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Influenza Tests

Also known as: Flu test; Rapid flu test; Influenza antigen test; H1N1; Rapid influenza diagnostic test; RIDT; Influenza by DFA
Formal name: Influenza rapid antigen test; Influenza type A and B antigen detection by direct fluorescent antibody (DFA); Influenza type A and B antibody (IgM, IgG); Influenza A and B culture; Influenza type A H1N1
Related tests: Strep; Blood culture; Pertussis; RSV

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

Sample Required?

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?

None

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.

The Test

How is it used?

Typically, cases of influenza are diagnosed without testing if it is the flu season and it is known that the flu has reached the community. If a person has clinical symptoms that are essentially the same as the symptoms of other people with known cases of the flu, then the doctor may make the diagnosis without testing. If it is outside the 48-hour window of antiviral treatment and there is no evidence of severe illness or secondary complications, the person may just be sent home to rest, drink fluids, and use over-the-counter remedies to soothe their symptoms.

However, the flu can be deadly and knowing that it is in the community can help doctors minimize its spread and treat patients to lessen its severity if it is diagnosed early, so testing may be performed for these reasons.

There are several influenza tests available that may be used to help make a diagnosis. The method used often depends on the circumstances and reason for testing.

  • Rapid influenza diagnostic test (RIDT) – used to detect the virus in nasal secretions and one of the most common methods used 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. It can help differentiate influenza from other viral and bacterial infections with similar symptoms that may be serious and must be treated differently. Rapid flu tests are best used within the first 48 hours of the onset of symptoms to help diagnose influenza and determine whether or not antiviral drugs are a treatment option, Sometimes rapid tests are ordered to help identify outbreaks.

    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. However, none of them are able to differentiate between the strains of influenza A, such as H1N1.

    The main disadvantages of the rapid influenza antigen test are that it will miss up to 30% of influenza cases, may only detect 10-70% of 2009 H1N1 influenza infections, and it will occasionally be positive when someone does not actually have the flu.

  • Direct fluorescent antibody stain (DFA) – detects influenza A or B virus in samples of nasal secretions. This method of testing is more specific than RIDT but requires specialized equipment to perform and special training to interpret, so it is usually not performed in a doctor's office. Samples are typically sent to a laboratory for testing. A negative result with this test does not rule out influenza. However, if positive, influenza A can be distinguished from influenza B with this method, and results may be available within several hours after the specimen is collected so the appropriate antiviral therapy can be initiated. This method cannot distinguish between seasonal influenza A, 2009 H1N1 influenza A, or avian influenza ("bird flu").
  • Viral culture has been considered the "gold standard" for diagnosing influenza, but it can take up to 3 to 10 days for results. It can be used for confirmation of a positive rapid test result. 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, may detect the presence of a respiratory virus in 24 to 48 hours, 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 and which antiviral agents are effective in treating these viruses. Identifying these outbreaks can assist health care workers in the prevention and treatment of the flu throughout a community. Influenza virus grown in culture can 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 or the 2009 H1N1 influenza virus. Viral cultures can also be used to identify other viral infections that cause clinical symptoms similar to the flu.

  • Real-time polymerase chain reaction (RT-PCR) – molecular testing is available that detects viral genetic material in nasal secretions and is the most sensitive test for influenza virus. However, this test is not widely available and results can take from one to several days. These tests may be ordered to diagnose influenza A infection, especially in those who are seriously ill (hospitalized) and to help track influenza outbreaks. Some false negatives can occur with this method. A new version of this test with specific reagents directed toward the 2009 H1N1 influenza virus was developed in April 2009 to allow the detection of this novel flu virus.
  • Influenza A or B antibody tests – these are blood tests performed to detect the body's immune response to an influenza infection. Acute and convalescent samples are required to help confirm that a recent infection was due to the flu. Antibody testing on single blood samples is not useful. These tests are not done routinely and are most often done for public health or research purposes.

The Centers for Disease Control and Prevention (CDC) recommends testing in certain situations:

  • For patients in the hospital 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

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When is it ordered?

Flu tests are ordered almost exclusively during flu season (late fall through early spring). Not everyone who has symptoms of influenza is tested. However, testing may be done within 48 hours of the onset of symptoms when a person has a respiratory infection and symptoms such as: headaches, fever, chills, muscle aches, weakness, fatigue, a stuffy nose, sore throat, cough, and, with some flu strains, diarrhea and vomiting, especially if the person is hospitalized, has a weakened immune system, or is otherwise at an increased risk of serious complications. If influenza has already been identified in the community, the doctor may order a flu test to confirm the diagnosis and validate possible antiviral 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) when the cause of the infection is unclear.

Sometimes testing may be done when someone who died had a severe, acute illness and influenza was the suspected cause.

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.

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What does the test result mean?

If a flu test is positive, it 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 doctor 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.

If the molecular test result (RT-PCR) is positive, then the person has influenza. Some of the tests that use this method can further identify the subtypes of influenza A, such as 2009 H1N1.

A viral culture, if positive, can be used to identify the virus present. If the virus present is influenza, further tests can be performed to characterize which strain is present and its susceptibility to antiviral agents.

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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 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 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.

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Common Questions

1.  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.

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2.  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. A 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 mortality rate. There is concern of a worldwide pandemic if this virus is able to mutate to allow human-to-human transmission of the virus.

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3.  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 protective antibodies formed from previous infections and vaccinations. The amount of the "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. 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 locally the next season. Each year the flu vaccine is produced based on their observations and experience, targeted to protect against the expected strain. It contains inactivated virus to protect 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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Article Sources

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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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