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Influenza tests
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Este artigo foi modificado pela última vez em 19 de Abril de 2023.
At a Glance
Why Get Tested?

To determine whether or not you have the influenza A or B; to help your doctor make rapid treatment decisions; and to help determine whether or not the flu has come to your community

When To Get Tested?

When it is flu season and your doctor wants to determine whether your flu-like symptoms are due to influenza A or B, or to other causes; within 48 hours of the onset of your symptoms, to help determine treatment options

Sample Required?

Depends on testing method being used; usually a nasopharyngeal (NP) swab, a nasal aspirate or nasal wash; under approved circumstances, a throat swab

What is being tested?
Influenza (the flu) is a viral respiratory infection that tends to be seasonal, beginning in late November and disappearing in early spring. It is a common respiratory illness that affects 30 to 50 million Americans each season, bringing headaches, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough. Symptoms of influenza tend to be more severe and longer lasting than the flu-like symptoms caused by the common cold. Influenza and its complications cause more than 100,000 hospitalizations and 20,000 deaths a year in the United States, especially in the very young, elderly, and in those with compromised immune systems or pre-existing lung disease.

There are two types of influenza, A and B, that infect humans and each virus can mutate to create different strains. Usually a single strain of influenza virus A will predominate during a particular flu season, although there may be a mixture of A and B causing outbreaks in the community at the same time.

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.)

Influenza testing can be used to help diagnose the flu and determine treatment options for an individual patient, and it can be used to help rule out the flu when looking for other illnesses. A rapid influenza antigen test to detect the virus in nasal secretions is one of the most common methods to diagnose this infection. Depending on the method, it may be completed in the doctor's office in less than 30 minutes or be sent to a laboratory, with the results available the same day. Depending on the particular type of test used, it can identify the presence of an influenza virus or differentiate which influenza virus is present (A or B). The main disadvantage of the rapid influenza antigen test is that it will miss up to 30% of influenza cases and it will occasionally be positive when the patient does not actually have the flu.

Other methods to detect influenza are more sensitive and specific. Nasal secretions can be sent to a laboratory where they are stained with fluorescent antibody to visualize the presence of the virus. Influenza A can be distinguished from influenza B within several hours after the specimen is collected, and the appropriate antiviral therapy can be initiated. This method does require a special microscope and skilled laboratory personnel to read and interpret the test. It is not usually performed in a doctor’s office.

Sometimes your doctor will order a viral culture. In this test, the influenza virus is actually grown and identified in the laboratory. It has the advantage of identifying which viruses (A, B, or another respiratory virus) and which strains of virus are present. A rapid culture method, known as shell vial culture, takes only 24 hours of incubation time to be read; however, a traditional viral culture performed in a large test tube may require several days before the virus can be detected. Growing the virus in culture is useful for documenting which strains of influenza are circulating in the community. Identifying these outbreaks can assist healthcare workers in the prevention and treatment of the flu throughout a community.

How is the sample collected for testing?

Sample collection technique is critical in influenza testing. The best sample is usually a nasal aspirate, but the most frequently used samples are the nasopharyngeal (NP) swab or nasal wash. The person collecting the sample will use a syringe to push a small amount of sterile saline into your nose, then either apply gentle suction (for the aspirate) or use gravity to collect the resulting fluid (saline and mucus) into a cup. To preserve the organisms in the sample, the sample should be put in a special container, referred to as "viral transport media" or VTM, for delivery to the laboratory.

The NP swab is collected by having you tip your head back, then a Dacron swab (like a long Q-tip ) 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 tear. Doctors usually use NP swabs on adults but may choose to do a nasal wash or aspirate on a child. In some circumstances, a doctor may use a throat swab, but this contains less virus than an NP aspirate and so may not be appropriate for use in rapid testing.

Accordion Title
Common Questions
  • 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 may diagnose you as having the flu without actually testing for influenza. If you are outside the 48-hour window of anti-viral treatment, they may just send you home to rest, drink fluids, and use over the counter remedies to soothe your symptoms. (This is what doctors did before flu testing and treatment were available.)

