Antibody tests are different to the PCR tests we’ve been using until now. But should you go for one? Watch this video to find out.
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In August the SA Health Products Regulatory Authority (SAHPRA) authorised the use of two different types of antibody tests for COVID-19.
One rapid, point-of-care antibody test kit.
You can take this test at an accredited laboratory service provider where you can get the results in 10 minutes.
A health worker will draw blood into a tube and the serum or plasma will be used for the test.
And three lab-based antibody tests. For these tests, you need to have a blood sample taken by a doctor or nurse that is then sent away to a lab.
What are COVID-19 antibody tests and how are they different to PCR tests?
PCR tests detect the genes of SARS-CoV-2, the virus that causes COVID-19.
They tell us whether you are infected at the time of taking the test.
PCR tests can’t tell us whether you’ve had the virus in the past.
Antibody tests don’t detect the presence of the new coronavirus.
Instead, they detect the proteins, or antibodies, that our bodies’ immune systems produce in response to SARS-CoV-2.
Our bodies’ produce different types of antibodies.
First they produce IgM antibodies.
In the case of SARS-CoV-2 infection, IgM antibodies usually appear between 5 and 7 days after infection.
But they are short-lived.
They normally disappear within a few weeks after infection.
In parallel with the IgM antibodies, the body makes IgG antibodies but these take longer to develop.
In the case of SARS-CoV-2, we don’t yet know how long IgG antibodies remain detectable after infection.
IgG antibodies indicate that you have been infected with SARS-CoV-2 in the past, but have likely recovered.
So, most of the time, IgG antibody tests can tell you whether your immune system has ever had to respond to SARS-CoV-2.
Why not in all cases?
Because our bodies respond to SARS-CoV-2 in different ways.
Some people take longer than others to develop antibodies.
Others may develop very low levels of antibodies that may be undetectable.
Can an antibody test tell you whether you have the virus at the time of taking the test?
The answer isn’t straightforward.
It depends on how long your body’s immune system takes to produce IgM and IgG antibodies after infection with SARS-CoV-2.
It also depends on how soon after infection you took a test.
If your body had produced IgM or IgG antibodies at the time of taking such a test, your test will mostly come out positive and tell us that you had been exposed to the virus.
- READ MORE: Q&A: Will rapid antibody tests save us?
If your immune system only produced IgM antibodies after you took the test, the test will come out negative.
But that doesn’t necessarily mean that you’re not infected.
So why do we need antibody tests?
81% of people with COVID-19 have no or mild symptoms.
Many of these people don’t get tested for SARS-CoV-2.
They mostly don’t feel sick enough to have a reason to go for a PCR test at the time of infection.
That means we don’t have accurate data on how many people have been infected.
Antibody tests will allow us to do surveys that will help us understand what proportion of society has been infected with SARS-CoV-2.
Why?
Because, unlike PCR tests, which are only positive for a short period during the infection, antibody tests are positive for a longer period after the infection and can tell us if someone who has recovered, had previously been infected with SARS-CoV-2 — even if they showed no symptoms.
Antibody tests can also tell us how the virus has spread.
[WATCH] Four differences between PCR and antibody tests
Such information can often tell us what proportion of a town or area has developed potential short-term immunity against SARS-CoV-2.
But we don’t yet know how long immunity lasts for — or how effective even short-term immunity is in protecting someone against re-infection.
There has been at least one confirmed case of reinfection with the coronavirus in Hong Kong.
A man, 33, got reinfected with SARS-CoV-2 within four and a half months after his first infection.
A positive antibody test therefore doesn’t necessarily mean you are immune against SARS-CoV-2.
How accurate are the antibody tests that have been registered in South Africa?
The accuracy of tests is measured in two ways.
How sensitive they are, in other words how often the test generates a positive result for people who have contracted SARS-CoV-2.
How specific they are, or the test’s ability to correctly generate a negative result for people not infected with SARS-CoV-2.
The sensitivity of the SAHPRA approved rapid antibody test registered in South Africa is 95% [range 89.6-98.8%].
This means if you test negative, it doesn’t necessarily mean you were not infected. It is more likely that the test will be positive in the case of people who had moderate or severe symptoms.
The specificity of the SAHPRA approved rapid antibody test registered in South Africa is 100% [range 93.4-100%].
This means that if you test positive, you have most likely contracted SARS-CoV-2.
Should you go for an antibody test?
“If someone in your home got the virus, but you didn’t experience any symptoms and did not go for a PCR test at the time, you can go for an antibody test now to find out whether you did or didn’t catch COVID-19.” – Professor Salim Abdool Karim, chair of the COVID-19 Scientific Ministerial Advisory Committee
Where can you get an antibody test and how much will it cost?
You’ll need a referral from your doctor to get an antibody test both in the public and private sector.
You can get a rapid, point-of-care antibody test at accredited laboratories and the cost will differ according to the service provider.
You can get a lab-based a test at:
Lancet Laboratories
Price: R230.00
Time it takes to get your results: 24 hours from the time the sample arrives in Johannesburg
Pathcare
Price: R224.10.
Time it takes to get your results 24 hours in Gauteng and the Western Cape and 48 hours elsewhere
Ampath
Price: R230.00
Time it takes to get your results: 24-48 hours
National Health Laboratory Service
Price: R150.00
Please note that antibody testing is subject to some restrictions.
