Summary

• When you are infected by the virus that causes COVID-19, or when you are vaccinated against COVID-19, your body makes antibodies to fight it.
• These antibodies appear in your blood about two weeks after your symptoms start and remain at low levels in your blood for a period of time.
• Testing a sample of your blood for antibodies can show if you have had a past infection or have been vaccinated.
• Because your body takes time to produce antibodies, these tests are likely to give a false negative result in the first few days of infection.
• They may also give a false positive result if they detect antibodies to other similar viruses including the virus that causes the common cold.

What are COVID-19 antibodies?

The virus that causes COVID-19 is called SARS-CoV-2 and is one of a family of viruses of that cause respiratory (lung and airway) infections called coronaviruses. SARS stands for severe acute respiratory syndrome.

When your body is infected by a virus, your immune system makes antibodies to fight it. Antibodies to SARS-CoV-2 are made from about two weeks after your symptoms appear. Also, when you are vaccinated against COVID-19 your body produces antibodies. Antibody (serology) testing looks for these antibodies in a sample of your blood. 

Timeline of infection by the SARS CoV-2 virus

After you are infected, the virus enters your bloodstream. Within a few days, you start to develop symptoms and both the symptoms and the amount of virus in your blood peak at about five days then subside. You are infectious from just before your symptoms appear.  

Antibodies 

At about 14 days post-infection, antibodies start to appear and can be detected in a blood sample taken at this time.

• IgM antibodies – these are produced early in the infection.
• IgG antibodies – these develop from two to four weeks after infection or vaccination. When they are detected in a test they are considered a reliable indicator of past infection.

It remains uncertain how long SARS-CoV-2 antibodies remain in your blood. There are also a few people who do not make antibodies for reasons which are also unclear.

Limitations of antibody tests

Because your body takes time to produce antibodies against the virus, these tests are likely to give a false negative result in the first few days of infection. These tests can also mistake antibodies we produce against other coronaviruses, such as the common cold, as COVID-19 antibodies and give a false positive result.

Tests are usually described in terms of their sensitivity and specificity. Developing laboratory tests is a balancing act. Tests need to be sensitive enough to detect the virus at low levels, but also specific enough not to give a positive result in response to something else such as another virus.

A test that is too highly sensitive will pick up people who have the virus but may also give positive results for people who don’t have the virus. A test that is too highly specific may miss people who have the virus. 

Because antibody testing can miss the virus in the early stages of infection, and mistake other similar viruses for SARS-CoV-2, a positive antibody test result does not necessarily mean you have immunity or that you are protected against a COVID-19 infection. They also cannot tell you whether you can infect other people. 

False negatives and false positives

You may come across these terms when COVID-19 testing is being discussed. 

• If the test gives a negative result in a person who is infected this is a false negative. 
• A person who does not have the infection but whose test gives a positive result is a false positive.

Why get tested

Antibody testing for SARS-CoV-2 is not routinely used in diagnosing and managing COVID-19. However, it can sometimes be used to help decide if your symptoms are due to a recent infection. They are also used to help decide whether health problems such as myocarditis (an infection of the heart), or long COVID symptoms such as extreme tiredness are due to a past COVID infection. This is helpful in directing treatment.

Having the test

Sample

Blood.

Any preparation?

None.

Your results

Reading your test report

Your results will generally be presented along with those of your other tests on the same form. You will see separate columns or lines for each test.  

Your COVID-19 antibody test results will be shown as either negative or positive. A positive result means that antibodies were found in your blood. 

Questions to ask your doctor

The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.

You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.

Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods, medications or supplements. These may affect your results. Ask:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.