What is the COVID-19 IgG Antibody test?
Antibody testing (or serological testing) is an examination of proteins in the bloodstream, to find out whether an individual has been infected with COVID-19. An infected person will have specific antibodies to pathogens they have been exposed to. The immune system produces antibodies as part of a larger process to defend itself from the virus.
That said, antibody tests may not be able to show whether the virus is currently infecting the body. Unlike a nasal or throat swab test, which looks for genetic signatures of the virus in the body, an antibody test looks for traces of the body's response to the virus.
Antibodies are abundant in the blood, so a sample of blood is collected by either a finger prick or a blood sample drawn with a needle. Two specific antibody types are sought out in an antibody test:
- IgM antibodies against SARS-CoV-2, which develops early on in an infection.
- IgG antibodies against SARS-CoV-2, which are mostly found after someone has recovered from the infection.
- This is a quantitative test for the detection of SARS-CoV-2 (COVID-19) IgG Antibodies.
- A positive test indicates exposure to SARS-CoV-2.
- A negative test indicates the absence of SARS-CoV-2 IgG antibodies hence immunity status cannot be established.
- IgG antibody usually becomes positive after 8 - 13 days of infection & peaks around 21 days.
IgG Antibody Test Interpretations
|Total Antibody (IgG+IgM*)||Positive||Antibodies have developed|
Exposure to SARS COV-2 is confirmed
|Symptomatic - Refer to RT PCR|
|Asymptomatic - Quarantine & repeat IgG after 7-10 days|
|Negative||Exposure not confirmed, Abs not developed||Symptomatic refer to RT PCR|
|Asymptomatic - confirmed negative|
*Note: ICMR does not recommend this test for diagnostic purposes. It is only for Sero-surveillance.
Why is my sample being tested?
You have been offered a test because:
- You may have previously tested negative for COVID-19 but did have some symptoms that we would normally expect from people with the infection.
- You have not had typical symptoms, but as many people are known to have had this virus and be asymptomatic, we are taking the opportunity to test people whilst they are having another, unrelated, blood test.
- You have been in close contact with an individual suspected of or confirmed to have COVID-19, or you may have been otherwise exposed to the virus.
- You have recovered from COVID-19.
Testing of your sample will help find out if you may have antibodies to COVID-19 and provide us with a greater understanding of the spread of the virus on the Isle of Man.
Our knowledge of the virus will grow as new scientific evidence and studies emerge. COVID-19 is a new disease, and our awareness of the body’s immune response to it is limited. We do not know, for example, how long an antibody response lasts, whether the antibodies produced are effective in neutralising the virus, or whether having antibodies means a person cannot transmit the virus to others.
Potential benefits include:
- The results can help give you some reassurance that you may already have had COVID-19.
- The results of this test may help us understand the spread of COVID-19 within the community.
What does it mean if I have a positive test result?
If you have a positive test result (antibodies are detected), you may have been infected with the virus that causes COVID-19 at some point in the past. There is still a small chance that the antibodies indicate past infection due to other coronaviruses.
The presence of IgG suggests that the infection happened weeks to months in the past. In some infectious diseases, having IgG gives immunity and protection from catching the disease again. In other diseases, this is not the case. With COVID-19, we do not know yet whether having measurable IgG gives immunity or, if it does, how long the immunity would last. How much it might protect you from catching COVID-19 in the future is unknown.
What does it mean if I have a negative test result?
A negative test result means that the antibodies to the virus that causes COVID-19 were not found in your sample. In most cases, this will mean that you have not been infected with the virus in the past. Some health conditions might make it difficult for your body to produce antibodies to fight the infection. However, there is a small chance for this test to give a negative result that is incorrect (false negative) in some people.
A negative result may occur if you are tested early after exposure to the virus and your body hasn’t had time to produce antibodies to fight the infection. This means that you could possibly still have had COVID-19 even though the test is negative.
- To ensure the safety of our patients and staff, please let us know if you have had a sore throat, fever or other symptoms within the last 2 weeks. If you are currently sick, please talk to your doctor about options for testing and care.
Limitations of antibody testing
Margin of error:
Antibody tests are important for establishing who has had the virus, especially because a percentage of people infected seem to show no symptoms. An antibody test can give results in under an hour. For instance, the ICMR-approved antibody testing kits show results within 30 minutes. The test can be used 7 to 10 days after someone is infected, but do have a larger percentage of error than swab tests.
Antibody tests that look for IgM antibodies are usually rapid finger-prick tests that can deliver results in under 20 minutes. But IgG tests require a blood sample to be sent to a lab – a process that could take a week to deliver results. IgG tests are more reliable than the IgM rapid test, but don't reveal as much information about someone's COVID-19 infection.
False negatives and false positive:
The many diagnostic kits in circulation currently are a necessity – but they are also products of fast-track approval and developing research into COVID-19, where the priority is public health demand and speed.
Considering the developing nature of ongoing COVID-19 research, there is no assurance of quality, or guarantee that a test result is 100 per cent accurate. Some kits will perform better than others.
There may be cases where exposure to COVID-19 isn't followed by the production of antibodies in an individual, showing up as a 'false negative'. Tests that aren't of high-quality may also show a 'false positive' if an individual has another, very similar virus in the body.
Currently, ICMR is the authority recommending the most suitable kit for widespread antibody testing after making these due considerations.
Overwhelming inaccuracies in testing asymptomatics:
There have been a few dozen studies that measure the accuracy of antibody tests compared with a reference standard to detect an ongoing or past COVID-19 infection. In a review of these studies, authors found that antibody tests one week after the first symptoms appeared only detected 30 per cent of positive COVID-19 cases. The accuracy increased in the second week, with 70 per cent of cases detected. It was highest in the third week, with over 90 per cent of cases detected.
We know that it takes anywhere between 1-3 weeks to develop antibodies after symptoms occur, which makes the test as good as a coinflip in people who don't show symptoms.
False sense of security:
While this isn't a challenge of antibody tests exclusively, it is one that needs to be considered. On getting a positive COVID-19 antibody test, a person may very well think that having COVID-19 antibodies gives them immune to the virus and won’t catch it again. There is currently no evidence to suggest that people who have recovered from COVID-19 are immune to catching it again.
Even if studies show antibodies are found to offer immunity, it is difficult to establish how much antibody is needed to offer this protection, or how long the protection might last. Scientists world over are carrying out research studies to answer these questions.