There are two types of COVID-19 tests in New Zealand. A PCR, which identifies genetic material from the virus and a rapid antigen test (RAT) which detects protein from the virus.
Reverse Transcription Polymerase Chain Reaction (PCR)
The most accurate test for detecting COVID-19 is a PCR test. This test detects genetic material from the virus in the sample. All viruses change over time and laboratories can sequence the genetic material from a PCR test to identify and understand which variant of COVID-19 a person has. Whole genome sequencing takes the genetic material detected from a sample run by PCR. It then generates a complete RNA sequence of the COVID-19 virus. This allows for tracing outbreaks and new variants.
How samples are taken
Samples for PCR tests can be taken by nasal swab, a nasal and throat swab or a saliva sample. Testing is done by accredited laboratories.
Nasal and throat swabs
These samples are taken from the back of your nose using a long flexible plastic swab. They are done at Community Testing Centres and GPs. Sometimes it may be a swab from both the throat and front of the nose.
Saliva samples are collected in a tube. You usually collect this sample yourself. They are easy to provide (a health worker does not need to take it), and more easily tolerated than a nasal swab.
Saliva testing is available as part of the public health response for certain settings, populations, and providers, such as in hospitals for people who cannot tolerate a nasal swab.
Saliva testing is also available from some private providers for pre-departure tests and surveillance testing.
Rapid antigen tests (RAT)
Rapid antigen tests detect specific proteins of the COVID-19 virus, such as the spike protein or nucleocapsid. They are less accurate than PCR tests, particularly if people have no symptoms or when the amount of virus in your system viral load is low. The main advantages of rapid antigen tests are that:
- you can do these yourself at home
- you will have results within 20 minutes.
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.