COVID Testing Explained

Since the start of lockdown in the UK back in March, the government has been “tweaking” its covid-19 testing strategy. Targeted testing, only symptomatic people, mass testing etc. Until last week (6th Nov, 2020), the only widely available tests were PCR (I’ll explain more in a moment). However, Liverpool is now the trial site of mass testing using a different type of covid test which is quicker and is being used on everyone, symptomatic or not. Here we’ll look at the different types of covid tests and explain what they’re doing.

The importance of a reliable and accessible covid test has been apparent in the control of viral spread. Firstly, we need to recognise that there are two distinct categories of covid test: virus tests and antibody tests (the video below gives a quick overview of these tests). If you’ve been lucky enough to have the pleasure of a stranger sticking a swab down your throat and up your nose, then you’ve had the virus test. An antibody test however, is a blood test which detects if you had an immune response to the virus (see Understanding Our Immune System).

Video courtesy of Imperial College Healthcare, NHS Trust.

The virus test is used to determine if, when you took the test, you had coronavirus in your throat and/or nose. These have also been referred to as antigen tests, which simply means something that triggers an immune response (which in this case is the virus). However, Imperial College London state antigen tests are technically slightly different to virus tests currently used. The most widely available virus test in the UK, to date, is the PCR (polymerase chain reaction) test which detects viral RNA (where DNA is a double strand, RNA is a single strand). As the virus may be present before symptoms arise, such tests can be very beneficial, warning people to isolate before spreading the disease even if they feel well, breaking the transmission chain. For PCR tests, when the swab reaches the lab it is added to a solution of primers (i.e. specially engineered genetic material which will bind to viral genetic material). If the correct viral RNA is present (i.e. covid RNA) a chain reaction starts and billions of copies of the genetic material (the RNA) are made, so it can be detected when analysed by scientists. Usually, it takes between 12 and 24 hours for the test to run, although those conducted in a hospital setting are faster. The UK government is currently only widely offering PCR tests to symptomatic individuals.

However, from last Friday (6th Nov, 2020), Liverpool became the first city to trial city-wide, mass testing covering everyone, whether they had symptoms or not. The pilot has adopted a different virus test though, the lateral flow test (LFT). The most common LFT you will have heard of is a pregnancy test, where you wee on the sample strip. Pregnant women will have the hormone hCG in their urine. If present, hCG will bind to antibodies within the test which have an enzyme attached to them (see diagram). There are further, immobilised antibodies in the test window (where you can see the result), a set on the test line and a set on the control line. If hCG is present, this will bind with the antibodies on the test line and change its colour. Excess antibodies (those without hCG) will bind to the antibodies on the control line, showing the test has worked correctly. Various biological samples can be used for this type of test, including urine, saliva, sweat, plasma, whole blood and more. For the covid LFTs, participants still have a throat and nasal swab as they would with the PCR test. This sample is then placed on the test device and results are given within 15 to 30 minutes. As with a pregnancy test, if the virus is detected and binds to antibodies in the test strip, the LFT will give a positive reading. A control line is also used to ensure correct liquid flow through the strip.

Image adapted from Compound Interest.

.What are the key differences? Well, the PCR tests are seen as the “gold standard” of testing as they can amplify the viral genetic material and thus detect a positive result from a weaker sample. The LFTs however have no amplification steps, meaning they are less accurate than PCR tests and less sensitive, leading to more false negatives. For example, if an individual was infected in the 24 hours prior to the test, they are unlikely to have much viral genetic material to detect; the virus needs time to replicate. Some have also criticised the government for hailing these tests as an answer to finding asymptomatic individuals. Again asymptomatic individuals, whilst able to pass on the virus, are unlikely to have high concentrations of the virus on their swabs and thus may have a negative LFT result (a false negative). The company providing the LFTs (Innova Tried and Tested) claims an average sensitivity between 88.8 % and 99.2%, but this doesn’t account for other factors (such as when the individual became infected etc.).

Despite the lower accuracy and sensitivity, leading to more false negatives, a mass testing regime such as that on trial in Liverpool, which is used alongside the more accurate PCR tests and contact tracing, could still be the “best way for the country to get out of this nightmare“, according to Simon Clarke (associate Prof. of cellular microbiology at the University of Reading). Others have stated that this trial is a validation study and “should not be the basis for national decision making” as these tests are assessing “infection not infectiousness“, John Deeks (Prof. of biostatistics at the University of Birmingham). The British Medical Journal (BMJ) has a great interactive covid-19 test calculator, where you can alter parameters to determine the chance of false negatives/positives HERE. The aim of the Liverpool study is to quickly diagnose as many people as possible so they can isolate and slow the spread of the virus. Hopefully, this second lockdown will provide enough time to collect evidence about such testing strategies, while we continue to wait for more vaccine results.

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