What Is Influenza?

Influenza, also known as the flu, is a respiratory disease caused by infection with the influenza type A, B, C or D virus. Influenza viruses are spread among people primarily through airborne droplets released when you talk, cough, or sneeze.

The flu is a common disease that is believed to affect around 8% of people in any given year. Flu viruses tend to spread more frequently during colder months and can lead to local seasonal epidemics. Much less often, new strains of flu spread rapidly across the world and cause a pandemic.

The flu can cause a wide range of symptoms including:

  • Fever
  • Cough
  • Congestion
  • Runny nose
  • Sore throat
  • Head and/or body aches
  • Fatigue

For most people, flu symptoms are mild and short-lived. In others, though, the flu can cause life-threatening complications. Children, older adults, pregnant women, and people with certain coexisting medical problems are at a higher risk of serious complications from an influenza virus infection.

There are multiple types of influenza viruses that can cause the flu. Influenza A and influenza B are the only types of influenza that can cause seasonal flu epidemics. An epidemic is a disease outbreak that occurs within a specific geographic area.

Influenza A viruses are classified into many subtypes based on specific proteins on the surface of the virus. As influenza viruses circulate, they undergo genetic changes that can affect their infectiousness and severity. Although uncommon, emergence of a new strain of influenza A can cause a flu pandemic. A pandemic is a global disease outbreak.

Influenza C is far less common and generally only causes mild symptoms while influenza D viruses primarily affect cattle.

A flu vaccine is developed every year based on the strains of influenza viruses that are expected to be circulating most widely in the colder months. Vaccination can reduce the impact of the flu but is not 100% effective. As a result, diagnosis and treatment of influenza can be important for many people.

It is important to note that “the flu” refers only to illnesses caused by an influenza virus infection. While some people may casually use the term “stomach flu” or refer to any condition with respiratory symptoms as a flu, these are inaccurate uses of the term.

The Role of Influenza Tests

Influenza tests can serve several different purposes for both individual patients and the overall population.

In individuals, flu tests are used mainly to diagnose whether a person has an influenza virus infection. This can help doctors determine the cause of the symptoms and differentiate between the flu and other viral or bacterial infections. With some tests, the same test sample can be used to detect the flu and other infections, including SARS-CoV-2, the virus that causes COVID-19.

For certain patients, diagnosing influenza can help optimize and speed up treatment and avoid unnecessary or unhelpful medications. For example, influenza testing can assess whether a patient may benefit from antiviral treatments that may be effective against the flu.

Identifying an influenza infection may also help slow the transmission of the virus. A person who is known to have the flu can follow recommendations to avoid spreading it to others. When respiratory infection outbreaks occur in places like schools, cruises, nursing homes, or hospitals, testing for the flu can aid in understanding and controlling the outbreak.

On a population level, influenza testing can help evaluate the type of virus, including the type or subtype of influenza, that is having the greatest effect in a community.

This data advances influenza research. Detailed analysis of large-scale flu testing is used for disease surveillance, which monitors how different strains of influenza evolve over the course of the year. This information enables the seasonal flu vaccine to be more targeted to the subtypes of influenza viruses that are most likely to be widespread in that year.

Who should get testing?

Flu tests are typically only performed when you have symptoms that are suggestive of a possible influenza virus infection. Several factors can influence whether flu testing is likely to be beneficial, including:

  • Whether there is extensive transmission of influenza viruses in your community
  • Whether you are at higher risk of severe complications from the flu
  • Whether the test result will change your treatment plan
  • Whether you have gotten sick as part of a respiratory disease outbreak in a school, nursing home, cruise ship, or similar situation where flu diagnosis may influence infection control measures

In many cases, especially if you have mild symptoms or are not at high risk of complications, a flu test is unnecessary because it will not change the course of your medical care. Similarly, if an influenza virus is spreading in your area and you have symptoms that are consistent with the flu, a diagnosis of influenza can be made without a flu test.

In contrast, if you have severe symptoms, are being hospitalized, or have an elevated risk of developing life-threatening complications, you are more likely to have flu testing. Testing is also generally done more often during seasonal periods when influenza virus transmission is high.

Getting test results

In many cases, you will get results from a flu test promptly. Many rapid tests are available that provide results within 10 to 30 minutes and without sending a sample to a laboratory. When laboratory analysis is needed, you may get results within a few hours or the next day.

