Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors

The availability and use of rapid influenza diagnostic tests (RIDTs) to detect influenza viral antigens in respiratory tract specimens by laboratories and clinics have increased in recent years.

• Rapid influenza diagnostic tests (RIDTs) are screening tests for influenza virus infection.
• They can provide results within approximately 15 minutes.
• More than 10 RIDTs have been approved by the U.S. Food and Drug Administration (FDA) (see Influenza Diagnostic Table).
• Some rapid influenza diagnostic tests utilize an analyzer reader device to standardize result interpretation.
  • One RIDT that uses an analyzer device is an immunoassay
  • One rapid immunofluorescence assay uses an analyzer device
• RIDTs differ in some important respects:
  • Some can identify influenza A and B viral antigens and distinguish between them in respiratory specimens.
  • Some can identify influenza A and B viral antigens but cannot distinguish between them in respiratory specimens.
  • Some tests are waived from requirements under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) and cleared for point-of-care use.
  • Most tests can be used with a variety of respiratory specimen types (see Influenza Diagnostic Table), but the accuracy of the tests can vary based on the type of specimen collected (for example throat swab versus nasal swab).
• FDA approval is based upon specific specimen types.
• RIDTs vary in terms of sensitivity and specificity when compared with viral culture or RT-PCR. Product insert information and research publications indicate that:
  • Sensitivities are generally approximately 50-70%
  • Specificities are generally approximately 90-95%
• Specimens to be used with RIDTs generally 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).

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 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 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 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 patient 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.)
• Otherwise, RIDTs do not address the public health need for influenza virus isolates that can only be obtained through the collection of 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 vaccine strains.

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Additional Information

• State Health Departments