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31Mar

A short FAQ on COVID-19 Testing

COVID-19 Testing FAQ

 

In case anyone is getting confused about Covid tests (and who isn't?), BIVDA Board member and acting COO John Bagshaw has put together the below FAQ on testing for COVID-19.

 

Antigen tests' as Michael Gove and now others have started calling them, are tests done on swabs which detect the presence of the virus. They are very sensitive, so if the test is negative you don't have the virus, or have caught it so recently that it has not had a chance to multiply yet. They use PCR technology and currently have to be done in laboratories. The testing capacity is being increased to try to cover the needs of front line clinical staff.

 

Antibody tests are performed on blood samples. The 3.5m tests the government claim to have 'bought' (although they can't/won't say who from or for how much, and seem to be still assessing them) are simple little cassettes which can be used anywhere. A lancet is needed to get a blood drop on the finger, which is then picked up and added into a hole at one end of the cassette. Some dilution fluid is then added to help it flow along a special paper inside. If there are antibodies present they are captured in a line and develop a colour, which is visible in a window in the cassette. There are three lines which can form. One is a control line, and must form for the test to be valid. The other two are two different types of antibody, called IgG and IgM. IgM antibodies are the first to be made in the body in response to an infection. Later IgG is formed and IgM declines. 

 

What do test results mean?

An 'antigen test', or PCR positive result means you have a current infection. A negative means either it is too early to detect - and that can be up to 7 days after infection - or it has been and gone, or that you have not been infected (yet).

 

An antibody test negative means you have not been infected, or you have but it is too early to detect antibodies. This can be for longer than PCR. It is also possible you don't produce many antibodies and therefore give a false negative result. The claimed sensitivity of these tests is around 97%, which means three out of one hundred people with antibodies will test negative for them. With millions to be tested that is a lot of wrong results. Worse, a similar proportion of people who test positive may not have antibodies to this virus. If they believe the result means they are immune, they may expose themselves to infection and spread it to others.

 

Antibody tests that are positive may be positive for IgG or IgM or both. A line for IgM only suggests you are in the early stage of infection. Lines for both IgG and IgM develop later, then when recovering IgG only, and IgG should stay positive for months and possibly years. In theory IgG positive people are immune. We don't know much about whether the virus will mutate enough to overcome immunity in the future, and, again, a small proportion will be false positives, therefore still at risk.

 

Can I do these tests on myself?

This is the big puzzle after Sharon Peacock of Public Health England told the Select Committee that they would be available from Boots and Amazon. The tests are not licensed as 'over the counter' (OTC) tests, and it is illegal currently to sell them as such. In any case how many people will want to stab themselves with a lancet, add buffer, wait 10 - 15 minutes then read and interpret the lines? We wait with interest to see what form the 3.5m tests will be in (they are usually in kits of 20 or 40 which would cost £200-£400 OTC) and how they will be made available, and to whom. Sorry, your guess is as good as mine at the moment!

 

Is there anything else coming that will help?

Maybe. In the 'flu crisis I helped supply 'antigen tests' which were like the antibody tests described above (so not PCR), but actually detect virus from nasal and throat swabs. They suffer the same lower sensitivity as the antibody tests, but they were used extensively in places such as on board cruise ships for rapid, on the spot indication of those infected who could then be isolated. Norovirus tests are also of this type. There are such Covid-19 antigen tests in development, and they may find a role similar to that in the 'flu epidemic. In addition there will be smart machines that can be placed in non-laboratory settings such as hospital waiting areas for triage, and will be able to run a PCR in an hour which will not only detect Covid-19 but also tell you if another infection is causing symptoms, e.g. 'flu. RSV (in children) or a rhinovirus or other Coronavirus strain which is harmless and gives you no more than a cold. These tests are more expensive, but the savings in isolation facilities and unnecessary time and treatment can be huge. 

 

The In Vitro Diagnostic Industry (companies that develop and supply clinical tests) is small compared with Pharma, but plays a tremendously important role in helping medical practice to diagnose, monitor and prevent diseases of all kinds. Represented by the trade association BIVDA in the UK, we owe a debt of gratitude to them for strenuous efforts to supply these vital tools to our doctors, nurses and scientists throughout the country.

 

John Bagshaw

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