How COVID-19 testing works

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

Page last updated: 28 September 2020

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See also: COVID-19 testing 

The 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.

Summary

The viral test is the main publicly-available test in New Zealand as this test is most reliable for diagnosing COVID-19. 

The test is done by taking a swab, most commonly from your nose or throat. 

The best time to test someone is during the first week of the illness . This is because it is when the highest quantity of the virus is in the body.

If you have symptoms consistent with COVID-19, such as cold or flu symptoms, contact Healthline (0800 358 5453) or your doctor to find out if you need a test.


Types of test for COVID-19

There are two types of test for COVID-19.

  • a viral test that shows if you are currently infected with the virus (also called a PCR test)
  • an antibody test that shows whether you were infected before (also known as a serology test).

In New Zealand, the main publicly-available test is the viral test. This test is most reliable for diagnosing COVID-19 if it is taken at the right time (during the first week of the illness). 

The antibody test may be helpful in future for finding out who has had COVID-19 in the past. But is not useful for diagnosing new infections, because it takes two to three weeks after catching a virus for your body to make the antibodies that this type of test detects. The antibody/serology test is not available publicly on request in New Zealand. This type of test is currently only used in limited cases, as directed by a public health doctor, to support investigation of possible new cases or to support patient management.

Read more about test results and what they can and cannot tell us.

Read more about antibodies and COVID-19


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 24 to 48  hours (depending on how quickly the sample can be transported to the testing laboratory and how many samples they have to process).


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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