Known as a false negative, this can happen for several reasons, including when you picked up the virus and the type of flu you have.
Of course, you should also always consider the possibility that your flu-like symptoms aren’t actually due to the flu and that you’re testing negative because you have something else, like COVID-19.
This article goes over what it means if you get a negative rapid flu test and what to do if you think the result may be wrong.
Why Your Rapid Flu Test May Be Inaccurate
Rapid influenza diagnostic tests look for influenza A and B viruses in fluid (secretions) from your respiratory tract.
A benefit of the tests is that the results are ready in as little as 15 minutes; other flu tests take time because they must be sent to a lab. While they’re faster, rapid tests are not as accurate.
Factors that can affect the outcome of a rapid flu test include:
How long you’ve been sick: Tests are the most accurate when the specimens are collected within three to four days of when a person’s symptoms started because that’s when the flu virus sheds the most. How the sample was collected: Each flu test has rules for collecting a specimen—for example, some have to be taken from the nose and others from the throat. These instructions must be followed to ensure the test is accurate. The type of influenza: Rapid flu tests are better at detecting influenza A than influenza B. Current flu activity: False negatives are more likely to happen when flu activity is high, but can happen at any time. False positives (results that indicate you have the flu when you really don’t) are more common when flu activity is low.
Rapid flu tests are a tool for providers, but results are not the only thing that matters in making a flu diagnosis. Your provider will consider all of these factors when reviewing your results to decide whether you could have the flu and need treatment.
Other Possibilities If You’re Truly Negative
If your rapid test comes back negative and it’s accurate, you do not have influenza. There are several different viruses that can cause flu-like illness and produce symptoms that are more severe than a common cold.
Some possibilities include:
Adenovirus COVID-19 (coronavirus) Parainfluenza Respiratory syncytial virus (RSV)
While these viruses can leave you feeling very sick for a few days or even a week, they are less likely to lead to serious complications, secondary infections, or death than the flu.
Treatment of Presumed Positive Cases
If your flu test is negative but you have classic flu symptoms and flu activity is high where you live, your provider may still diagnose you with the flu and and prescribe an antiviral drug, such as Tamiflu (oseltamivir), Relenza (zanamivir), Rapivab (peramivir), and Xofluza (baloxavir marboxil).
For most people, taking antivirals without a confirmed case of the flu is not as big of a risk as not treating influenza.
When they’re started within the first 48 hours of getting flu symptoms, antivirals have been shown to lessen the severity of symptoms, shorten the duration of illness by about one day, and reduce the risk of flu complications.
If your symptoms do not improve with antiviral treatment, you probably did not have the flu after all. In that case, you might need different treatment. However, many over-the-counter (OTC) medications for symptom relief work for many different flu-like illnesses.
Do I Need to Get Retested?
Some providers won’t do rapid flu tests because of the limited accuracy. Instead, they’ll treat you based on your symptoms. However, additional flu tests can be helpful in some situations, especially if your provider thinks your flu test was a false negative.
For example, retesting is helpful for pregnant people and people who are immunocompromised or live with someone who is. In these cases, confirming a flu diagnosis helps providers make the right treatment decisions.
More accurate flu tests can be done by special labs, but they are rarely used to make a flu diagnosis in the general population aside from confirming the cause of a new respiratory illness outbreak.
When these tests are done, the results usually get sent to the Centers for Disease Control and Protection (CDC) to keep tabs on circulating flu strains and the rate of flu activity throughout the country.
Summary
If you have flu symptoms but a rapid flu test is negative, you may truly have another illness (like COVID-19 or RSV) and not the flu. However, these tests are not always accurate and there are several factors that affect the results. You may get a false negative result, which means you do have the flu but the test didn’t pick it up.
If your provider thinks you have the flu based on your symptoms and the level of flu activity in your community, they will probably treat you without a flu test or even with a negative test.
