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To determine whether or not you have an influenza infection (flu), especially if you are hospitalized, have a weakened immune system, or are otherwise at an increased risk of serious complications from influenza; to help your healthcare practitioner make treatment decisions; to help determine whether or not the flu has spread to your community
When it is flu season and a healthcare practitioner wants to determine whether your symptoms are due to seasonal influenza A or B or to another cause; within 3 to 4 days of the onset of signs and symptoms, such as fever, headache, muscle aches, sore throat, weakness, fatigue, cough, sore throat, stuffy nose and sometimes runny nose
Depending on the test used, a respiratory sample collected on a swab from the nasopharynx or a nasal aspirate; sometimes, a nasal or throat swab
Influenza (the flu) is a common viral respiratory infection that causes an illness ranging from mild to severe, and sometimes can be fatal. Influenza testing detects the presence of the virus in a sample of respiratory secretions.
Influenza tends to be seasonal, usually beginning in late fall and disappearing in early spring. According to the Centers for Disease Control and Prevention (CDC), influenza affects millions of Americans each season. Signs and symptoms like headache, fever, chills, muscle pains, exhaustion, a stuffy nose, sore throat, and a cough tend to be more severe and longer lasting than the symptoms caused by the common cold.
Two types of influenza virus, A and B, cause annual flu pandemics and most epidemics. Type C can cause mild respiratory illness and is not thought to cause epidemics. There are numerous subtypes of influenza A viruses, and they are named using two designations based on the antigenic components of the virus, hemagglutinin (H) and neuraminidase (N). The most common influenza A viruses currently infecting humans have the subtypes H1N1 and H3N2. (For more on this, see the condition article on Influenza.)
It is common for healthcare practitioners to diagnose and treat the flu without laboratory testing, especially during peak influenza season and when influenza has already been documented in an area. However, influenza testing can help rule out other illnesses and reduce the chances of people using unnecessary antibiotics, while increasing the chances that they will receive anti-viral therapy early in the illness, when it is most effective.
Also, if there is an outbreak of respiratory illness in a confined setting like a hospital, nursing home, or school, diagnostic testing will help determine the cause of the outbreak. Influenza testing also helps local and state health departments and the CDC track influenza in communities. Since the flu virus changes every year, testing also helps the CDC to monitor the subtypes and strains of flu that are circulating that year, to collect information for developing flu vaccines, and to monitor strains for resistance to anti-viral drugs.
There are several different kinds of influenza tests and they serve different purposes. Read the "How is it used?" section to learn more.
Sample collection technique is critical in influenza testing, and different kinds of influenza tests rely on different collection techniques. The best sample is usually a nasal aspirate, but a swab from the nasopharynx or nasal secretions may also be used. In some circumstances, a healthcare practitioner may use a throat swab, but this contains less of the virus than a nasopharyngeal aspirate and so may not be appropriate for use in rapid testing where sensitivity is a concern.
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 nasopharyngeal swab is collected by having the person tip his or her 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 may tickle a bit and cause the eyes to tear.
No test preparation is needed.
Influenza testing may be used to help diagnose an influenza infection (flu) and help make treatment decisions. Sometimes influenza testing is used to help document the presence of influenza in the community or to help determine whether an outbreak of flu is occurring, especially in settings such as nursing homes and schools. Testing may be used to identify the type and/or the strain of influenza causing infections.
Not all cases of influenza require testing for antiviral treatments to be prescribed. Healthcare practitioners may diagnose and treat influenza without testing if the person has typical signs and symptoms of the flu, plus it is the flu season and it is known that the flu has reached a community. If it is outside the 3 to 4 day window when antiviral treatment is most effective and there is no evidence of severe illness or secondary complications, the person may simply be sent home to rest, drink fluids, and soothe symptoms with over-the-counter remedies.
However, the flu can be deadly. Testing can help healthcare practitioners minimize its spread and prescribe antiviral drugs to treat patients to lessen its severity, if it is diagnosed early. The Centers for Disease Control and Prevention (CDC) recommends testing in certain situations:
Testing is also performed to look for new strains of viruses to help prevent possible pandemics, to monitor for antiviral resistance, and prepare for the next year's flu vaccine.
