Influenza A & B Antigen

Proteins from the influenza viruses that cause seasonal flu.

Influenza antigen tests detect proteins from the two virus families behind seasonal flu — influenza A and B — and tell you which one you have. Since flu, COVID-19, and common colds can feel nearly identical in the first days, knowing which virus you’re dealing with changes what you do next.

What the test detects

A nasal swab is checked for influenza viral proteins, with separate result lines for type A and type B. Influenza A is typically the driver of larger seasonal waves; B circulates alongside it, often later in the season. Combination tests check for SARS-CoV-2 from the same swab.

Why timing matters

Viral levels peak in the first few days of symptoms, which is when antigen tests read most reliably. It’s also when knowing matters most: flu antivirals work best when started within 48 hours of symptom onset, so an early answer keeps that option open.

What a result can tell you

A positive result identifies which virus is behind your symptoms — useful for treatment conversations with your provider, and for protecting household members at higher risk. It also spares you the guessing that tends to fill the first days of feeling unwell.

What it can’t tell you

Rapid antigen tests are less sensitive than lab PCR, so a negative with strong symptoms doesn’t rule flu out — especially later in the illness. The test also can’t tell you how severe your case will be. If symptoms are intense, prolonged, or you’re in a higher-risk group, contact a healthcare provider regardless of the result.