Why are swab tests much more accurate than blood tests?
In: Biology
Cold viruses like coronaviruses does not target blood cells. So you can have a full blown infection in your airways and yet not any blood infection. This is why the most accurate test of coronavirus is swabbing the throat and nose where the actual virus is and not the blood where the virus is not. The blood tests that have been developed work differently in that they do not look for the virus directly but rather looks for antibodies that your immune system have made in response to the virus infection. These will be mainly present in the blood. However the antibodies only show up after you have had the virus for some time and your immune system have targeted it. On the other hand the blood test will show a positive result even long after the immune system have beaten the infection. We do not yet know for exactly how long as we have only been testing people for three months. So a negative blood test for coronavirus may mean that you have only gotten the infection recently and a positive blood test may mean you have had coronavirus but no longer. Both tests are of course usefull but in different ways.
I assume you mean to test for Coronavirus infection, because in general, blood tests tend to be reasonably accurate.
But blood tests can only test for what’s in your blood. Most viruses don’t infect your blood, and Coronavirus isn’t one of them. So we take samples from where it infects — the nose and back of the throat. Then we can test for the virus directly. These types of tests are generally quite easy to make and laboratories are already set up to do hundreds or thousands of them, they just need to know the Coronavirus specific ‘code’, and they can test for it.
Blood tests for coronavirus test for antibodies, which are molecules that we make to help clear the virus. Most tests for antibodies are very accurate, because antibodies are highly targeted seeker molecules, they can find *exactly* what they’re looking for. However, it takes a long time to develop a really good antibody test, so that’s why currently swabs are probably better. Eventually, it’s likely that a high quality blood test will be developed, but it won’t necessarily be able to detect an infection that’s just started, so in that case there’ll always be a role for swabs.
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