The results from a flu test are reported as either positive or negative. The result may be communicated to you directly, and in some cases you may receive a formal test report that shows the result.

Interpretation of your results involves more than just seeing whether the test was positive or negative. The doctor also reviews the type of test used, how and when the test was conducted, your symptoms, and the extent of influenza virus transmission in your area.

These considerations are important because no flu test is 100% accurate in identifying influenza infection. It is possible for tests to return a false-negative or a false-positive result. A false-negative result is when you test negative but actually have an influenza infection. A false-positive result is when you test positive but don’t actually have an infection.

Several factors can play an important role in the possibility of a false-negative or false-positive result on influenza tests:

  • Type of test: The chance of a false-positive or false-negative result is higher with certain types of tests, such as rapid antigen flu tests. False negatives are more common than false positives, especially with rapid tests.
  • Level of community flu transmission: If your area is experiencing a seasonal flu epidemic, there is a much higher probability that you’ve been exposed to an influenza virus. For this reason, false negatives occur more often when community flu transmission is high, and false positives occur more often when community spread is low.
  • Timing of the test: The ability to detect signs of an influenza infection change depending on when an infection occurred. Amounts of the virus in the respiratory tract are usually highest during the first three to four days, so testing after this period can increase the chances of a false negative.
  • Type of test sample: Most influenza tests are flu swab tests, meaning that a cotton swab is used to obtain a sample from your nose, throat, or the area behind the nose and throat known as the nasopharynx. The type of sample taken must match the requirements outlined by the test manufacturer. The quality of the test sample can affect test results.
  • Vaccination status: Some types of flu vaccines use a live but weakened version of the virus to enable the body’s immune response to influenza. These vaccines do not cause the flu, but they may cause a positive result if influenza testing is performed within a few days after receiving this kind of vaccine.

The doctor reviews these factors and your symptoms when deciding whether you should be diagnosed with the flu. In general, a positive test result is suggestive of an influenza infection, but does not always mean that you actually have the flu.

In interpreting test results, the doctor may also consider whether other viral or bacterial infections are present. It is possible to have the flu at the same time as another infection. Some tests are designed to check for flu and other viruses, and the results of these tests will list separate results for each component of the test.

Types of Influenza Tests

The first step in diagnosing influenza is almost always a physical examination that includes a review of any active flu-like symptoms. During seasonal flu epidemics, additional testing beyond a physical exam may not be needed to diagnose influenza.

When other tests are needed, they focus on identifying signs of an influenza infection. There are several different tests that can check for the presence of an influenza virus. The choice of which test to prescribe depends on the purpose of testing, how quickly results are needed, test availability, and laboratory capabilities.

One important way that influenza tests can be categorized is based on where the analysis is conducted:

  • Rapid, point-of-care tests, sometimes called rapid influenza diagnostic tests (RIDTs),  are able to determine whether the test is positive or negative without sending the sample to a lab. These tests usually provide results within 10 to 30 minutes. The design of these tests allows them to be used in many settings including doctor’s offices, health clinics, and pharmacies.
  • Laboratory tests require that the sample be analyzed using equipment or methods that require the use of a medical lab. These tests often take several hours to complete and are not as widely available as rapid tests.

Another important way of categorizing tests is based on how they detect signs of an infection:

Antigen tests use a sample from the respiratory tract and look for antigens of an influenza virus. Antigens exist on the surface of the virus and trigger an immune response. The presence of influenza antigens can be an indication of an active flu infection.

Most antigen flu tests are rapid, point-of-care tests that don’t require sending results to a laboratory. This is the most common type of flu testing.  Rapid antigen tests may provide a visual, color-coded result, or the result may be shown on a small device that analyzes the sample. In general, rapid antigen tests are the most likely to deliver a false negative test result.

Antigen testing can also be done in a laboratory. Laboratory-based antigen testing can take longer but may provide more accurate results.

Some versions of rapid antigen testing can determine whether the infection is with influenza A or influenza B. Other rapid antigen tests cannot distinguish between these types of influenza. No rapid antigen tests that are currently available can distinguish between subtypes of influenza A.

Molecular flu tests look for traces of genetic material, or DNA, from the influenza virus in a sample from your respiratory tract. These may also be called nucleic acid amplification tests (NAATs) or referred to by a specific laboratory method, such as reverse transcription polymerase chain reaction, (RT-PCR).

