SARS-CoV-2 Antigen

Proteins from the virus that causes COVID-19.

Antigen tests look for proteins on the surface of the SARS-CoV-2 virus itself. They’re fastest and most reliable when viral levels are high — which is also when you’re most likely to pass the virus on. That’s what makes a rapid antigen test practical: it answers the question that matters right now.

What an antigen test detects

Rather than looking for your immune response, an antigen test detects the virus directly — specifically the nucleocapsid protein it carries. A nasal swab picks up the sample, and results appear within minutes.

When it works best

Antigen levels track with viral load, which typically peaks in the first days of symptoms. That’s when a rapid test is most sensitive — and conveniently, when knowing your status matters most for protecting the people around you.

What a result can tell you

A positive result is dependable: it means viral proteins are present and you should follow current isolation guidance. A negative result with symptoms is best treated as provisional — repeating the test after 48 hours significantly improves reliability, which is why kits include more than one test.

What it can’t tell you

Antigen tests are less sensitive than lab PCR, especially early in infection or after symptoms fade — a single negative doesn’t fully rule out infection. They also can’t assess immunity or past infection. If you’re at higher risk or symptoms worsen, contact a healthcare provider promptly, since early treatment options are time-sensitive.