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Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors

Information for Clinical Laboratory Directors

Rapid influenza diagnostic tests (RIDTs) detect influenza viral antigens in respiratory tract specimens. Available RIDTS detect and differentiate between type influenza A and B viruses, but do not specifically identify or differentiate subtypes of influenza A viruses. RIDTs are not recommended for use in hospitalized patients with suspected influenza. Molecular assays, including RT-PCR, are recommended for testing respiratory tract specimens from hospitalized patients because of their high sensitivity and high specificity.

  • RIDTs can provide results within approximately 15 minutes.
  • Some tests are waived from requirements under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) and cleared for point-of-care use.
  • Most RIDTs are immunoassays that utilize antibodies against the nucleoproteins of influenza A and B viruses to detect viral antigens.
  • One RIDT is an immunofluorescence assay
  • Some RIDTs utilize a digital analyzer reader device to standardize result interpretation.
    • RIDTs that utilize an analyzer reader device have higher sensitivities compared to RT-PCR than RIDTs without reader devices.
    • RIDTs without analyzer reader devices have low to moderate sensitivities compared to RT-PCR.
    • All RIDTs have high specificities compared to RT-PCR.
  • RIDTs are approved for specific kinds of respiratory specimens
    • Specimens to be used with RIDTs should be collected as close as is possible to the start of symptoms (e.g., less than 4 days after illness onset). In very young children, influenza viruses can be shed for longer periods; therefore, in some instances, testing for a few days after this period may still detect influenza viruses. Immunosuppressed persons may have detectable influenza viruses in respiratory specimens for prolonged periods (weeks to months).
  • For a list of currently available RIDTs and the respiratory specimens approved for the tests by the U.S. Food and Drug Administration (FDA), see Table 2: Rapid Influenza Diagnostic Tests (RIDTs).
Recent Developments
  • In 2017, the FDA re-classified RIDTs from class I to class II devices, and now requires RIDTs to meet specific minimum criteria for sensitivity and specificity.
Sensitivity
  • Compared to RT-PCR, FDA-cleared RIDTs must achieve 80% sensitivity for detection of influenza A and influenza B viruses.
  • Compared to viral culture, FDA-cleared RIDTs must achieve 90% sensitivity for detection of influenza A and 80% sensitivity to detect influenza B viruses.
Specificity
  • Compared to RT-PCR, FDA-cleared RIDTs must achieve 95% specificity for detection of influenza A and influenza B viruses.
  • Compared to viral culture, FDA-cleared RIDTs must achieve 95% specificity for detection of influenza A and influenza B viruses.
Interpretation of Testing Results

Proper interpretation of testing results, particularly negative testing results is very important.

Predictive Value Depends Upon Prevalence

The positive and negative predictive values vary considerably depending upon the prevalence of influenza (level of influenza activity) in the patient population being tested.

  • False-positive (and true-negative) influenza test results are more likely to occur when disease prevalence is low, which is generally at the beginning and end of the influenza season.
  • False-negative (and true-positive) influenza test results are more likely to occur when disease prevalence is high, which is typically at the height of the influenza season.
Clinical Considerations of Testing When Influenza Prevalence is Low

When influenza prevalence is relatively low, the positive predictive value (PPV) is low and false-positive test results are more likely. By contrast, when influenza prevalence is low, the negative predictive value (NPV) is high, and negative results are more likely to be true.

If Influenza Prevalence is… And Specificity is… Then PPV is… False Pos. rate1 is…
VERY LOW (2.5%) MODERATE  (80%) VERY LOW (6-12%) VERY HIGH (88-94%)
VERY LOW (2.5%) HIGH  (98%) LOW  (39-56%) HIGH (44-61%)
MODERATE (20%) MODERATE (80%) LOW  (38-56%) HIGH (44-62%)
MODERATE (20%) HIGH  (98%) HIGH (86-93%) LOW (7-14%)

The false positive rate is the number of false positives divided by the number of total positives, or 1-PPV.

The interpretation of positive results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population being tested (e.g., level of influenza activity in the community). If an important clinical decision is affected by the test result, the RIDT result should be confirmed by a molecular assay, such as reverse transcription polymerase chain reaction (RT-PCR).

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Clinical Considerations of Testing When Influenza Prevalence Is High

When influenza prevalence is relatively high, the NPV is low and false-negative test results are more likely. When influenza prevalence is high, the PPV is high and positive results are more likely to be true.

If Influenza Prevalence is… And Sensitivity is… Then NPV is… False Neg. rate2 is…
MODERATE (20%) LOW  (50%) MODERATE (86-89%) MODERATE (11-14%)
MODERATE (20%) HIGH (90%) HIGH(97-99%)  LOW (2-3%)
HIGH (40%) LOW  (50%) MODERATE (70-75%) MODERATE (25-30%)
HIGH (40%) HIGH (90%) HIGH  (93-94%) LOW (6-7%)

The false negative rate is the number of false negatives/number of total positives, or 1-NPV.

The interpretation of negative results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population being tested (e.g., level of influenza activity in the community). If an important clinical decision is affected by the test result and influenza is still suspected, then the RIDT result should be confirmed by a molecular assay, such as RT-PCR.

Selecting Tests

Many factors should be considered when selecting a test, including the following:

  • Tests with high sensitivity and high specificity will provide higher positive and negative predictive values, respectively.
  • Tests that are CLIA-waived can be used at the point-of-care; tests that are classified as moderately complex are required to be performed in a clinical laboratory.

Information about these characteristics can be found in product inserts and scientific articles, and by contacting the manufacturers.

Changes in Recommended Procedures Can Affect Test Results

Modification by the user can affect test performances and increase false-positive and/or false-negative rates. Such modifications include:

  • Using specimens for which the test is not optimized
  • Using swabs that did not come with the rapid influenza diagnostic test kits [unless recommended (see package insert for specific instructions
  • Improper storage or prolonged storage before specimens are tested
When Is Use of Rapid Diagnostic Tests Beneficial?
  • Testing during an outbreak of acute respiratory disease can determine if influenza is the cause and to guide prompt implementation of infection prevention and control measures.
  • During influenza season, testing of selected patients presenting with acute respiratory illnesses compatible with influenza can help establish whether influenza is present in a specific outpatient population and help health-care providers determine how to use their clinical judgment for diagnosing and treating respiratory illness. (Testing need not be done for all patients.)
  • For outpatients with suspected influenza, rapid molecular assays are recommended over RIDTs because of higher sensitivities of rapid molecular assays to detect influenza A and B viruses in respiratory specimens.
  • For hospitalized patients with suspected influenza, molecular assays are recommended because of their high sensitivities, and RIDTs are not recommended.
  • RIDTs do not address the public health need for influenza virus isolates that can only be obtained through the collection of respiratory specimens for viral culture. Influenza virus isolates are essential for determining the match between circulating influenza virus strains and those virus strains contained in the vaccine and for aiding in the selection of new influenza vaccine virus strains.
  • A positive RIDT result for influenza A virus cannot identify or differentiate between seasonal influenza A viruses circulating among humans and influenza A viruses circulating among animals (e.g. avian influenza A viruses or swine influenza A viruses). If human infection with a novel influenza A virus of animal origin is suspected on the basis of recent exposures to poultry or pigs, the state public health department should be consulted to arrange specific RT-PCR testing for novel influenza A viruses.

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Table 1: Influenza Virus Testing Methods

Table 2: Rapid Influenza Diagnostic Tests (RIDTs)

Table 3: Nucleic Acid Detection Based Tests

Additional Information

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