How COVID-19 testing works

Find out about the three types of COVID-19 test, the science behind how they work, and the best time to test.

Page last updated: 13 December 2021

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See also: COVID-19 testing 

Information on this page is included in the factsheet COVID-19 testing, testing uncertainty and what we know about infection and immunity, available from What we know about infection and immunity.


Types of test for COVID-19

A key part of limiting transmission of COVID-19 is a testing strategy that is accurate and situationally appropriate. Prompt testing and identification of infected individuals is vital for preventing outbreaks and continued, onward transmission.  

There are three main reasons to get tested for SARS-CoV-2, the virus that causes COVID-19.

  1. Diagnosis: You have COVID-19 symptoms or are at risk of developing COVID-19 (because you are a contact of a case or have been at a location of interest) – the test will tell you if you have COVID-19. 
  2. Surveillance: You feel fine but are at risk of catching the virus at work or bringing it into your workplace – the test is a check-up. Regular surveillance testing is compulsory for border workers and others working in higher-risk situations. Surveillance testing is also undertaken by workers whose work is essential to a business operation and may place them in a high-risk situation for contracting or transmitting the virus. 
  3. Screening: You feel fine but may have the virus. This is a one-off test.

There are numerous methods and sample types for testing for SARS-CoV-2 and their applications can vary depending on the situation. 

Methods 

Some tests are more accurate than others, which is the key measure for determining which situation they are most useful. An ideal test is one that always correctly identifies both someone with the disease and someone who doesn’t have it.  

The most accurate test for detecting SARS-CoV-2 uses a method called Reverse Transcription Polymerase Chain Reaction (RT-PCR), which detects genetic material from the virus in the sample. Another type of test, rapid antigen tests, detect specific pieces (i.e., antigens) on the outside of the SARS-CoV-2 virus, such as the spike protein. Rapid antigen tests are not as accurate and have poorer performance for detecting SARS-CoV-2 compared to RT-PCR. 

Samples 

A common method for collecting a sample to test for COVID-19 is to have a health worker swab the back of your nose using a swab with a small cotton-bud. These are referred to as nasopharyngeal swabs and are the types of samples taken at Community Testing Centres (CTCs) and GPs. An alternative is a combination throat/front of the nose swab, or as is the case with many rapid antigen tests, a front of the nose swab.

Saliva is also an alternative type of sample and is collected in a tube. The sample is usually self-collected. Saliva is an increasingly popular choice with workers undergoing mandatory surveillance testing. This is because saliva samples are easy to provide (a health worker is not required to collect the sample), more easily tolerated than a nasopharyngeal swab, and shift workers have more options to drop their sample off at a time that suits them. 

For whole genome sequencing, which is a vital tool in tracing outbreaks, the sample type test must be a nasopharyngeal or a combination throat/nasal swab.  

Rapid antigen testing 

The rapid antigen tests approved for use in New Zealand, often abbreviated ‘RATs’, generally use a front of the nose swab and work by detecting the presence of specific proteins from the virus rather than its genetic material.  

Rapid antigen tests are less accurate than RT-PCR tests, particularly if people are without symptoms or when the viral load is low. The main advantage of rapid antigen tests is that the sample does not need to go to a lab and results can be returned in 30 minutes or less.  

Rapid antigen tests are most useful for screening and surveillance testing. Increasing the frequency of a RAT for surveillance, eg, every 2-3 days or daily, improves their reliability compared to a one-off RAT test. 

They may be used to rapidly screen and triage people entering high-risk environments, such as patients entering a hospital emergency department where vulnerable patients need to be protected from infection. 

From 15 December 2020 to 17 January 2022, RATs will be used to screen asymptomatic unvaccinated people aged over 12 years and 3 months who are leaving Auckland. Domestic travel carriers may require proof of a negative RAT result for asymptomatic unvaccinated people.

RATs are also used voluntarily by some businesses for surveillance purposes as an occupational health and safety measure to better protect staff and customers. 

Rapid antigen tests are being rolled out for wider use in New Zealand and will become a more common surveillance and screening test.

