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Rapid Diagnostic Testing for Influenza: Information for Clinical Laboratory Directors

Information for Clinical Laboratory Directors

The availability and use of commercial influenza rapid diagnostic tests by laboratories and clinics have substantially increased in recent years.

  • Rapid influenza diagnostic tests (RIDTs) are screening tests for influenza virus infection.
  • They can provide results within 15 minutes.
  • More than 10 RIDTs have been approved by the U.S. Food and Drug Administration (FDA) (see Influenza Diagnostic Table).
  • RIDTs differ in some important respects:
    • Some can identify influenza A and B viruses and distinguish between them.
    • Some can identify influenza A and B viruses but cannot distinguish between them.
    • Some tests are waived from requirements under the Clinical Laboratory Improvement Amendments of 1988 (CLIA).
    • Most tests can be used with a variety of 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 approximately 50-70%
    • Specificities are approximately 90-95%
  • Specimens to be used with RIDTs generally should be collected as close as is possible to the start of symptoms and usually no more than 4-5 days later in adults. 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 be useful.
Accuracy Depends Upon Prevalence

The positive and negative predictive values vary considerably depending upon the prevalence of influenza in the community.

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

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

The false pos. rate is the number of false positives/number of total positives, or 1-PPV.

The interpretation of positive results should take into account the clinical characteristics of the case. If an important clinical decision is affected by the test result, the rapid test result should be confirmed by another test, such as viral culture or polymerase chain reaction (PCR).

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Clinical Considerations of Testing When Influenza Prevalence Is High

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

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

The false neg. 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. If an important clinical decision is affected by the test result, then the rapid test result should be confirmed by another test, such as viral culture or PCR.

Selecting Tests

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

  • Tests with high sensitivity and specificity will provide better positive and negative predictive values.
  • Types of specimens that provide the most accurate results.

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

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)].
When Is Use of Rapid Diagnostic Tests Beneficial?
  • Testing during an outbreak of acute respiratory disease can determine if influenza is the cause.
  • During influenza season, testing of selected patients presenting with 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 isolated 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 viruses and those viruses contained in the vaccine and for aiding in the selection of new vaccine strains.
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Table 1: Influenza Virus Testing Methods
Method1Types DetectedAcceptable Specimens2Test TimeCLIA Waived3
Viral cell culture (conventional)A and BNP4 swab, throat swab, NP2 or bronchial wash, nasal or endotracheal aspirate, sputum3-10 daysNo
Rapid cell culture (shell vials; cell mixtures)A and BAs above1-3 daysNo
Immunofluorescence, Direct (DFA) or Indirect (IFA) Antibody StainingA and BNP4 swab or wash, bronchial wash, nasal or endotracheal aspirate1-4 hoursNo
RT-PCR5 (singleplex and multiplex; real-time and other RNA-based) and other molecular assaysA and BNP4 swab, throat swab, NP2 or bronchial wash, nasal or endotracheal aspirate, sputumVaried (Generally 1-6 hours)No
Rapid Influenza Diagnostic Tests6 (antigen)A and BNP4 swab, (throat swab), nasal wash, nasal aspirate<30 min.Yes/No
  1. Serologic (antibody detection) testing is not recommended for routine patient diagnosis.
  2. Ref: Leland, et al. 2007, Clin Micro Rev 20: 49-78. Approved respiratory specimens vary among FDA cleared influenza assays.
  3. Ref: http://www.cms.gov/Regulations-and-Guidance/Legislation/CLIA/index.html
  4. NP = nasopharyngeal
  5. Reverse transcriptase polymerase chain reaction, including FDA-approved test systems, reference laboratory testing using ASR or lab-developed reagents
  6. Chromatographic- and/or fluorescence-based lateral flow and membrane-based immunoassays
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Table 2: Characteristics of Rapid Influenza Diagnostic Tests1
Procedure
(Manufacturer/Distributor)
Influenza Virus
Types Detected
Approved Specimens2CLIA Waved3
3M™ Rapid Detection
Flu A+B Test4
(3M)
A and BNP5 swab/aspirate
Nasal wash/aspirate
No
Alere™ Influenza A & B4
(Alere)
A and BNasal swabYes
BD Veritor™ System for Rapid Detection of Flu A+B CLIA-waived4,8
(Becton Dickinson)
A and BNP5 swab/nasal swabYes
BD Veritor™ System for Rapid Detection of Flu A+B Moderately Complex4,8
(Becton Dickinson)
A and BNP5 wash/aspirate/swabNo
BinaxNOW® Influenza A&B4
(Alere)
A and BNP5 swab,
Nasal wash/aspirate/swab
Yes
BioSign® Flu A+B4

(Princeton BioMedtech)
A and BNP5 swab/aspirate/wash, nasal swabNo
Directigen™ EZ Flu A+B4
(Becton-Dickinson)
A and BNP5 wash/aspirate/swab
Throat swab
No
OSOM® Influenza A&B4
(Sekisui Diagnostics)
A and BNasal swabNo
QuickVue® Influenza Test6
(Quidel)
A or BNasal wash/aspirate/swabYes
QuickVue® Influenza A+B Test4
(Quidel)
A and BNP5 swab
Nasal wash/aspirate/swab
Yes
SAS™ FluAlert A&B
(SA Scientific)
A and BNasal wash/aspirateNo
SAS™ FluAlert A4
(SA Scientific)
A onlyNasal wash/aspirateYes
SAS™ FluAlert B6
(SA Scientific)
B onlyNasal wash/aspirateYes
Sofia® Influenza A+B 4,7,8
(Quidel)
A and BNP5 aspirate/swab/wash
Nasal wash
Yes
TRU FLU®4
(Meridian Bioscience)
A and BNP5 aspirate/swab
Nasal wash
No
XPECT™ Flu A&B4
(Remel/Thermofisher)
A and BNasal wash/swab
Throat swab
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.
  7. Immunofluorescence assay.
  8. Requires use of a separate analyzer reader device.

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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