Several types of influenza tests are available. The method used often depends on availability and reason for testing.
Rapid tests vary in their ability to detect influenza. Some types can only detect influenza A; others can detect both A and B but not distinguish between the two. Still others can detect and distinguish between influenza A and B. Some of them are able to further differentiate between the strains of influenza A, such as H1N1.
Additional laboratory tests may be used in conjunction with influenza testing to help rule out other types of infections with similar symptoms and/or if the cause of the infection is unclear. Examples include:
Flu tests are mostly ordered during flu season (late fall through early spring), especially when a person is hospitalized, has a weakened immune systems, or is otherwise at an increased risk of serious complications. Testing is usually ordered within 3 days of the onset of signs and symptoms, such as:
When influenza has not yet been documented in the community, a healthcare practitioner may order a rapid flu test both to document the presence of influenza in the area and to help diagnose an individual's current illness.
Testing may be ordered during outbreaks of respiratory illnesses when influenza is the suspected cause.
Sometimes testing may be done when someone who died had a severe, acute illness and influenza was the suspected cause.
A positive flu test means that the affected person most likely has influenza A or B, and treatment with antiviral medication may be prescribed to minimize symptoms. However, it may not tell the healthcare practitioner which strain of influenza is causing the infection, how severe the symptoms are likely to be, or whether or not a person may experience any secondary complications.
A negative influenza test may mean that the person has something other than influenza, that the test is not detecting the influenza strain, 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 a person has had the flu for several days and less virus is being shed. New strains of influenza may be present that diagnostic tests cannot reliably detect.
Some rapid tests can further identify the subtypes of influenza A, such as H1N1, and some identify other respiratory viruses, such as RSV.
Viral cultures, if positive, identify the virus present. If the virus present is influenza, further tests can be performed to determine which strain is present and its susceptibility to antiviral agents.
Treated or untreated, most influenza infections will go away within one or two weeks, although fatigue and a cough may last a while 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 people who are immunocompromised 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.
Yes. Influenza tests may not detect every case of the flu. Rapid antigen tests generally detect 50-70% of influenza cases, while molecular tests detect 66%-100% of cases, depending on the test used. A negative result may mean that the test is not detecting the influenza strain or that there is not sufficient virus in the sample to allow it to be detected. Also, new strains of influenza may be present in the sample that tests cannot reliably detect.
Your healthcare provider may order a strep test to check for strep throat, an RSV test to check for 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. Your healthcare practitioner may also order blood tests such as a comprehensive metabolic panel (CMP) or complete blood count (CBC) to monitor the health and function of organs such as your lungs and kidneys.
The flu is an important general and public health concern because it can be deadly and because every few decades an especially lethal influenza strain emerges. The worst on record is the 1918 Spanish flu pandemic, which killed 20 to 50 million people worldwide, and more than 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.
In more recent years, there has been international concern about the H5N1 and H7N9 subtypes of influenza A, commonly called avian (bird) flu, and H1N1 (swine) flu. Some of these subtypes have caused outbreaks in humans, and there is concern that they may in the future cause more widespread and serious outbreaks. Read the article on Influenza to learn more about these.
The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months old and older get a flu shot each season. In particular, people who are at high risk of complications from the flu should receive the vaccine. These include young children, the elderly, residents of nursing home and other long-term care facilities, pregnant women, and people with pre-existing conditions such as asthma, COPD, heart disease, and liver or kidney disorders. See the CDC page on Key Facts About Seasonal Flu Vaccine to learn more.
Yes, it is possible to get the flu after being vaccinated since no vaccine is 100% effective. However, if you get the flu after being vaccinated, it is usually a milder illness with a quicker recovery. It also takes a few weeks after vaccination before antibodies are made to the vaccine that can provide protection. Occasionally, a new strain circulates that is not in the current vaccine, so you would not be protected against infection with that strain of the virus.