Molecular tests can be rapid, point-of-care tests or laboratory-based tests. Molecular tests conducted in a laboratory tend to be more accurate, but both rapid and lab-based molecular tests miss fewer cases of influenza virus infection than antigen tests. For this reason, molecular tests may be used to confirm the results of antigen tests.

Many molecular flu tests are designed to distinguish between influenza A and B infections. Some of these tests can also identify specific subtypes of influenza A and/or detect infection with an unknown or new influenza A subtype.

Some laboratories offer molecular tests that check for influenza as well as other respiratory infections, including COVID-19. This is known as multiplex testing, and it is most often used in patients who are hospitalized or at higher risk from infection because of a weakened immune system.

Viral culture tests are conducted in a laboratory and involve using your test sample to grow more copies of the virus in a vial. These tests can take from 1-10 days to complete, so they are rarely used for diagnosis or guiding treatment.

The main use of viral culture tests is to identify specific types and subtypes of influenza that are being transmitted. This can help researchers and public health officials monitor epidemics and design annual flu vaccines. In some cases, viral culture tests can be used to confirm the results of other tests, especially in specific outbreaks, such as in a nursing home or similar environment.

Serology tests look for antibodies to influenza viruses in a blood sample. Antibodies are developed as part of the immune system’s reaction to a viral infection. It can take time for antibodies to form, though, so serology tests are used to identify a prior infection.

Because they do not reliably detect active infections, antibody serology tests are not used for diagnosis or to influence treatment in people with influenza. Instead, serology testing is principally done for research purposes to evaluate the extent of prior influenza infections in a certain area or group of people.

In addition to tests to detect an influenza infection, flu testing may involve blood tests, chest x-rays, monitoring of oxygen levels, and other tests that can evaluate potential complications from an influenza infection.

Getting Tested for Influenza

Tests for influenza infection are often ordered by a doctor and conducted in a hospital, doctor’s office, or laboratory. Rapid flu tests may also be done in outpatient settings like health clinics or pharmacies.

Testing for influenza normally involves a swab of the area in the back of the nose, the inside of the nostril, and/or the throat. For most people, obtaining a sample is not painful and can be done in under a minute.

In addition to flu swabs, testing can be done with nasopharyngeal aspirates, which are samples removed through the nose with a suction device. Because they are more difficult and uncomfortable to obtain, nasopharyngeal aspirates are not commonly used for diagnostic testing.

The only type of flu testing that does not use a sample from the respiratory tract is serology testing, which is not used for flu diagnosis. Serology tests require a blood sample that is typically taken from a vein in the arm.

At-home testing

A few options are available for at-home flu tests, and they can be divided into two main categories:

  • Self-tests allow you to take a swab sample and conduct the test analysis at home. These are rapid antigen tests that offer results in about 10-15 minutes.
  • Self-collection tests involve taking a nasal swab and/or saliva sample and sending it to a lab where it can be analyzed. Most self-collection tests involve molecular analysis such as with RT-PCR.

Some at-home flu tests also include the ability to test the same sample for COVID-19.

Some at-home flu tests are only available when ordered by a doctor while others can be purchased without a prescription.

It is important to remember that the results of flu tests require careful interpretation that accounts for individual symptoms as well as the type of testing and the extent of community flu transmission. For this reason, at-home influenza tests should always be reviewed with a doctor.

Comparing Tests

Because there are multiple types of influenza tests, many people are unsure about the different kinds of tests and why one may be used instead of another.

For normal flu diagnosis, most experts prefer to use molecular influenza tests, including with RT-PCR, over antigen tests. Molecular tests have a lower rate of false negative results, are more often able to identify flu subtypes, and can be designed to test for other viruses at the same time.

However, molecular testing is not always available. Some medical facilities or laboratories may not have the capability to conduct molecular flu tests or to perform testing promptly. In these cases, rapid antigen testing provides a convenient and lower-cost way to identify an influenza infection. In some cases, molecular tests can be used to verify the results of an antigen test.

While molecular tests are strongly suggestive of an active viral infection, viral culture tests are the only definitive way to know that a sample contains influenza virus capable of replicating itself and spreading. However, because viral culture tests take longer to provide results, they are not frequently used in patient care.

Serology tests are different from other flu tests because they use a blood sample and because they are not designed to detect a current infection. Instead, these tests are used for disease research by identifying the extent of prior influenza cases in a given place or group of people.

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