Antigen tests: explained.

An antigen test (aka Ag test, rapid test) looks for the presence of the virus that causes COVID-19 itself in a sample taken from your nose (or sometimes throat). They are an excellent tool for figuring out if you are infected–especially if there’s a good chance you are. This post will describe what an antigen test is and how it is different from the other types of tests out there.*

*Edited to clarify false positives and false negatives in Omicron context and link to Omicron Ag test update post.

We’re getting a lot of questions about antigen testing for COVID-19. We have already answered several antigen test questions, which you can access at the bottom of this post. Note that many of these posts were written before Omicron. We recommend reading the Omicron update to understand some of the subtle but important changes to rapid test performance and interpretation.

Here, we’ll talk basics. Let’s get nerdy.

👩🏽‍🔬 Three main types of tests 👩🏽‍🔬

First of all, there are three main types of tests related to COVID-19. The first two are used to diagnose a current, active infection. They are: antigen tests and molecular (PCR) tests. The third type is an antibody test, and it detects whether your immune cells have a memory of being exposed to the viral particles of COVID-19 (or just part of it, via a vaccine) sometime in the past. Note: antibodies and antigens are related to one another and are similar-sounding words, but they are not the same. More on that below. We are talking about antigens here.

👩🏽‍🔬 Antigen tests 👩🏽‍🔬

Antigen COVID-19 tests are sometimes also called viral tests, Ag tests, rapid tests, and lateral flow tests.

They are available in clinical settings and over-the-counter for home use. There are many brands on the market, including BinaxNOW from Abbott Diagnostics, iHealth Labs, and others.

(Note: there are a few other types of rapid COVID tests available for home use too, so don’t get confused–an antigen test will be labeled as an antigen or Ag test. At-home molecular tests are similar to PCR and will be labeled as a molecular test.)

Typically, the antigen test kit is going to include 📖 instructions, 🖌️ a swab, 🧪 some liquid in a little tube or dropper, and a cassette or card with a 🎞️ test strip. Instructions will describe how to swab yourself to get a sample, mix the swab with the liquid, and then use the test strip to get a result. You need to follow the instructions closely, and each brand works a bit differently.

🔎 At the end of the process, you’ll typically interpret the results based on whether there is a line in the “test” or “T” position or not. A second line appears in the “control” or “C” position to show that the test worked at all.

Two lines = is a positive result and one line — is a negative result. No lines mean you didn’t do it right, and the test cannot be interpreted. It doesn’t matter how faint the test line is. If it’s there at all, it is a positive result. (Check your kit instructions for specifics on reading the results).

👩🏽‍🔬 What an antigen test detects 👩🏽‍🔬

When you get COVID-19, the virus gets inside your cells and hijacks your cellular machinery to start making millions and millions of copies of itself. It forces your cells to make so many copies that they explode. Virus (and lots of other stuff) is everywhere. It’s messy. Each one of those millions of viruses has a bunch of proteins on the inside and others on the outside.

An antigen test uses a chemical reaction to bind and “light up” a specific protein that is part of the virus, usually from the N or nucleocapsid protein. (These tests do NOT look for the infamous spike protein that all the vaccines use to train our immune systems).

If it finds enough of that protein on your sample, the test is positive. You can read this to mean that there was a lot of this specific virus on your swab–particularly if you have symptoms of COVID-19 or are riding out the peak numbers of the omicron wave.

👩🏽‍🔬 Let’s talk about false positives and false negatives 👩🏽‍🔬

Antigen tests are a very useful tool, but they’re imperfect. Both false positives and false negatives are possible.

False positives are hardly common. Yet, they are a realistic possibility when tests are used in situations when there’s very low chances of a true positive–like for asymptomatic screening in low-COVID areas. So, when you get an unexpected positive result on an antigen test, you may want to confirm it with a second antigen test OR a PCR test.

That said, if the chances that you really are infected are high in the first place, a positive antigen test result is very likely to be correct. Examples of high likelihood of a true positive include when you have symptoms, your infection has been confirmed with another test, you have a known recent exposure or you are living with someone who has COVID-19, or your local case numbers are breaking records.

False negatives are a serious concern with rapid tests, especially for Omicron. With Omicron, early evidence suggests that you could be contagious for a couple days before the virus takes hold in your nose and is detectable with a rapid antigen test. Thus, negative rapid tests don’t deliver the same sure-fire “green light” they used to, even for that same day.

A negative antigen test only tells you that you don’t have tons of virus *right now* – in the part of the body you swabbed. It could be lurking elsewhere, like the saliva, as we have seen with Omicron. If you get a negative result and you’re, say, in quarantine because you have a known exposure–you should test again in a day or two because there’s a chance that there just wasn’t enough viral material in your nose to detect YET. If you’ve been lighting up antigen tests with consistent positives throughout your illness and have finally gotten a negative test, that’s a great sign that you have cleared the virus and are no longer infected.

👩🏽‍🔬 Antibodies are not the same as antigens 👩🏽‍🔬

Antibodies and antigens are related to one another and are confusingly similar-sounding words, but they are not the same.

🦠 An antigen is the thing that makes your immune system react. In this case, it’s the proteins on the COVID-19 virus.

🩸 An antibody is a protein made by your own body that detects/destroys things that can make you sick.

To oversimplify, antigens = bad guys. Antibodies = your immune system. Antibody tests and antigen tests do not look for the same thing at all.

When you get vaccinated, your body’s natural, beautiful immune system notices that something isn’t quite right at the site of your injection. The vaccine contains antigens, and your body makes antibodies unique to those antigens, like making a key for a lock. It stores the key in its memory.

When you get vaccinated, you do temporarily have some antigens in your body, BUT they are not the same ones that the antigen tests look for. And, the antigen test does not react with your antibodies at all. That’s why an antigen test is not affected by whether you’ve been vaccinated.

Look forward to another informative post on antigen tests which will talk about whether a positive antigen test means you’re contagious. That is coming on Wednesday! In the meantime, please enjoy this collage of actual antigen tests by Those Nerdy Girls. Yes, we’re using these things like they’re going out of style.

⭐UPDATE on detecting Omicron with Rapid Antigen Tests

⭐ Should I take a COVID-19 rapid antigen test? Part 1 in a Series

⭐ My kid has a runny nose. Should we get tested?

⭐ Do antigen tests detect the Omicron variant?

⭐ Should I get tested after X (fill in the blank)?

⭐ Do you have practical advice for families around testing for COVID-19?

⭐ Overview of types of tests related to COVID-19

and

⭐ False positives and false negatives on rapid home tests

… to name a few.

Some further reading & viewing:

CDC – Testing for SARS-CoV-2 Infection