Antibody Tests Explained

April 22, 2021

This article has not been updated recently

Antibody tests check to see if you have previously had COVID-19, and are not the same as PCR, lateral flow or LAMP testing, which detect whether you’re currently infected with coronavirus.

What are antibodies?

Antibodies are biological molecules produced by special white blood cells in your body which control immunity. These are called B cells. Antibodies recognise and stick to structures inside or on the surface of viruses and bacteria known as antigens, neutralising the threat and keeping you safe and well.

Your immune system is stocked with tens of millions of these white blood cells, each of which produces a different antibody that recognises a unique antigen.

This means that whatever harmful virus or bacteria you might have encountered during your lifetime, you probably have some B cells that can make antibodies against it.

When you catch a virus like SARS-CoV-2, which causes COVID-19, the B cells that produce the specific antibodies for the virus multiply rapidly, and then travel around your body to tackle the infection.

After the infection is over, the remaining antibodies are gradually broken down over several months if they are no longer needed. But some so-called ‘memory’ B cells can remain in the body, ready to spring into action if you catch the same virus again.

What is a COVID antibody test and how does it work?

COVID antibody tests detect antibodies in your blood, specific to  the coronavirus. 

There are two main types of antibody tests:

  • Anti-N tests look for antibodies that recognise a molecule inside the virus called the nucleocapsid (N). These are only produced if you have caught COVID-19 previously, and show natural immunity. 
  • Anti-S tests detect antibodies against the spike protein (S) on the surface of the virus. Because COVID vaccines are based on the spike protein, anti-S tests detect antibodies produced both through natural infection and vaccination. 

To do an antibody test, you prick your finger with a needle device provided in your kit and squeeze some blood into a special tube. This is then sent off to a lab for analysis. You will get your results back in three to seven days.

What does the result of your COVID antibody test mean?

There are three possible results from the test:

  • Positive: anti-N antibodies were detected
  • Negative: anti-N antibodies were not detected
  • Void: the test didn’t work properly 

A positive antibody test result means it’s likely you've had coronavirus before. However, it doesn’t mean that you’re now immune, as it’s not clear how good anti-N antibodies are at neutralising the coronavirus. (Anti-S antibodies are much more protective, which is why the spike is used in vaccines.)

A negative test result means you have no detectable levels of COVID-specific anti-N antibodies in your blood. 

If you were infected, you might not have produced anti-N antibodies - not everybody does. They also fade over time, so if you were infected in early 2020 they may now have disappeared.

Anti-N antibody tests can only reveal if it’s likely you’ve had COVID-19, and can’t tell you if your vaccine is working, since they only show the natural immune responses produced by your body.

Whatever your result, you could still catch the virus again or spread it to others, especially if you haven’t been vaccinated yet. It’s essential to keep following public health guidelines for now to help keep yourself, your loved ones and your wider community safe.

Even though cases of COVID-19 are falling around the UK, it’s important to download and use the ZOE COVID Symptom Study app to log daily health reports and the after effects of your COVID vaccine.

You’ll be helping with life-saving science to help us understand COVID-19 immunity and responses to vaccination, as well as acting as a vital early warning system to detect any hotspots across the UK due to new variants.

Stay safe and keep logging.

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