How Do Doctors Test for Flu? RIDTs

rapid influenza diagnostic tests
Your doctor may test for the flu using rapid influenza diagnostic tests (RIDTs)

Influenza, or the flu, is an illness of the respiratory system that spreads easily through water droplets in the air from coughs or sneezes of infected people. Influenza A is more common than influenza B. 

Your doctor will usually be able to tell whether you have the flu based on your symptoms. But in addition to doing a complete physical examination, they may want to order a test to confirm whether the influenza virus is causing your illness and not something else.

Rapid influenza diagnostic tests (RIDTs) can detect influenza viral antigens in respiratory specimens. Other tests include:

  • Immunofluorescence antibody test
  • Viral culture
  • Reverse transcriptase polymerase chain reaction (RT-PCR) test for influenza deoxyribonucleic acid (DNA)

The RT-PCR can identify the presence of influenza A and B viral particles in respiratory specimens (nasal and throat fluids).

Why do you need a rapid influenza diagnostic test?

Symptoms of influenza often show up suddenly, about 1-4 days after getting infected. Most people can recover from the virus within a few days to less than 2 weeks. 

Your doctor may suggest RIDTs if you have been experiencing symptoms such as:

How are samples collected for rapid influenza diagnostic tests?

There are two main methods of getting a sample for testing:

  1. Swab test: Your doctor will use a special swab to take a sample from your nose or throat.
  2. Nasal aspirate: Your doctor  will inject a saline solution into your nose and then remove a sample with gentle suction.

These samples are checked for the presence of viral particles. A positive result means that the rapid influenza diagnostic test (RIDT) detected the influenza viral particle in the fluid. However, this does not necessarily mean the virus is alive and thriving in the body because it may also detect a dead viral particle.

  • A positive result is most likely a true positive result if the respiratory specimen was collected close to illness onset (within 4 days) during periods of high influenza activity. 
  • A negative result simply means that the RIDT did not detect an influenza viral antigen and could be a false-negative.

What are the pros and cons of rapid influenza diagnostic tests?

Advantages of the test

  • RIDTs  produce quick results in less than 15 minutes and are simple to perform.
  • Some RIDTs are cleared for office/bedside use.

Disadvantages of the test

  • Sub-optimal test sensitivity and false-negative results are common, especially when influenza activity is high.
  • Sensitivity of RIDTs used to detect influenza B viral antigens is lower than those used to detect influenza A viral antigens.
  • Although specificity is high, false-positive results can also occur, especially during times when influenza activity is low.
  • Some RIDTs distinguish between influenza A and B viruses, while others do not.
  • RIDTs that provide results on the type of influenza virus (A or B) do not provide information on influenza A virus subtype or specific virus strain. RIDTs cannot distinguish between seasonal influenza A virus infection and novel influenza A virus infection (due to infection with bird or variant influenza A viruses).

What factors can influence results of rapid influenza diagnostic tests?

Many factors can influence the accuracy of RIDTs, including:

  • Clinical signs and symptoms consistent with the flu
  • Prevalence of influenza infection in the population tested
  • Time from illness onset to collection of respiratory specimens for testing
  • Type of respiratory specimen tested
  • Accuracy of the test compared to a reference test (the “gold standard” is RT-PCR or viral culture)


Which illness is known as a viral upper respiratory tract infection? See Answer

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