What tests are available for coronavirus infection?
There are two main types.
- The first, an antigen test, detects the presence or absence in the body of the new coronavirus, which causes the disease Covid-19.
- The second, an antibody test, looks for signs that someone has been infected in the past by searching for an immune response.
Since Chinese scientists in mid-January 2020 published the full genetic code of the virus – officially known as Sars-CoV-2 – laboratories anywhere in the world have been able to use antigen testing to identify its genes in samples from patients.
They use a procedure called polymerase chain reaction (PCR) that has been standard practice in labs for 30 years. PCR samples can come from several different sites in the patient. Simplest is the nasal swab taken from well inside the nose. The back of the throat is another option. For patients in hospital, a sample from the lower respiratory tract may give the best results. Antigen testing reveals whether someone has a current infection and could therefore pass Covid-19 on to others.
In contrast, antibody (or serological) tests work on blood samples to detect the immunity conferred by past infection. The test kits use proteins from the virus as “glue” to trap antibodies present in blood. A finger prick provides enough blood to detect two types of antibodies against Sars-CoV-2.
The first, IgM antibodies, are short lived and may indicate that the virus is still present. They usually disappear a couple of weeks after infection.
In their place emerge a second more durable type of antibody, known as IgG antibodies.
No one knows yet how long these will last and provide resistance to reinfection — anything from a few months to a few years seems possible.
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