COVID-19 antibody testing explained

Dr Unnati Desai answers your questions about the COVID-19 IgG antibody test, covering everything from how it works, to what the results mean.

What are antibodies?

Antibodies are a key part of the immune system – the body’s natural defence system. They help protect us from harmful substances (pathogens) such as viruses, bacteria and other foreign invaders that would otherwise cause us harm or disease.

The immune system detects proteins (antigens) on the surface of all of our cells. Early in life our immune system learns to identify our own proteins on the surface of our cells and it ignores these.

When our body is exposed to a pathogen for the first time, the immune system won’t recognise the new antigen on its surface and our body will launch an immune response. This involves producing antibodies that circulate in your blood and lock onto that specific foreign antigen to mark the cells as a problem to be eradicated.

What is the IgG antibody?

There are 5 main types of antibodies, each with their own role to play. The IgG antibody is the one that marks pathogen cells to be destroyed.

Some of these antibodies will stay in circulation afterwards, so that if you’re exposed to the same pathogen again, the immune system can quickly redeploy the antibodies and destroy the pathogen without us becoming unwell. This is immunity.

Sometimes the immune system can’t retain enough of these antibodies, or any at all, so we are at risk of reinfection.

What is the COVID-19 antibody test?

The COVID-19 antibody test is a blood test that looks for the the IgG antibody in your blood. If you were exposed to the SAR CoV-2 virus, which causes COVID-19, your immune system should have produced COVID-19 IgG antibodies.

When is the best time to take the COVID-19 antibody test?

Because it takes time for the immune system to develop the IgG antibodies, the blood test can’t be done too early. Studies tell us that over 90% of people who have had COVID-19 will have produced the IgG antibody 14 days from the start of the illness.

So the best time to take the COVID-19 IgG antibody blood test is 14–21 days after you first become unwell. How long the IgG antibody will continue to be detected in the blood isn’t yet fully known because the virus hasn’t been around for long enough.

What does the test involve?

A physiologist will take a sample of your blood from a vein in your arm – the same as in a normal blood test. It’s a very quick and relatively pain-free process.

Once your blood sample has been taken, it will be sent to a Nuffield Health pathology lab for testing. The results will tell us that whether your blood sample has the Covid-19 IgG antibody (a positive result) or not (a negative result).

How accurate is the COVID-19 antibody test?

The COVID-19 antibody test we use at Nuffield Health is very accurate and it’s been approved by Public Health England.

This test has a sensitivity of 100% (meaning the test will currently identify COVID-19 IgG antibody if it is present in the blood 100% of the time) and a specificity of >99.8% (meaning the test will correctly determine that there are no antibodies in the blood nearly all the time).

Understanding my results

What does a positive antibody test mean?

A positive COVID-19 IgG antibody test means that you previously had or have been exposed to the virus that causes COVID-19, and that your immune system developed antibodies in response to it.

In most cases, exposure to the COVID-19 virus would have resulted in symptoms of an infection. However, a small percentage of the population may carry the virus and develop antibodies to it without developing symptoms (asymptomatic carriers). A positive result does not mean:

  • you now have immunity to the virus – it’s still unclear whether the COVID-19 IgG antibodies confers lifelong immunity and there’s evidence of some people developing the illness more than once
  • you can’t pass the virus onto others – it’s possible to have had a past exposure, resulting in IgG antibodies, but contract the virus again
  • you can ignore social distancing, good hygiene and infection control measures.

What does a negative antibody test mean?

A negative COVID-19 IgG antibody test means that your immune system hasn’t developed antibodies in response to the virus that causes COVID-19. This could be for a few reasons:

  • You haven’t been exposed to the COVID-19 virus (a true negative result)
  • You have been exposed to the COVID-19 virus, but your immune system hasn’t produced antibodies in response to the virus (a false negative result). This can happen because:
    • the test was performed too soon after the onset of the infection (IgG antibodies are produced after 14 days from the onset of infection, with optimal levels after day 21)
    • your immune system responded to the COVID-19 virus without producing the IgG antibodies.

What should I do after receiving my result?

Whatever your COVID-19 antibody result is, government advice on social distancing, good hygiene with appropriate handwashing, infection control measures using face masks, and staying at home unless travel is essential, remains the same for everyone.

This is because there’s very little evidence so far on whether the antibody provides immunity or protection from reinfection. Until there’s a clear correlation, everyone must continue to follow government guidance.

If you’d like to book a COVID-19 IgG antibody test with us, find out more here.

Last updated Tuesday 11 May 2021

First published on Thursday 3 September 2020