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Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors

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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Table: Influenza Virus Testing Methods
Method1 Types Detected Acceptable Specimens2 Test Time CLIA Waived3
Rapid Influenza Diagnostic Tests4 (antigen detection) A and B NP5 swab, aspirate or wash, nasal swab, aspirate or wash,  throat swab <15 min. Yes/No
Rapid Molecular Assay [influenza viral RNA or nucleic acid detection] A and B NP5 swab, nasal swab <20 minutes6 Yes/No6
Immunofluorescence, Direct (DFA) or Indirect (IFA) Florescent Antibody Staining [antigen detection] A and B NP4 swab or wash, bronchial wash, nasal or endotracheal aspirate 1-4 hours No
RT-PCR7 (singleplex and multiplex; real-time and other RNA-based) and other molecular assays [influenza viral RNA or nucleic acid detection] A and B NP5 swab, throat swab, NP5 or bronchial wash, nasal or endotracheal aspirate, sputum Varies (1 to 8 hours, varies by the assay) No
Rapid cell culture (shell vials; cell mixtures; yields live virus) A and B NP5 swab, throat swab, NP5 or bronchial wash, nasal or endotracheal aspirate, sputum; (specimens placed in VTM8) 1-3 days No
Viral tissue cell culture (conventional; yields live virus) A and B NP5 swab, throat swab, NP5 or bronchial wash, nasal or endotracheal aspirate, sputum (specimens placed in VTM8) 3-10 days No
  1. Serologic (antibody detection) testing is not recommended for routine patient diagnosis and cannot inform clinical management. A single acute serum specimen for seasonal influenza serology is uninterpretable and should not be collected. Serological testing for detection of antibodies to seasonal influenza viruses is useful for research studies and requires collection of appropriately timed acute and convalescent serum specimens and testing of paired sera at specialized research or public health laboratories.
  2. Approved clinical specimens vary by influenza test. Consult the manufacturer’s package insert for the approved clinical specimens for each test. Ref: Leland, et al. 2007, Clin Micro Rev 20: 49-78. Approved respiratory specimens vary among FDA cleared influenza assays.
  3. Clinical Laboratory Improvement Amendments (CLIA) of 1988. http://www.cms.gov/Regulations-and-Guidance/Legislation/CLIA/index.html
  4. Chromatographic- and/or fluorescence-based lateral flow and membrane-based immunoassays. Some approved rapid influenza diagnostic assays utilize an analyzer reader device.
  5. NP = nasopharyngeal
  6. Rapid molecular assays can provide results in approximately 20 minutes. Alere i Influenza A&B was FDA-cleared for use with both nasal swabs (direct) and NP or nasal swabs in VTM, and CLIA-waived for use with nasal swabs (direct) only. Roche Cobas Influenza A/B was cleared and CLIA-waived by FDA for use with nasopharyngeal swabs only.
  7. Reverse transcription polymerase chain reaction, including FDA-approved test systems, reference laboratory testing using ASR or lab-developed reagents. Some approved molecular assays can produce results in approximately 60-80 minutes.
  8. VTM = Viral transport media
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Table: Characteristics of Rapid Influenza Diagnostic Tests (Antigen Detection Only)1
Procedure
(Manufacturer/Distributor)
Influenza Virus
Types Detected
Approved Specimens2 CLIA Waived3 Uses Analyzer Reader Device
BD Directigen™ EZ Flu A+B4
(Becton-Dickinson & Co.)
A and B NP5 wash/aspirate/swab
Throat swab
No No
BD Veritor™ System for Rapid Detection of Flu A+B4 (CLIA-waived),
(Becton Dickinson & Co.)
A and B NP5 swab/
nasal swab
Yes Yes
BD Veritor™ System for Rapid Detection of Flu A+B4 (Moderately Complex), (Becton Dickinson & Co.) A and B NP5 wash/aspirate No Yes
Binax NOW® Influenza A&B4 Test
(Alere Scarborough, Inc.)
A and B NP5 swab,
Nasal wash/aspirate/swab
Yes No
BioSign® Flu A+B4
or
OraSure QuickFlu Rapid A+B Test
or
Polymedco Poly stat Flu A&B Test
or
LifeSign LLC Status Flu A&B (Princeton BioMedtech Corp.)
A and B NP5 swab/aspirate/wash, nasal swab No No
ClearView Exact II Influenza A&B Test or
Alere Influenza A&B Test
(Alere Scarborough, Inc.)
A and B Nasal swab Yes No
OSOM® Influenza A&B4 Test
(Sekisui Diagnostics)
A and B Nasal swab No No
QuickVue® Influenza A/B Test6
(Quidel Corp.)
A and B Nasal wash/aspirate/swab Yes No
QuickVue® Influenza A+B Test4
(Quidel Corp.)
A and B NP5 swab
Nasal wash/aspirate/swab
Yes No
RAMP Influenza A/B Assay
or
3M™ Rapid Detection Flu A+B Test4
(Response Biomedical Corp.)
 
A and B NP5 swab/aspirate
Nasal wash/aspirate
No Yes
SAS™ FluAlert A&B Test4
(SA Scientific, Inc.)
A and B Nasal wash/aspirate No No
SAS™ Influenza A Test4
(SA Scientific, Inc.)
A only Nasal wash/aspirate Yes No
SAS™ Influenza B Test4,6
(SA Scientific, Inc.)
B only Nasal wash/aspirate Yes No
Sofia® Analyzer and Influenza A+B FIA4
(CLIA-waived) (Quidel Corp.)
A and B NP5 swab
Nasal swab
Yes Yes
Sofia® Analyzer and Influenza A+B FIA4
(Quidel Corp.)
A and B NP5 aspirate/ wash No Yes
TRU FLU®4
(Meridian Bioscience, Inc.)
A and B NP5 aspirate/swab
Nasal wash/swab
No No
XPECT™ Influenza A/B4
(Remel Inc./Thermo Fisher Scientific)
A and B Nasal wash/swab
Throat swab
No No

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  1. List may not include all test kits approved by the U.S. Food and Drug Administration. Discontinued tests not included.
  2. Approved respiratory specimens according to manufacturer's package insert. Note that test performance may vary if other respiratory specimens are used.
  3. Ref: http://www.cms.gov/Regulations-and-Guidance/Legislation/CLIA/index.html
  4. Distinguishes between influenza A and B virus infections.
  5. NP = nasopharyngeal.
  6. Does not distinguish between influenza A and B virus infections when used alone.

Disclaimer: Use of trade names or commercial sources is for identification only and does not imply endorsement by the Centers for Disease Control and Prevention or the Department of Health and Human Services.

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Additional Information

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