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Flu test
Rapid flu test
Influenza antigen test
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Influenza tests
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Este artigo foi modificado pela última vez em 10 de Julho de 2017.
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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).