Influenza (Flu) Tests
If your doctor wants to determine whether your flu-like symptoms are due to flu, another virus, or other causes. Usually, only patients with severe infection are tested. When influenza is at a high level in the community, doctors will often diagnose respiratory infections as influenza based on symptoms, without taking a sample to test.
Usually a nose/throat swab or a nasopharyngeal aspirate
Influenza (the flu) is a viral infection that tends to be seasonal, beginning in late November and disappearing in early spring. It is a common respiratory (affecting the lungs) illness that causes headaches, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough. Symptoms of flu tend to be more severe and longer lasting than the flu-like symptoms caused by the common cold. Flu and its complications can lead to hospitalisations even death, especially in the very young, the elderly, and in those with lowered immune responses or pre-existing lung disease.
There are three types of influenza, known as A, B and C, each of which can change many times to create multiple strains. Influenza virus A is the most common and causes the most severe symptoms. Type B is less common and causes less severe symptoms, whilst type C usually causes only a mild illness (similar to the cold) and is usually not included in influenza tests.
Flu testing relies on detecting virus that is being shed in the respiratory secretions of the person infected. Detectable virus is usually only shed for the first few days that a person is ill, so most testing must be done during this time period. Anti-viral medications have been developed to treat either influenza A alone, or both A and B. These medications, if given within 48 hours of the onset of symptoms, can reduce the severity of symptoms and reduce the time that a patient is sick by about a day. (They will not help if given later and they will not work against other viruses or against bacterial infections). These medications will only be prescribed if the person is at particular risk of complications and it is known that influenza A or B are circulating in the community. For otherwise healthy people, the treatment is to stay in bed and rest, drinking plenty of fluids, until the symptoms have cleared up
Flu testing is also useful for documenting that the flu (A and/or B) has reached a community, and for identifying outbreaks in particular populations, such as a nursing home, school, or neighbourhood. Identifying these outbreaks can assist healthcare workers in the prevention and treatment of the flu throughout a community. Influenza sub-typing tests are used to determine which sub-type of flu a person has, such as H1N1 (swine flu) or H3N2 which is useful in tracking the spread of flu in a season.
How is the sample collected for testing?
A nasal swab is collected by having you tip your head back, then a swab (like a long cotton bud) is gently inserted into one of your nostrils until resistance is met (about 1 to 2 inches in), then rotated several times and withdrawn. Sometimes a doctor may use a throat swab in place of or as well as a nose swab. In children, a nasopharyngeal aspirate is often taken instead of a swab, by using a syringe or other suction device to insert and immediately withdrawing a small amount of saline into the back of the nose while the head is tipped back. These procedures should not be painful but may tickle a bit and cause your eyes to well-up. Collection of a good quality sample is crucial for the flu tests to work well.
Influenza virus can also be detected in bronchoalveolar lavage (BAL) fluid which is collected from the lower respiratory tract (lungs) of more seriously ill people.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
If it is the flu season, and:
- the flu has reached your community,
- you have clinical symptoms that are essentially the same as the symptoms of other patients with known cases of the flu,
- you have no evidence of secondary complications,
then your doctor will diagnose you as having the flu from your symptoms alone and laboratory tests for flu will not be necessary. It is likely the doctor will send you home to rest, drink fluids, and use over-the-counter remedies to soothe your symptoms.
The flu test is used to help diagnose influenza A and B, and to differentiate them from other viral and bacterial infections which may be serious and must be addressed separately. Knowing the particular type of strain of flu that is present in the community can help your doctor minimise its spread.
There are several types of influenza test available, each with advantages and disadvantages. The type of test used often depends on the circumstances and reasons for testing.
- Influenza nucleic acid amplification tests detect the genome of influenza viruses in respiratory samples. The most commonly used test of this type is real-time PCR and this has become the “gold standard” in the UK. Specific PCR assays are used to detect influenza A and B and to sub-type influenza A. Often these are combined with PCRs for other common respiratory viruses such as RSV. PCR tests have very high sensitivity and specificity and results are usually available within 24 hours, but they are often more expensive than other types of test.
- Viral culture is used to detect live viruses by growing them in cells cultured in the laboratory. It has the advantage that many different virus types can be cultured and a supply of the virus can be isolated for further studies. However, culture has been replaced by PCR in many laboratories as it requires technical expertise and can take 2 – 10 days for a result.
- Rapid influenza antigen tests detect the antigens of influenza viruses and can be used at the point of care (e.g. in the GP surgery or A&E) or in the laboratory. They have the advantage that a result can be available in as little as 10 minutes, but produce a large number of false-negative results and therefore require confirmatory testing in the laboratory by either PCR or viral culture.
- Direct immunofluorescence tests are used in some laboratories to rapidly detect influenza antigens with fluorescently labelled antibodies. They have similar advantages and disadvantages to rapid antigen tests.
