How long after having flu symptoms can a person be tested and get positive result?

It can take up to 7 days after flu symptoms start for a PCR (polymerase chain reaction) test to come back positive. A PCR test is a molecular test that detects the presence of the influenza virus in a person's respiratory sample. It is the most sensitive and specific test for diagnosing the flu, but it can take several days to get the results.

A rapid influenza diagnostic test (RIDT) can detect influenza virus antigens directly from respiratory specimens. RIDT results can be obtained within 15–30 minutes, but they are less sensitive than PCR tests and can sometimes produce false-negative results.

If you have flu symptoms, it is best to see a doctor as soon as possible to be tested for the flu. Early diagnosis and treatment can help prevent serious complications such as pneumonia and hospitalization.