At a glance

Why get tested?

To identify the type or strain of flu virus that you have

When to get tested?

If your doctor wants to determine whether your flu-like symptoms are due to a particular type or strain of the influenza virus, or due to other causes

Sample required?

Usually a nose or throat swab

What is being tested?

Influenza (the flu) is a viral infection that tends to be seasonal, beginning in late July 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 (see Common questions#3). 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).

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.

In the flu test, DNA from the influenza virus is detected using a special technique known as PCR (Polymerase Chain Reaction). In addition, the influenza virus is actually grown and identified in the laboratory. The test can identify which viruses (A, B, or some other virus) and which strains of virus are present. It is 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 and in the manufacture of the annual influenza vaccine.

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. This is not painful, but it may tickle a bit and cause your eyes to well-up. Sometimes a doctor may use a throat swab.

The Test

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.

When is it requested?

The doctor may request a flu test for patients who are at risk of complications (such as 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 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 can give your doctor information about the likely severity of the strain infecting you and can provide useful information for your doctor to have and to share with other doctors as they treat your family, friends, and neighbours.

Negative flu tests may mean that you have something other than influenza, 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.

Common Questions

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.

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 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).

Where can I find out about bird flu or swine flu?

Last Review Date: May 1, 2012