At a Glance
Why Get Tested?
To determine whether or not you have an influenza viral infection; to help your doctor make rapid treatment decisions; 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 symptoms are due to seasonal influenza A or B, due to the 2009 H1N1 influenza A virus, or to other causes; within 48 hours of the onset of your symptoms, to help determine treatment options
Depends on the requirements for the manufacturer's product being used; usually a nasopharyngeal (NP) swab or a nasal aspirate; under approved circumstances, a throat swab; sometimes, for antibody testing, a blood sample drawn from a vein in the arm
Test Preparation Needed?
The Test Sample
What is being tested?
Influenza (the flu) is a viral respiratory infection that tends to be seasonal, usually beginning in late fall and disappearing in early spring. It is a common respiratory illness that affects 30 to 50 million Americans each season. Symptoms of influenza (headaches, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough) tend to be more severe and longer lasting than the flu-like symptoms caused by the common cold. According to the CDC (Centers for Disease Control and Prevention), about 36,000 people a year die from flu-related complications in the U.S. and more than 200,000 people are hospitalized, especially the very young, elderly, those who are pregnant, and those with compromised immune systems or pre-existing lung disease.
There are two types of influenza, A and B, that commonly infect humans, and each virus can mutate to develop 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. For the 2009-2010 flu season, there was a seasonal influenza A strain as well as a new strain identified as 2009 H1N1 influenza A (sometimes called swine flu). According to the CDC, this new strain has caused the majority of influenza infections in 2009-2010, and the World Health Organization (WHO) has declared the 2009 H1N1 virus a pandemic, since it caused multiple influenza outbreaks throughout the world at the same time.
Flu testing relies on detecting either the virus that is being shed in the respiratory secretions or detecting antibodies directed against the virus in the blood of the person infected. In nasal secretions, 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. Antiviral 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 person is sick by about a day. Influenza testing can be used to help diagnose the flu and determine treatment options for an individual person, and it can be used to help rule out the flu when looking for other illnesses.
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 sample is the nasopharyngeal (NP) swab. For an aspirate, the person collecting the sample will use a syringe to push a small amount of sterile saline into the nose, then apply gentle suction to collect the resulting fluid (saline and mucus). To preserve the organisms in the sample, the sample is put into a special container, referred to as "viral transport media" or VTM, for delivery to the laboratory.
The NP swab is collected by having the person tip their head back, then a Dacron swab (like a long Q-tip® ) is gently inserted into one of the 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 the 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.
For antibody testing, a blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Sources Used in Current Review
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(2010 February 10). Questions & Answers, 2009 H1N1 Flu ("Swine Flu") and You. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov. Accessed March 2010.
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(2009 November 13). Fact Sheet for Healthcare Providers: Interpreting ELITech Molecular Diagnostics 2009-H1N1 Influenza A virus Real-Time RT-PCR Test Results ARUP Laboratories [On-line information]. PDF available for download at http://www.arup-lab.com/Testing-Information/resources/HotLines/H1N1-HealthcareProviders-InterpretingTestResults.pdf through http://www.arup-lab.com. Accessed March 2010.
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(Updated 2010 March 8). CDC Estimates of 2009 H1N1 Influenza Cases, Hospitalizations and Deaths in the United States, April 2009 – February 13, 2010. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/estimates_2009_h1n1.htm through http://www.cdc.gov. Accessed March 2010.
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Sources Used in Previous Reviews
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CDC (2002 September 20, Updated). Flu Facts for Everyone. Centers For Disease Control (CDC) [On-line information]. Available online at http://www.cdc.gov/nip/Flu/Public.htm through http://www.cdc.gov.
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