Influenza (flu) tests | Pathology Tests Explained

Influenza or flu testing is used to decide whether your flu-like symptoms are due to the influenza virus or other causes, most often the common cold. It is mostly used in people who are at risk of developing severe complications from influenza such as the very young, the elderly, people from indigenous communities, those with lowered immune systems, and those who already have a chronic condition such as heart, lung, kidney disease or diabetes.

The test will show which of three possible viruses are present, influenza virus A, B or C, the most common being A. Identifying the type of virus will guide the choice of anti-viral treatment if this is needed. Some laboratories will also test for other similar flu-like viruses as part of the same test, such as parainfluenza, RSV or human metapneumovirus.

Influenza testing is also used to identify the virus type if there is an outbreak in a local community or in a place where many people are living together, such as a hospital, nursing home or school. This is so that measures can be taken to control and the spread of the virus.

What is being tested?

Influenza (the flu) is a viral infection that tends to be seasonal, beginning in mid-late autumn and disappearing in spring. It is a common respiratory illness (affecting the lungs) that may cause 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 or even death, especially in the very young, the elderly, and in those with lowered immune responses or pre-existing lung disease.

There are three overall 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, while type C usually causes only a mild illness (similar to the cold).

Flu testing relies on detecting virus (or its genetic material) that is being shed in the respiratory secretions of the person infected. Detectable virus is usually only shed for the first few days that someone is ill, so most testing must be done during this period. Anti-viral medications have been developed to treat influenza A and B. These medications, if given within 48 hours of the onset of symptoms, may reduce the severity of symptoms and reduce the time that a patient is sick by about a day. (Evidence suggests they will not usually 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 risk of complications and is known or suspected to have influenza A or B (e.g. while these 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 receded.

In the flu test, genetic material known as RNA from the influenza virus is detected using a special technique known as PCR (Polymerase Chain Reaction). In some cases, the influenza virus is actually grown and identified in the laboratory. The test will usually identify influenza A and B viruses and potentially a particular strain of influenza virus. It may be combined with tests for other viruses. This test is useful for documenting that the flu (A and/or B) has reached a community and for identifying outbreaks in particular populations, such as nursing homes, schools, or neighbourhoods. 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 it used?

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.

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 people with known cases of the flu and
you have no evidence of secondary complications,

your doctor will most likely diagnose you as having the flu from your symptoms alone and laboratory tests for flu will not be necessary. You will probably be sent home to rest, drink fluids, and use over-the-counter remedies to soothe your symptoms.

When is it requested?

The doctor may request a flu test for people who are at risk of complications (such as elderly or particularly frail people) 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 to document the presence of influenza in the area as well as help diagnose their patient. They may also order a flu test along with other viral studies, such as RSV (respiratory syncytial virus - which often infects young children and the elderly), or with bacterial tests such as a streptococcus test (to check for group A streptococcus, which causes a sore throat) if the cause of the infection is unclear.

Uncommonly, a person contracts influenza outside of the normal flu season. This may happen when they travel 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 result mean?

A positive flu test can provide useful information for your doctor in deciding how to treat your illness and also for public health officials to share with other doctors as they treat others in the community, including 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 enable 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). With current testing this problem of a falsely negative test is less common. 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 go away within one or two weeks, although the tiredness and 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. It is uncommon, but possible for anyone to get complications from the flu, but the very young, the elderly, and people 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 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 which 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. They may also order a non-laboratory based test such as chest x-ray to look for the presence of pneumonia.

Why is the flu important?

The flu can be deadly and 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 per cent 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).