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. Photo source: National Institute of Allergy and Infectious Diseases

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

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 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:

• RSV test—to detect respiratory syncytial virus, a virus that often infects young children and the elderly
• Strep test—to check for group A streptococcus, the bacteria that cause strep throat

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:

• Headaches
• Fever, chills
• Muscle aches
• Weakness, fatigue
• Stuffy nose
• Sore throat
• Cough
• 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.

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.