Antigen vs. Antibody: Differences and COVID-19 Testing

An antigen test looks for an active infection, whereas an antibody test looks for signs of a past infection. There are a few different ways you can check for infection with SARS-CoV-2, the virus that causes COVID-19—knowing when, how, and where to get tested can get confusing with so many options.

In this article, you will learn the difference between antigen and antibody testing for COVID-19 and when each of these tests is most appropriate and accurate.

What Is an Antigen?

An antigen is any foreign or toxic substance your immune system knows does not belong in your body. Antigens can be viruses, allergens, or even chemicals, prompting the body to mount an immune response to fight illness or infection. Antigens are required to trigger the process of producing antibodies.

What Is an Antibody?

An antibody is a protein your immune system produces after sensing a foreign substance in your body. When your immune system detects a virus, bacterium, allergen, or another substance, it creates proteins (antibodies) to neutralize similar substances the next time you encounter them.

Antigen vs. Antibody Testing for COVID-19

Scientists created specific antigen and antibody tests in response to the COVID-19 pandemic. Each test serves a particular purpose and is used in different situations.

What Are COVID-19 Antigen Tests?

Antigen tests are commonly used for detecting and diagnosing respiratory infections like influenza, respiratory syncytial virus (RSV), and SARS-CoV-2.

Since the presence of antigens are the earliest evidence that a virus has entered your body, antigen testing helps detect a current viral infection. Antigen tests are usually used as point-of-care tests. In the case of COVID-19 testing, you might have this test done in a lab, at your healthcare provider's office, or even in your home.

These tests usually use a swab to take a fluid sample from your nose or throat. The sample is mixed with a chemical called a reagent in the test kit, and the results are ready in 15 to 30 minutes.

Antigen tests are not typically as sensitive as real-time reverse transcription polymerase chain reaction (RT-PCR) and other nucleic acid amplification tests (NAATs) that identify genetic pieces of a virus. Still, they help make quick test results widely accessible.

What Is Antibody Testing?

Antibody testing determines whether your body has an immune response to an antigen. In the case of COVID-19, antibody testing is available but isn't recommended for regular use among the general public.

Antibody testing should not be used to detect or diagnose a current or active COVID-19 infection. These tests also shouldn't be used to determine if an unvaccinated person has been infected and requires immunization or whether vaccination against COVID-19 was effective.

Finally, antibody tests should not be used to justify exemptions on personal protective equipment (PPE), such as face masks, to prevent COVID-19 infections.

The purpose of antibody testing in the case of COVID-19 is to:

• Identify a past infection or vaccination against COVID-19 in specific clinical situations
• Help diagnose multisystem inflammatory syndrome in children (MIS-C) and adults (MIS-A)
• Monitor population-level immunity and infection rates of SARS-CoV-2

How Are They Used in Vaccines?

Antibody testing is generally more useful for public health purposes than for making a clinical diagnosis. Although these tests can be used in the clinical setting to detect past infections, their real value is in measuring the level of antibodies produced in response to different antigens. This information can be used for disease surveillance and to help tailor vaccines or boosters to protect against the most current variants.

Choosing the Right COVID-19 Test

If you are trying to determine if you are currently infected with COVID-19, an antigen test is the best choice. You can take these at home using a rapid antigen test or have the test done in a lab or by a healthcare provider. Your results will indicate current infection with the SARS-CoV-2 virus.

Although you can purchase antibody tests for home use, you should not use them to test for an active infection. Even if you are trying to test for a previous infection, the results of an antibody test may not be accurate until one to three weeks after you first became infected.

Summary

Antigen tests can detect a current COVID-19 infection and are available in labs, clinics, healthcare provider offices, or as at-home tests. Antibody tests won't identify an active infection, but they may help determine if your body has created antibodies against SARS-CoV-2 from a previous infection or vaccination.

The CDC recommends antibody testing to gauge vaccine efficacy and population-based immunity rather than for tracking individual immunity.