is used in a context where the disease is very common, the likelihood that a positive result is true is higher and the likelihood that a negative result is true is lower. If an LFT (or any diagnostic test) is used in a context where the disease is very rare, the likelihood that a positive result is true is lower and the likelihood that a negative result is true is higher. Whether or not the test is being used in an area with many COVID-19 cases is also important. In people who did not have COVID‐19, LFTs correctly ruled in infection in 99.5% of people with COVID-like symptoms, and 98.9% of those without them. This is because the specificity of LFTs – their ability to accurately diagnose uninfected individuals – is higher, and therefore false positives are highly unlikely. However, if you get a positive test result, you can be more confident that you really are infected. So, if you have any symptoms of COVID-19, you should behave as if you are infected, which will mean self-isolating and ideally book a PCR test if possible. Most people who are infected with coronavirus remain contagious for 4–8 days, with most transmission occurring two days before to five days after developing symptoms.īecause of this relatively low sensitivity, a negative test result cannot guarantee that you aren’t infected – false negatives are reasonably common. This would mean that in for every 100 people infected with COVID-19 who had symptoms, only 72 of them would test positive on a LFT. A recent Cochrane Review, which combined the results of multiple studies assessing the accuracy of LFTs, found that the average sensitivity of such tests was 72% among people with COVID-19 symptoms, and 58% for people without symptoms. This is the ability of the test to accurately diagnose people who are infected with SARS-CoV-2. When people talk about the accuracy of different COVID-19 tests, what they often have in mind is their sensitivity. However, because they are less accurate than PCR tests, the results need to be more carefully interpreted. They’ve been suggested as a means of avoiding self-isolation or ensuring that it is safe to visit vulnerable people. Lateral flow tests (LFTs) can tell you if you’re infected with COVID-19 within minutes, rather than waiting 24-48 hours for the results of a PCR test.
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