How do we test for Covid-19

from flickr user Prachatai under a Creative Commons CC BY-NC-ND 2.0 Licence

In the current Covid-19 outbreak, there are a number of outstanding questions that are key to managing the outbreak and minimising the loss of life caused by Covid-19 infections. How many people are currently infected with Covid-19? How many people have been infected overall? How quickly is it spreading?

Only by getting more widespread testing of the currently infected and previously infected can we start to solve these problems. There are two main types of test currently available for Covi-19 and they work in very different ways.

The throat swab test

The most widely used Covid-19 test to date is those tests you’ve seen that are carried out on throat swabs, like in the drive-through testing stations in south korea. To explain how these work, they work by detecting the genetic material of the Covid-19 virus. Covid-19, like the ‘flu virus is a retrovirus. This means its genetic material isn’t DNA but its close relative RNA. In our cells, the genes in our DNA are “transcribed” into an RNA message which is then “translated” into a functional protein. When our cells are infected with a retrovirus, the virus hijacks our cells, turning the cells’ machinery into a virus producing factory by turning the RNA back into DNA. These cells then act on the instructions in this DNA, produce lots of new virus particles, then explode, and it is this process that causes us to be ill. The kind of test carried out on those throat swabs are called reverse transcription polymerase chain reaction (RT-PCR). The RNA from the virus is converted into DNA (the “reverse transcription” bit). This is then copied lots of times and the copies are detected using a fluorescent probe. In healthy cells, there will be no viral DNA so there is no signal, but in infected cells, the viral RNA is present, so a signal is detected.

The blood test

The other test that has been in the news is the antibody blood test. When we infected with a virus, our body fights the virus by our white blood cells producing antibodies which bind and inactivate the virus. The blood tests work by mixing a small amount of blood with a sample containing proteins from the virus. If antibodies against Covid-19 are present, they recognise and bind to the virus proteins and this triggers a colour change. When a person is initially infected their white blood cells will not have had time to produce antibodies against the virus so they would show up as negative. On the other hand, they will detect antibodies in a person long after they have had the infection. This allows us to identify people who have previously been infected with Covid-19 even if they experienced mild symptoms or no symptoms at all.

The two types of test have different applications and are both critical to monitoring the disease. The antibody test will be critical after the initial Covid-19 outbreak to determine how many people have been infected and therefore have immunity. This will be important in planning lockdown measures in the near future.

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