More information on rapid antigen testing.

RT-PCR and RAT process and sample collection  

To summarise, after a sample is collected, it is processed in one of two ways: 

  1. RT-PCR, which identifies genetic material from the virus  
  2. RAT, which identifies proteins from the virus  
Sample Collection Methods 

RT-PCR test  

Rapid antigen tests 

Nasopharyngeal swab 

Nasopharyngeal 

Combined throat and nasal swab 

Front of nose swab 

Saliva  

Saliva (not yet available in NZ) 

RT-PCR tests, which are the most accurate kind of test, can be used for both surveillance and diagnosis. 

RATs should only be used for screening and surveillance.  


How the viral test works

The viral test looks for the virus itself. It does this by searching for the virus’ genes in a sample taken from the person being tested, usually a swab from the nose or throat. Genes are small sections of DNA – information carried inside the cells of all living organisms.

In technical terms the viral test is called a ‘PCR’ test. PCR stands for polymerase chain reaction, which is a laboratory method to make large numbers of copies from a very small sample of genetic material. So, the test can find really small amounts of virus genes in a sample.


Collecting samples

If a person needs to be tested for COVID-19, a sample is taken from them. 

The most common method of collecting a sample is to swab the back of your nose (a swab is a bit like a small cotton-bud but with a longer stick). This type of testing is used in isolation and quarantine facilities and also in community settings, either at Community Based Assessment Centres (CBACs) or at your local GP clinic.

Sometimes the sample may be taken from the back of the throat instead of the nose, but this sort of sample is less likely to find the virus 

A sample may also be collected from liquids from your lungs. The fluid is either coughed up or may need to be extracted. This method is not very common, and these samples are usually only taken when people are in hospital.

The sample is then sent to the laboratory and tested to see if it contains any genes from the SARS-CoV-2 virus – the virus that causes COVID-19. The result is usually available within 48 hours (depending on how quickly the sample can be transported to the testing laboratory and how many samples they have to process). If you are awaiting test results and have not received a result after four days, please contact your GP or the place your test was done.


The best time to test

The time for a viral test to detect COVID-19 is when the highest quantity of the virus is in the body (medical experts call this quantity ‘the viral load’).

When the person first catches the virus, there’s not much of it in their system. Over the first week, the virus multiplies in their body. A day or two before the person has any symptoms, the amount of virus can already be high enough to detect in a person’s nose and throat.

The best time to test someone is during the first two to four days of the illness (this is usually around eight days after they caught the virus, but times vary between individuals) because the viral load is at its highest.

Once they start to recover, the virus left in the body starts to die off as the body’s defences kill it. But there may be bits of virus genetic material left in the person’s nose and throat for some time after the person recovers (even weeks), at levels high enough to detect but not necessarily enough for accurate testing.

Levels of virus and antibodies after catching COVID-19 and the likelihood they will be detected during testing

Graph showing how levels of the virus that causes COVID-19 increase and then decrease after a person is infected.

Graph showing how levels of the virus that causes COVID-19 increase and then decrease after a person is infected.

After infection, levels of the virus found in the nose and the lungs increase over 7-10 days. Around this time, most people start having symptoms(however, some people do not have any symptoms).

During the first week after symptoms start, the person is likely to be most infectious.

Towards the end of the first week of symptoms, the level of virus in the body will start to decrease, with the levels found in the nose decreasing faster than the levels in the lungs.

There may still be low levels of the virus in the body five to six weeks after symptoms begin.

Around seven to ten days after symptoms start, levels of proteins called antibodies start to increase rapidly in the body.

The levels of two types of antibody are shown here. The IgM antibody, which is the first antibody the body produces when it detects an infection, increases over three weeks and then levels of it start to decrease. At the same time, the IgG antibody increases and its levels stay relatively high over time.

The best time to test someone to see if they have COVID-19 is when the levels of virus are highest. A person is likely to test positive for COVID-19 using the viral test for around three weeks after symptoms start, although the virus may be detected after this too.

The best time to test for antibodies, to see if someone had COVID-19 in the past, is ten days or longer after the symptoms began.

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