When is it requested?
Flu tests are mostly requested during flu season (late autumn through to early spring). Not everyone who has symptoms of influenza requires testing, however the doctor is more likely to request a flu test for patients already in hospital or patients who are at risk of complications (such as very young, immunocompromised, elderly or particularly frail patients) who present with a respiratory infection and symptoms such as: headaches, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough.
When influenza has not yet been reported in the community, the doctor may order a flu test both to document the presence of influenza in the area and to help diagnose their current patient. He may also order a flu test along with other viral studies, such as RSV (respiratory syncytial virus - a virus that often infects young children and the elderly), or with bacterial tests, such as a streptococcus test (to check for group A streptococcus, the bacteria that cause a sore throat) if the cause of the infection is unclear.
In rare instances, someone will contract influenza outside of the normal flu season. (This may happen when someone travels outside of the UK to a part of the world where the flu is currently infecting that area's residents.) In this case the doctor may request a flu test to confirm the diagnosis.
What does the test result mean?
A positive flu test means that the affected person most likely has influenza A or B, and treatment with antiviral medication may be prescribed to minimise symptoms. However, it may not tell the healthcare professional which strain of influenza is causing the infection, how severe the symptoms are likely to be, or whether or not a person may experience any secondary complications.
Negative flu tests may mean that you have something other than influenza, that the test is not detecting the strain of influenza that is causing the infection or that there is not sufficient virus in the specimen to allow it to be detected. This may be due to either a poor specimen collection or because you have had the flu for several days (in the later stages of influenza less virus is shed). Your doctor will use your negative result along with other clinical findings to recommend the treatment best for you.
Is there anything else I should know?
Treated or untreated, most influenza infections will go away within 1 or 2 weeks, although tiredness and a cough may last a little longer. A few people, however, may develop serious secondary complications. These complications often arise just as the flu symptoms are fading. Anyone is susceptible to complications from the flu, but the very young, the elderly, and patients who have lowered immune responses or pre-existing lung disease are most affected. Complications such as pneumonia, sepsis (widespread infections that can be detected in the blood), and encephalitis (an inflammation of the brain) can be very serious and may require immediate medical treatment.
What other tests might my doctor request to diagnose my flu-like symptoms?
Your doctor may request a streptococcus test to check for streptococcus infection, blood cultures to check for bacterial infections in the blood, an RSV test (respiratory syncytial virus; a virus that often infects young children and the elderly), or a sputum culture to look for bacterial and/or fungal causes of a respiratory infection. They may also order blood tests such as a FBC (Full blood count) to monitor body organ function.
Why is the flu important?
Because the flu can be deadly and because every few decades an especially lethal influenza emerges. The worst on record is the 1918 Spanish flu pandemic (large scale epidemic), which killed more than 20 million people worldwide. That is why prevention is stressed and why research for additional treatments is ongoing.
A strain of influenza A, named for its unique viral antigens (H5N1), was first detected circulating in birds and chickens in Asia. Since October 2022 bird flu has been found in the UK in commercial poultry farms, and wildfowl and seabird flocks. Bird flu has been known toinfect humans who have had close contact with infected birds and has a high mortality rate. There is concern of a worldwide pandemic if this virus is able to mutate to allow human-to-human transmission of the virus.
Can I still get the flu if I've had the vaccination?
Yes. Influenza circles around the globe and moves through communities. As it travels, it undergoes spontaneous changes that allow it to evade the protections of last year's flu vaccine and re-infect you. The amount of change varies from year to year. Bigger changes in the flu virus often result in more severe illnesses. Doctors and researchers carefully track the influenza virus as it moves through the world and try to anticipate the strain(s) that will eventually appear in the UK the next season. Each year the flu vaccine is produced based on their observations and experience and targeted to protect us against the expected strain.
In most cases the flu vaccine will prevent the flu but it requires a few weeks before it provides protection and it is not 100% effective. In addition, sometimes the flu will 'break through' - there will have been enough change in the virus so that it appears slightly different to the body's immune system, decreasing the effectiveness of the vaccine's protection. Alternatively, the flu that predominates may end up being an unexpected strain, not the one that the vaccine was developed to protect against. Usually in these cases the vaccination will at least reduce the severity of the infection. In individual high-risk patients (those with heart, kidney, and lung disease for instance), doctors may bolster the protection by prescribing anti-viral treatments that can provide short term protection while influenza moves through the community (although this comes at a price in terms of cost and treatment side effects).
Is there a blood test for the flu?
Yes, there are blood tests available that detect and measure influenza antibodies, immune proteins produced in response to an infection. However, these are not useful for diagnosing the flu at the time of infection as it takes several days for the body to develop antibodies. They may be used to determine whether someone has developed antibodies following vaccination, but this is usually only in research settings.