Flu (Influenza) Test
Reviewed
Also called: rapid flu test, influenza antigen test, rapid influenza diagnostic test, RIDT, Flu PCR, Type A and B Antigen Detection
A flu (influenza) test checks a sample of fluid from your nose or throat to find out if you are infected with a flu virus. It is a laboratory test that can be done as a rapid antigen test (RIDT) or a molecular test (PCR). The test detects seasonal influenza A and B viruses. Some tests also check for other respiratory viruses like COVID-19 or RSV. Results help guide treatment and infection control decisions.
Specimen: Nasopharyngeal swab, nasal aspirate or wash, conjunctival swab, sputum, bronchoalveolar lavage (BAL), or self-collected nasal swab (for at-home tests).
Why this test may be ordered
- To determine if your respiratory symptoms are caused by influenza A or B, especially if you are at high risk for serious complications.
- To help your healthcare provider decide if antiviral medicine is appropriate.
- To monitor flu and COVID-19 outbreaks in the community (public health use).
Preparation and collection
No special preparation for most collection methods. For a sputum culture, you may be asked to drink extra water the night before and possibly avoid eating for 1-2 hours before the test. For bronchoscopy or BAL, you may need to fast for up to 12 hours and stop certain medicines. Follow your provider's instructions.
A health care professional may collect a sample using a nasopharyngeal swab (inserting a long swab into the nostril to reach the nasopharynx and rotating for 10-15 seconds), a nasal aspirate or wash (instilling saline and suctioning), a conjunctival swab (swabbing inside lower eyelid), or sputum culture (coughing deeply into a cup). For severe respiratory disease, a bronchoalveolar lavage (BAL) during bronchoscopy may be used. At-home tests involve self-collecting a nasal swab and either running the test at home (rapid antigen) or mailing the sample to a lab (self-collection kit).
For nasopharyngeal swab, you may feel a gagging sensation or a tickle. A nasal aspirate or wash and a conjunctival swab may feel uncomfortable; these effects are temporary. There is no risk to providing a sputum culture. For bronchoscopy or BAL, there is very little risk; serious complications are rare but may include bleeding in the airways, infection, or a pneumothorax (collapse of part of your lung).
Understanding results
Your results will be reported as positive or negative. A positive result means you most likely have the seasonal flu. A negative result means you may not have the flu, but the flu cannot be ruled out due to the possibility of a false negative.
- Positive: You most likely have the seasonal flu. Limitation: False positives are very uncommon according to the source. Next step: Limit contact with others to avoid spreading the virus. Get plenty of rest and drink lots of liquids. Over-the-counter pain relievers may help with fever and aches (children and teens should not take aspirin). You can return to usual activities when your symptoms have been improving for at least 24 hours and you have not had a fever and are not taking fever-reducing medicine. Your provider may prescribe antiviral medicine depending on your health and how long you've been sick.
- Negative: You may not have the flu. A different virus or bacteria may be causing your symptoms. Limitation: False negatives are possible, especially with rapid antigen tests or if the sample was collected when the virus level was low. Next step: If you have flu-like symptoms, ask your healthcare provider what to do next. You may need more testing, especially if your first test was a rapid antigen test.
- False negatives are possible, especially with rapid antigen tests or if the sample is collected when the virus level is low.
- False positives are very uncommon but can occur.
What to do next
If you have a high risk of serious illness, contact your healthcare provider at the first sign of symptoms. Antiviral medicine works best when started within 2 days of symptoms. To lower your risk of getting the flu, wash hands often, avoid close contact with sick people, and get a yearly flu vaccine.
- Do you have symptoms of a respiratory infection such as fever, chills, muscle aches, fatigue, headache, runny or stuffy nose, sore throat, or cough?
- Are you at high risk for serious flu complications (e.g., age under 5, age 65+, or chronic conditions like asthma, diabetes, heart disease)?
- Have you been in contact with animals that could have had bird flu (e.g., birds or cows) and have flu symptoms?