Each year, the flu vaccine, containing inactivated virus, is targeted to protect against the expected strains, based on the observations and experience of healthcare practitioners and scientists. Healthcare providers and researchers carefully track the influenza virus circulating worldwide and try to anticipate the strain(s) that will eventually appear locally the next season.
As influenza travels through communities around the world, it undergoes spontaneous changes (called "antigenic drift") that allow it to evade the protective antibodies formed from previous infections and vaccinations. The amount of "antigenic drift" varies from year to year. Bigger antigenic drifts, known as "antigenic shifts," result in more severe illnesses since more people will be susceptible to the virus.
Sometimes the flu strain will have significant antigenic drift during the season so 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), healthcare practitioners may bolster the protection by prescribing antiviral treatments before the person gets sick to provide short-term protection while influenza moves through the community.
Sources Used in Current Review
(October 26, 2016) Centers for Disease Control and Prevention. Influenza Signs and Symptoms and the Role of Laboratory Diagnostics. Available online at https://www.cdc.gov/flu/professionals/diagnosis/labrolesprocedures.htm. Accessed December 2016.
(September 23, 2016) Centers for Disease Control and Prevention. Influenza Virus Testing Methods. Available online at https://www.cdc.gov/flu/professionals/diagnosis/table-testing-methods.htm. Accessed December 2016.
(October 25, 2016) Centers for Disease Control and Prevention. Rapid Diagnostic Testing for Influenza: Information for Health Care Professionals. Available online at https://www.cdc.gov/flu/professionals/diagnosis/rapidclin.htm. Accessed December 2016.
(October 14, 2016) Centers for Disease Control and Prevention. Key Facts About Seasonal Flu Vaccine. Available online at https://www.cdc.gov/flu/protect/keyfacts.htm. Accessed January 2017
(October 25, 2016) Centers for Disease Control and Prevention. Guidance for Clinicians on the Use of RT-PCR and Other Molecular Assays for Diagnosis of Influenza Virus Infection. Available online at https://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm. Accessed December 2016.
(July 19, 2016) National Institute of Allergy and Infectious Diseases. Influenza. Available online at https://www.niaid.nih.gov/diseases-conditions/influenza. Accessed December 2016.
Sources Used in Previous Reviews
Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].
Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.
Ault, A. (2002 October 7). CDC Urges Flu, Pneumococcal Shots for Seniors. MedlinePlus & Reuters Health Information [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/news/fullstory_9746.html.
NIAID (2001 July). Flu. Fact Sheet, National Institute of Allergy and Infectious Diseases, National Institutes of Health [On-line information]. Available online at http://www.niaid.nih.gov/factsheets/flu.htm.
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.
CDC (2002 July 9, Reviewed). Influenza: The Disease. National Center for Infectious Diseases [On-line information]. Available online at http://www.cdc.gov/ncidod/diseases/flu/fluinfo.
CDC (2002 July 9, Reviewed). Antiviral Drug Information. National Center for Infectious Diseases [On-line information]. Available online at http://www.cdc.gov/ncidod/diseases/flu/fluviral.htm.
CDC (2002). Laboratory Diagnostic Procedures for Influenza, Diagnosis of influenza. National Center for Infectious Diseases [On-line information]. Available online at http://www.cdc.gov/ncidod/diseases/flu/flu_dx_table.htm.
CDC (2001 November 27, Modified). Inhalational Anthrax, Influenza, or Influenza-like Illness? Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/nip/Flu/MMWR_summary_11-9.htm.
Hurtado, R. (2002 March 1). Influenza. MedlinePlus Health Information, Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000080.htm.
Montato, N., et. al. (2000 December 1). Updated Treatment for Influenza A and B. American Family Physician [On-line information]. Available online at http://www.aafp.org/afp/20001201/2467.html.
Kingston, B. and Wright, C. (2002 January 1). Influenza in the Nursing Home. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20020101/75.html.
Baylor College of Medicine (2002). Virology Question of the Month, How accurate are the new rapid tests for flu? Diagnostic Virology Laboratory Newsletter [On-line newsletter]. Available online at http://www.bcm.tmc.edu/pedi/infect/dvl/nws10_2001.htm.
