Explainer: What are the three COVID-19 tests?

Sam Wood
Students in Kent labs

In response to the regular news updates of the tests required for the COVID-19 pandemic, the School of Biosciences' Professor Mark Smales, Professor of Industrial Biotechnology, explains what these test are for and what is involved. He said:

‘The different tests work on the basis of either detecting the presence of the virus and confirming an individual is currently infected, or detecting a “signature” left by our immune system after recovery – showing that an individual once had the virus but has now recovered and may have some immunity to it. Therefore, it is important that tests are accurate.

The Polymerase Chain Reaction (PCR) test

‘The Polymerase Chain Reaction (PCR) test identifies those currently infected by the virus by detecting the presence of the genetic material of the virus, its Ribonucleic acid (RNA). During the PCR test, the RNA is actually “copied” into DNA, allowing the amplification or the making of lots of copies of DNA from a very small number of RNA molecules. Therefore, the test can detect small amounts of the viral RNA through this amplification process.

‘The presence of an RNA sequence specific to the SARS-CoV-2 virus (which causes COVID-19) only is identified by the PCR test to confirm the presence of the virus. This test needs to be undertaken in a laboratory by a skilled operator and relies on a swab sample to actually collect the virus. A problem here is that there is less virus present early in the infection and it may be possible that a swab does not collect any virus. To solve this, the sample is usually collected by placing a long Q-tip type swab through to the back of the nasal cavity or a swab taken from the back of the throat. This procedure can be rather uncomfortable and result in coughing so it is vital the collector is protected from the individual being sampled.

‘Once the sample has been collected it can take from hours to several days for results to be returned, depending on how long it takes to get the sample to the lab and processed, and how many samples are queued. Once a person has fully recovered from the infection, virus RNA should no longer be present and a person will test negative.’

The Antibody test

‘The antibody test looks to detect antibodies the immune system has raised against the virus and that continue to circulate in the blood, potentially offering immunity if these persist. These tests focus on detecting antibodies the immune system has raised against particular proteins the virus has.

‘Antibodies are not detected until sometime after infection and therefore when someone is tested for the presence of antibodies is important in the accuracy of a test. Our immune system makes a type of antibody known as IgM early in response to infection that can be found circulating in the blood, and then a little later IgG antibodies start to emerge. Reports suggest that accurate identification of the presence of IgM and IgG antibodies is evidence that a patient has been exposed to the virus and potentially holds longer-term protection against reinfection. However, current data suggests this is best begun several weeks after infection in order to ensure the highest reliability.

‘These tests may look to detect both IgM and IgG antibodies in the serum of recovered patients. These tests usually work by taking a small blood prick sample and can be done in minutes and even at home – depending on the format. Whether the presence of such antibodies offers an individual longer term immunity will only become known as we track recovered individuals in the months and years to come’.

The Antigen test

‘Finally, there is a 3rd test that has been badly confused for many; antigen tests for the presence of the virus. The PCR test is often incorrectly referred to as an antigen test but this detects RNA as described above and is not an antigen test. Antigen tests check for the presence of a protein component on the virus, most often by having an antibody created specifically for the target virus protein component. These, like the PCR test, detect the presence of the virus but do not give information on whether an individual has raised an immune response like the antibody test. These can be much simpler to run than the PCR test and less invasive for sampling, but also generally much less sensitive.

‘It would no doubt be simpler to describe tests as in one of two buckets, those that detect the presence of the virus and confirm infection and those that determine whether you have previously been infected (the antibody tests). All these tests are important tools in managing the pandemic for individuals and the wider population. Such tests help guide the management and treatment of infected and recovered individuals and provide vital clues about immunity and the continued spread and transmission of the virus in the community.’

Professor Mark Smales, School of Biosciences, University of Kent

Professor Mark Smales is Professor of Industrial Biotechnology at the University of Kent. The group headed by Mark has a number of on-going projects whose objectives are to advance understanding of biotechnological products and processes at the fundamental biological or chemical level to enable their manipulation and control for improved (a) biotherapeutic recombinant protein yields and quality, (b) manufacture of gene therapies, (c) re-tuning of cell metabolism via synthetic biology approaches, (d) development of diagnostics. He has recently been nominated to sit on the World Health Organisation (WHO) expert group for assays development for COVID-19 vaccines.

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