Testing for COVID-19: what does it tell us?

Testing for COVID-19 can give us some much-needed information to help understand and treat some symptoms of this infection on an individual, local, national and global level.

Testing for COVID-19: what does it tell us?

COVID-19 is an infectious disease caused by a new coronavirus known as SARS-CoV-2. The first known cases of COVID-19 were reported in December 2019. Although it may feel like the virus has been in the news, non-stop, for a long time, COVID-19 is a very new disease. We are still learning about the way the virus spreads through the community, and how the body responds to the coronavirus during and after an infection

 

What can testing tell us?

Testing can give us some much-needed information to help understand and treat some symptoms of this infection on an individual, local, national and global level.

Medical tests are rarely 100% reliable. Even among established medical tests that have existed for decades, there is always a small chance that someone may test positive for a condition they don’t have (false positive) or have an illness that remains undetected by the testing process (false negative).

This is why medical tests should be done in consultation with a health professional. A health professional, such as your GP, can decide which test is best suited to help answer your concerns and any health-related queries, and can guide you in making future decisions about your health, based on your test results alongside your medical history.

 

Types of test

There are two types of tests which help identify people who have been infected with COVID-19.*

Currently the most reliable way to test someone for an active (acute) COVID-19 infection is by conducting a reverse transcriptase polymerase chain reaction (PCR) test. A COVID-19 PCR test can detect the SARS-CoV-2 virus if it’s present in a swab sample (usually taken from the back of your nose and throat).

There have been cases of people with COVID-19 whose PCR-test results were initially negative (also known as a false negative result). This can happen for a number of reasons including if the swab sample is unsuitable, or if the sample is mishandled before it’s examined for traces of the virus. This highlights the importance of following your health professional’s advice, which may still include a period of isolation even if your test result is negative.

The second test is known as a serology test. COVID-19 serology tests look for antibodies to SARS-CoV-2 in your blood and may be able to tell if you’ve had a COVID-19 infection in the past.

Antibodies and immunity

Antibodies are generally produced in response to an infection or vaccination. They are important, because the presence of antibodies to disease-causing bacteria or viruses is usually an indicator of immunity to the diseases those microbes cause. If someone is immune to a disease, because they have had it in the past or been vaccinated against it, that means the ‘memory cells’ of their immune system have the capability to quickly produce the necessary antibodies to ward off the germs which cause that disease, without making you seriously ill.

It’s important to note that having antibodies to a virus or bacteria that causes a disease does not always mean that a person will be immune to that illness in the long term. Some diseases can only be caught once, but for others, immunity (including the number of antibodies in the body for that disease) wears off over time.

At the moment, our understanding of COVID-19 is too limited to draw any conclusions about immunity from the results of a serology test. We don’t yet know whether having COVID-19 means you are unable to catch it again, or if it does, for how long that immunity might last.

* Update: 11 July 

Since this article was first published, areas in Victoria have started to trial a saliva-based test. To find out more about the test and how it works read this explainer on the Doherty Institute’s website.

 

What kind of tests for COVID-19 are available in Australia?

In Australia, a PCR test is generally used to detect whether you are currently infected with COVID-19. It can spot the presence of genetic material from the SARS-CoV-2 coronavirus from a swab, usually done at the back of your nose and throat. A positive test result indicates that you currently have COVID-19. In Australia, when you hear about people “getting tested for COVID-19” this usually means a PCR test, unless otherwise specified.

The Therapeutic Goods Administration (TGA) has approved a number of serology tests. These tests involve taking a small blood sample, to look for antibodies to SARS-CoV-2. In the media, these serology tests for COVID-19 have also been described as:

  • finger prick tests – because of how the blood sample is taken,
  • antibody tests – because they are looking for SARS-CoV-2 antibodies, and
  • point-of-care tests – if they are performed by a heath professional who is also providing care (eg, a GP clinic).

Usually blood for serology testing is collected at a pathology company’s premises.

 

Frequently asked questions about PCR tests

What happens during a PCR test?

In general, a health professional will take a swab sample from the back of your nose and throat.

You may also be asked to provide a sample of sputum (mucus from your airways), by coughing and spitting into a sample container. Your sample is sent to a laboratory and you will usually receive your results in 5 days.

A health professional may require you to collect a specimen for a COVID-19 test by yourself. This is not a home testing kit because the testing itself will be conducted in an accredited pathology laboratory.

If you need to collect a specimen by yourself, read the Department of Health’s information sheet on how to self-collect a COVID-19 swab

How do I get tested for COVID-19?

The criteria for testing in Australia will depend on where you live. At a minimum you need fit one or more of these:

  • experiencing COVID-19 like symptoms such as fever and cough
  • have been in contact with a confirmed COVID-19 case
  • have returned recently from overseas.

Speak to your doctor or call the National Coronavirus Hotline on 1800 020 080 about whether you meet the requirements for testing in your area.

If you are concerned about flu-like symptoms use the healthdirect Coronavirus (COVID-19) Symptom Checker to see the next steps you need to take.