Locke, J. (2001 July 9). Rapid Infectious Disease Testing (Strep and Flu Diagnostics) - The Promise and Pitfalls. Clinical Laboratory Strategies [Unpublished version of published article]. Available E-mail: [email protected]
Harvard (2002 October). Fleeing crummy? Is it the flu? Harvard Health Letter [Newsletter]. Associated online at http://www.health.harvard.edu.
College of American Pathology (2002 October 09). CLIA Waived Test List. CAP Laboratory Improvement, Surveys/EXCEL (On-line serial). Available online at http://www.cap.org/html/lip/waived.html.
(Updated 2009 December 24). H1N1 Flu: Diagnosis and Lab Testing. Centers for Disease Control and Prevention, [On-line information]. Available online at http://www.cdc.gov/h1n1flu/diagnosis/. Accessed March 2010.
Chang, H. (2009 November 4). Influenza. JAMA. 2009;302(17):1926 On-line information]. Available online at http://jama.ama-assn.org/cgi/reprint/302/17/1926.pdf. Accessed March 2010.
(Updated 2009 December 7). Updated Interim Recommendations for the Use of Antiviral Medications in the Treatment and Prevention of Influenza for the 2009-2010 Season. Centers for Disease Control and Prevention, [On-line information]. Available online at http://www.cdc.gov/h1n1flu/recommendations.htm. Accessed March 2010.
(2009 September 29). Questions & Answers, Influenza Diagnostic Testing During the 2009-2010 Flu Season. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/diagnostic_testing_public_qa.htm. Accessed March 2010.
(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.
Derlet, R. et. al. (Updated 2009 December 8). Influenza. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/219557-overview. Accessed March 2010.
Bronze, M. S. (Updated 2010 January 27). H1N1 Influenza (Swine Flu) eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/1807048-overview. Accessed March 2010.
Katz, M. et. al. (2009 March 13). Use of Rapid Tests and Antiviral Medications for Influenza Among Primary Care Providers in the United States. Medscape Today from Influenza Resp Viruses. 2009;3(1):29-35 [On-line information]. Available online at http://www.medscape.com/viewarticle/588778. Accessed March 2010.
(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. Accessed March 2010.
Hillyard, D. et. al. (Updated 2010 February). Influenza Virus. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/InfluenzaVirus.html?client_ID=LTD. Accessed March 2010.
Mayo Clinic Staff (2009 September 30). H1N1 flu (swine flu). MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/swine-flu/DS01144. Accessed March 2010.
Mayo Clinic Staff (2009 September 11). Influenza (flu). MayoClinic.com [On-line information]. Available online at http://www.mayoclinic.com/health/influenza/DS00081. Accessed March 2010.
(2009 August 10). Interim Guidance for the Detection of Novel Influenza A Virus Using Rapid Influenza Diagnostic Tests. Centers for Disease Control and Prevention [On-line information]. Available online at http://www.cdc.gov/h1n1flu/guidance/rapid_testing.htm. Accessed March 2010.
(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. Accessed March 2010.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1225, 1578.
Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 729.
Reviewed March 2013. Influenza Virus. Arup Consult. Available online at http://www.arupconsult.com/Topics/InfluenzaVirus.html. Accessed November 2013.
Reviewed December 2012. Laboratory Support for Surveillance of Vaccine-Preventable Diseases. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/vaccines/pubs/surv-manual/chpt22-lab-support.html. Accessed November 2013.
Reviewed March 2012. Guidance for Clinicians on the Use of RT-PCR and Other Molecular Assays for Diagnosis of Influenza Virus Infection. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm. Accessed November 2013.
Reviewed December 2011. Influenza Symptoms and the Role of Laboratory Diagnostics. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/professionals/diagnosis/labrolesprocedures.htm. Accessed November 2013.
Reviewed October 2013. Overview of Influenza Surveillance in the United States. Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/flu/weekly/overview.htm. Accessed November 2013.
Updated February 2012. Diagnostic Influenza Tests. Medscape. Available online at http://emedicine.medscape.com/article/2053517-overview. Accessed November 2013.