print   Print full article COVID-19 testing – what’s happening now and new developments

Around the world the main type of test being used to detect the coronavirus that causes COVID-19 is called RT-PCR – the reverse-transcriptase polymerase chain reaction.  This type of testing for the genetic material of viruses and other microbes has come into common use only in recent years.

Other types of tests are in development and some starting to be used, most notably serology tests. Serology tests detect the antibodies your immune system makes to an infection. Some of these antibodies stay in your blood for months and years. This type of testing is quick to perform, sometimes using just a finger prick blood sample and a small test kit that can be used anywhere. When this type of testing is automated it allows us to test thousands of people. It will be most useful in allowing us to see the true scale of the COVID-19 epidemic.  We will be able to see how many people have had the virus to help us understand how much immunity we have as a community.

RT-PCR testing and how it works
The coronavirus is made up of a genetic material called RNA which is surrounded by proteins and lipids (fats). When the virus enters your cells it uses RNA to replicate itself.

PCR testing detects DNA.  This means that the first step in the testing process is to convert the viral RNA into DNA using the enzyme reverse transcriptase. Once this is done chemicals are used to amplify the DNA so it can be read by the PCR instrument. Hence the name RT-PCR.

The test measures the amount of the virus’s genetic material in the sample.  Usually, the sample will be a nasal and throat swab but can also be sputum or fluids collected from deeper in the lungs.
 
The RT-PCR test can only tell whether the person has live or very recently dead virus in their body at the time of sample collection.

When the person recovers from the infection the results of a RT-PCR test quickly become negative.
 

Serology – a different type of test

Serology tests are different from the RT-PCR test. Instead of measuring genetic material from the actual virus, they measure your body’s response to being infected by the virus.  Serology tests have been used for many years to detect infectious diseases. Serology tests for the coronavirus that causes COVID-19 are being rushed into development with tests already in use in some countries around the world. Several have just been approved for use in Australia and may be available for testing patients in the near future.

How serology testing works

A key step in the human immune system’s response to infection is the production of antibodies to the infection. These antibodies are proteins produced by blood cells called lymphocytes. Antibodies are produced specifically against the virus. They bind very tightly to the virus to help deactivate it so that the body can get rid of it. The body stimulates the lymphocytes to multiply until there are enough of them to produce large quantities of specific antibodies against the virus.

The first type of antibody produced is called IgM and this may appear within days of the initial infection. IgM antibodies typically fade away a few months after the infection but sometimes they persist for longer. Other types of antibodies produced are IgG, and in respiratory infections like coronavirus, IgA.

IgG antibodies usually stay around in the bloodstream much longer after you have recovered – for years or even life-long.
 

What the results can show

The antibody tests recently approved for use in Australia can measure both IgM and IgG anti-coronavirus antibodies. 

If your test results show IgM anti-coronavirus antibodies, it suggests that you have a current or recent infection. If both IgM and IgG are present this means much the same thing. However, if only IgG is present this suggests that you have had an infection at some time in the past and may be immune to further infection by this coronavirus. However, it is not yet certain that people who have been infected will develop immunity to reinfection. This should become clearer in coming months.

The ways that serology testing can be used

Using an antibody test to detect an active infection

Currently, diagnosis of active infection with the virus causing COVID-19 is done with the RT-PCR test. IgM serology tests have the potential to be used to make the diagnosis of a current infection. However, data provided by the companies supplying the new antibody tests show they vary in how soon patients show positive results. This may range between 5 and 10 days after symptoms appear before IgM antibodies can be detected. This means that the currently available serology tests are not really useful for diagnosis in a person with very recent onset of symptoms and the RT-PCR test is required in this case.

Using an antibody test to detect past infection and measure community immunity

IgG serology tests are extremely useful for assessing whether someone has previously been infected and developed an immune response to COVID-19. If this immune response protects someone from being reinfected then this can be used in several ways. If health authorities discover that a large proportion of the population has already developed immunity this can help them predict the likely course of the epidemic. If most schoolchildren are discovered to be IgG positive and hence immune, this could possibly inform decisions on closing or reopening schools.

If a healthcare worker knows that they are now immune to COVID-19 they might be able to be deployed to work in frontline caring positions without risk of becoming infected. It may even be possible to harvest antibodies from people with very high levels and use them as a therapeutic drug to treat critically ill patients.

Quick and easy to perform

These immunoassay antibody tests have some other advantages. They are usually simpler to perform than RT-PCR tests. Some antibody tests in development can be done on a fingerprick blood sample. Currently, tests of this type cannot be sold to consumers at home but could be used in workplaces under supervision. Antibody tests are also likely to be cheaper than RT-PCR tests and large numbers of tests may be automated and thus have a faster turnaround time. The test may require simpler instruments to carry them out so that the testing equipment can be available in more laboratories than those with the required facilities for PCR testing. One concern is the potential for false-positive test results. Many people will have been exposed to other less dangerous coronaviruses . Preliminary data from manufacturers and some labs indicate that this may not be a major problem but we need more data to be certain for each commercially available test.

Different types of tests for COVID-19 are in the pipeline

Many organisations, universities and companies are currently working on developing new tests for the virus causing COVID-19. These will probably include tests that use different methodology from both RT-PCR and serology. Some companies are working on tests that detect viral components such as proteins, nucleic acids or combinations of these in patient samples directly without an amplification step or with newer types of amplification technology that require simpler instruments. This is a rapidly changing area and as data becomes available on these tests LTOAU will try to keep you up to date as more tests become available.

A list of approved COVID-19 tests is available on the TGA website.





 

Last Review Date: September 2, 2020