Where can I be tested?

Speak with your health professional about the closest testing centre to you. Testing in Australia usually takes place in a designated pathology collection centre, a hospital, or a respiratory clinic. There are also mobile and drive-through clinics in some areas.

Each venue has its own rules about whether you need to make an appointment or have a GP referral with you. Call ahead before your visit to ensure you have the right documents and identification with you.

Find out more about respiratory clinics in your area

What do I do after the test?

After your test a health professional will give you clear instructions on what you need to do. In general, you will be told to stay home and wait for your results. While you wait, avoid close contact with people in your household and practice good hygiene.

How long will it take for me to get results?

This will vary and depends on where you are tested. In general, it takes between 1 and 5 days. You should get a phone call or SMS if you have a negative result. If your result is positive you will get a phone call with details about what you need to do next.

If your PCR test results are positive, you will be required to self-isolate but you may also need medicine or treatment depending on your symptoms. This will be clarified when you receive your results.

I've read about false negative and false positive PCR results, what does this mean?

As mentioned earlier, no medical test is 100% perfect. They can provide helpful information to support a diagnosis, or to help guide treatment, but they shouldn't be used on their own.

Very rarely a PCR test may not find traces of SARS-CoV-2 , even though the person has been infected with the virus. This is what we call a 'false negative' result. 

These results may occur because the person has only recently been infected, or is at the end of the infection, and the amount of SARS-CoV-2 virus in their body is low at the time of testing. There could also be issues with the testing process such as a swab collecting cells not yet infected by the virus. 

On the other hand, a person could be told they have SARS-CoV-2, when they don’t. This is called a 'false positive', and also happens very rarely. False positives may be due to contamination of the swab, or occur when a person has recovered from the infection but still has a small amount of the virus in their body, even though they are not infectious anymore. 

Receiving a false negative or a false positive test results could be distressing. A false negative result may increase the risk of spreading infection in the broader community, while a false positive result can cause increased anxiety and an unnecessary period of self-isolation.

Tests are an important part of healthcare, but the above explains why diagnosis should not rely on a test alone, and why health professionals will consider other information about the person, such as symptoms, contact with other infected people, and recent overseas travel.

 

Frequently asked questions about serology tests

Can I get a serology test?

You will have to speak to your GP to get more information about serology testing.

I’ve seen at-home testing kits advertised. Can I use them to see if I have COVID-19?

No. In Australia, the advertising and supply of self-tests for serious infectious diseases, such as COVID-19, is prohibited under therapeutic goods legislation.

The appeal of these tests is obvious – pricking your finger in the comfort of your own home sounds more pleasant than waiting in a queue to be swabbed.

However, misinterpreting the results of an at-home test could have dangerous consequences for your health and the health of those living in your household.

Approved serology tests currently available in Australia can only be used in a clinical environment so a health professional can explain the results to you.

Why aren’t serology tests used for diagnosing an active (acute) COVID-19 infection?

The results of COVID-19 serology tests have a limited role in diagnosing an active (acute) infection. This is because your body takes time to produce antibodies to a virus.

If your serology test result is negative, the SARS-CoV-2 virus may still be in your body, as it can take two weeks from the time you were infected with SARS-CoV-2 to develop antibodies. Not only could you have COVID-19, despite a negative serology test result, you could also infect others during this time.

On the other hand, if your serology test result is positive for antibodies it could mean either that you have COVID-19 or that you have recovered from it. Serology tests can’t tell you how long ago you had the disease.

Many serology tests have been manufactured during the COVID-19 pandemic. Some are more reliable than others. There are tests that may have several limitations, for example, some might also react to seasonal coronaviruses – such as those that cause a common cold – and produce a positive result, without detecting the specific antibodies to SARS-CoV-2.

Why are serology tests used at all, if their results are so inconclusive?

Testing during a pandemic is crucial. PCR tests are important on an individual level because they can identify the people who are infected and require treatment, while serology tests are useful for collecting data on a larger scale.

Serology tests are easier, faster and cheaper than PCR tests, which is why they are appealing – especially in countries with fewer testing resources than Australia.

Even though they are not always conclusive, they are still informative. At the very least, statistics from the results of a specific serology test can be used to help the manufacturer of the test improve their product. But the potential of serology tests is far greater than production quality.

Serology tests used alongside PCR tests can help us understand the rate and pattern of infection. People who have antibodies to COVID-19 without ever experiencing disease symptoms may help us estimate the number of people who have had the disease and learn more about why some people are at a greater risk of being infected with the SARS-CoV-2 virus than others.

Serology tests will play a crucial role in the future when we have conclusive evidence about immunity to COVID-19. This will include information about the types and number of antibodies that can determine immunity, how long immunity lasts, and under what circumstances.

Once we know these things, people with antibodies to SARS-Cov-2 may feel safer in public, especially where social distancing is not realistic, such as travelling in a workplace lift.

 

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