    So why order the flu test at all? Because the flu can be deadly, because knowing it is in the community can help your doctor minimize its spread and because there is treatment available that can lessen its severity if it is diagnosed early. (Of course the best treatment is prevention - getting a flu shot.) 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 treated differently.

    Rapid (same day) flu tests are best used within the first 48 hours of symptoms to help diagnose influenza and determine whether or not anti-viral medications should be used, or they are ordered within the first week to help identify outbreaks. Viral cultures are usually used to track flu outbreaks and to identify the particular strain that is causing them (sometimes the influenza will mutate enough to make the flu vaccine less effective and sometimes an unexpected flu strain will predominate). Viral cultures are also be used to identify other viral infections that cause clinical symptoms similar to the flu. Currently, the rapid influenza antigen test and the fluorescent antibody detection of influenza cannot distinguish seasonal influenza from avian influenza (“bird flu”). Influenza virus grown in culture could be sent to a public health laboratory to determine if the strain of influenza A is the antigenic type (H5N1) found in birds and chickens.
  • When is it ordered?
    Flu tests are ordered almost exclusively during flu season (late October through March) when a patient presents with a respiratory infection and symptoms such as: headaches, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough. If influenza has already been identified in the community the doctor may order a flu test to confirm the diagnosis and validate possible anti-viral treatment.

    When influenza has not yet been documented in the community, the doctor may order a rapid flu test both to document the presence of influenza in the area and to help diagnose their patient's current illness. He may also order an influenza test along with other viral studies, such as RSV antigen test (respiratory syncytial virus - a virus that often infects young children and the elderly), or with bacterial tests, such as a strep test (to check for group A streptococcus, the bacteria that cause strep throat) if the cause of the infection is unclear.

    In rare instances, someone will contract influenza outside of the regular flu season. (This may happen when someone travels outside of the continental U.S. to a part of the world where the flu is currently infecting that area's residents.) In this case, the doctor may order a respiratory viral culture in order to have a more definitive diagnosis.
  • What does the test result mean?
    If your flu test is positive, it means that you most likely have influenza A (or B), but it does not tell your doctor how severe your symptoms are likely to be or reveal whether or not you may experience any secondary complications.

    Negative influenza antigen 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 fatigue and a cough may last awhile longer. A few people, however, may develop serious secondary complications. These complications often arise just as influenza symptoms are fading. Anyone is susceptible to complications from the flu, but the very young, the elderly, and patients who are immune compromised or who have pre-existing lung disease are most affected. Complications such as pneumonia, sepsis, and encephalitis can be very serious and may require immediate medical treatment.
  • What other tests might my doctor order to diagnose my flu-like symptoms?
    Your doctor may order a strep test to check for strep throat, blood cultures to check for bacterial infections in the blood, an RSV antigen 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 CMP (Comprehensive Metabolic Panel) or CBC (Complete blood count) to monitor body organ function.
  • Why is the flu such a big deal?
    It is 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, which killed more than 20 million people worldwide, 500,000 in the United States alone. In 1957 and 1968 hundreds of thousands died in the U.S. from Asian and Hong Kong flu variants. The new strain of influenza A, named for its unique viral antigens (H5N1), is circulating in birds and chickens in Asia. It has infected humans who have had close contact with the infected birds and has a high morality rate. There is concern of a world-wide 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 (called "antigenic drift") that allow it to evade the protections of last year's flu shot and re-infect you. The amount of the antigen drift varies from year to year. Bigger drifts 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 United States the next season. Each year the flu vaccine is produced based on their observations and experience, targeted to protect us against the expected strain. It contains inactivated virus to protect us against both influenza A and B.

    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 "breakthrough" -- there will have been enough antigenic drift during the season that the virus will appear slightly different to the body's immune system, decreasing the effectiveness of the vaccine's protection. Or, the flu that predominates may end up being an unexpected strain, not the ones that the vaccine was developed to protect against. Usually in these cases the vaccination will at least lessen the severity of the infection. In individual high risk patients (those with heart, kidney, and lung ailments for instance), doctors may bolster the protection by prescribing anti-viral treatments as prophylactics -- they can provide short term protection while influenza moves through the community (although this comes at a price in terms of cost and medication side effects).
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