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
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.
How is the sample collected for testing?
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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is the test used?
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:
- For patients in the hospital, or with high-risk conditions, who are suspected of having the flu
- For cases in which an influenza diagnosis will guide care or affect actions taken to control the spread of the virus to others in close contact and/or in the community
- For people who had severe symptoms and died from a suspected case of influenza
Several types of influenza tests are available. The method used often depends on availability and reason for testing.
- Rapid flu tests—depending on the method, a flu test may be completed in a healthcare practitioner's office or near a hospital patient's bedside in 20 minutes or less, or the sample may be sent to a laboratory, with the results available the same day.
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.
- Rapid influenza diagnostic test antigen detection – these tests detect viral antigens in nasal secretions. One main disadvantage of the rapid influenza antigen test is the high rate of false-negative results. Rapid antigen tests generally detect 50-70% of influenza cases. Therefore, the CDC recommends not withholding treatment from people with suspected influenza, even if they test negative. If confirmation is needed, a negative test may be followed by a viral culture or a more sensitive molecular test. Rapid antigen tests will occasionally be positive when someone does not actually have the flu.
- Real Time Reverse Transcription Polymerase Chain Reaction (RT-PCR) and other molecular tests – these tests detect viral genetic material (RNA) in respiratory samples. They are generally more sensitive and specific for the influenza virus than rapid antigen detection tests. They can more accurately detect the virus when it is present and rule it out when it is not. Depending on the test used, they will identify 66% to 100% of influenza cases. Some versions of molecular tests performed in laboratories simultaneously test for the presence of multiple respiratory viruses, such as influenza, respiratory syncytial virus (RSV), and rhinovirus.
- Viral culture—in this test, the virus is actually grown and further identified in the laboratory as influenza A or B and the strain present, or as another respiratory virus. The availability of viral cultures is decreasing as most laboratories adopt molecular assays to detect viral infections. Viral cultures are costly and more difficult to perform and take up to 3 to 10 days to provide a result, which makes them less useful for determining whether or not someone has the flu and for making treatment decisions. Viral cultures may sometimes be used for confirmation of a positive or negative rapid test result. Influenza virus grown in culture can be sent to a public health laboratory to determine if the strain of influenza A is H5N1, found in birds and chickens, or the 2009 H1N1 influenza virus.
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:
When is it ordered?
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:
- Fever, chills
- Muscle aches
- Weakness, fatigue
- Stuffy nose
- Sore throat
- With some flu strains, diarrhea and vomiting
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.
What does the test result mean?
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.
Is there anything else I should know?
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.
Can I test negative and still have the flu?
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.
What other tests might my doctor order to diagnose my flu-like symptoms?
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.
Why is the flu such a big deal?
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.
Should I get a flu shot?
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.
Can I still get the flu if I've had the vaccine?
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.
How do they decide what strains are in the vaccine each year?
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.
On This Site
Conditions: Influenza, Pneumonia, Lung Diseases
In the News: Influenza B Appears Early in Flu Season (2020), Widespread Flu Affecting Most of the U.S. This Season (2018)
Elsewhere On The Web
Centers for Disease Control and Prevention (CDC): Influenza (Flu)
Infectious Diseases Society of America: Influenza
CDC: Test Your Flu IQ
CDC: Swine Influenza (Swine Flu)
CDC: Avian Influenza (Bird Flu)
MedlinePlus Health Information: Flu
World Health Organization: Influenza