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Reliability and Interpretation of Rapid Influenza Test Results

The reliability of rapid influenza diagnostic tests depends largely on the conditions under which they are used, and are entirely based on the experience with seasonal influenza.

  • For detection of seasonal influenza virus infection, sensitivities of rapid diagnostic tests are approximately 50-70% when compared with viral culture or RT-PCR, and specificities of rapid diagnostic tests for influenza are approximately 90-95%. Sensitivity and specificity of these tests for detection of the novel H1N1 flu virus are unknown.
  • False-positive (and true-negative) results are more likely to occur when influenza is uncommon in the community, which is generally at the beginning and end of an outbreak.
  • False-negative (and true-positive) results are more likely to occur when influenza is common in the community, which is typically at the height of an outbreak.
  • Test sensitivity may vary depending on when in the course of illness the specimen is collected. Respiratory specimens for testing should be collected in the first 4-5 days of illness when viral shedding is greatest.

Given these limitations, the decision of whether or not to test patients with rapid influenza diagnostic tests should be based upon the patient’s presenting symptoms, whether or not cases of novel H1N1 have been confirmed in the area, and/or the patient’s risk for severe disease or other complications.

  • How to interpret a positive test result:

A patient testing positive for influenza B by rapid diagnostic test likely has been infected with seasonal influenza B virus that is continuing to circulate or is a false-positive result. Such a patient is unlikely to have novel H1N1 virus infection.


There are several possibilities when a patient tests positive for influenza A by rapid antigen test:

  •  
    • The patient might have novel H1N1 virus infection
    • The patient might have seasonal influenza A virus infection or
    • The patient might have a false positive test result.

 

  • Information provided by states and local health authorities should be consulted to determine whether public health authorities are advising that patients who test positive on a rapid influenza antigen test need additional testing. In areas with many new confirmed cases of novel H1N1 flu infection and where community spread of H1N1 is occurring, patients who test positive on a rapid influenza diagnostic test can be treated empirically with antiviral medications if clinically indicated (see Interim Guidance on Antiviral Recommendations for Patients with Novel Influenza A (H1N1) Virus Infection and Their Close Contacts ) without further testing.. In areas with no or few confirmed cases of novel H1N1 flu, a nasopharyngeal swab/aspirate or nasal aspirate should be collected and sent to the state public health laboratory for RT-PCR to determine if the patient has H1N1 infection, seasonal influenza A virus infection, or a false-positive test result. See Interim guidelines for specimen collection and Interim biosafety guidelines for laboratory workers.
  • How to interpret a negative result:

    Novel H1N1 flu virus infection cannot be excluded when a patient tests negative for influenza A by rapid antigen test. If the patient has an epidemiologic link to a confirmed case (i.e. had close contact with a confirmed case), or has either traveled to or resides in a community where there are one or more confirmed novel H1N1 cases, further testing and treatment should be based upon clinical suspicion, severity of illness, and risk for complications. If there is no epidemiologic link and the patient has mild illness, further testing and treatment are not recommended.

 

 

Preferred respiratory specimens

The following should be collected as soon as possible after illness onset: nasopharyngeal swab, nasal aspirate or a combined nasopharyngeal swab with oropharyngeal swab. If these specimens cannot be collected, a nasal swab or oropharyngeal swab is acceptable. For patients who are intubated, an endotracheal aspirate should also be collected. Bronchoalveolar lavage (BAL) and sputum specimens are also acceptable. Specimens should be placed into sterile viral transport media (VTM) and immediately placed on ice or cold packs or at 4°C (refrigerator) for transport to the laboratory.  Recommended infection control guidance is available for persons collecting clinical specimens in clinics and other clinical settings and for laboratory personnel.

Swabs

Ideally, swab specimens should be collected using swabs with a synthetic tip (e.g. polyester or Dacron®) and an aluminum or plastic shaft. Swabs with cotton tips and wooden shafts are not recommended. Specimens collected with swabs made of calcium alginate are not acceptable. The swab specimen collection vials should contain 1-3ml of viral transport medium (e.g. containing, protein stabilizer, antibiotics to discourage bacterial and fungal growth, and buffer solution).

Storing clinical specimens

All respiratory specimens should be kept at 4°C for no longer than 4 days. 

Shipping clinical specimens

Clinical specimens should be shipped on wet ice or cold packs in appropriate packaging. All specimens should be labeled clearly and include information requested by your state public health laboratory. Suspected case specimens shipped from the state public health laboratory to CDC should include all information required for seasonal influenza surveillance isolate or specimen submission.  

Recommended tests

Real-time RT-PCR is the recommended test for confirmation of novel influenza A (H1N)1 cases. Currently, novel influenza A (H1N1) virus will test positive for influenza A and negative for H1 and H3 by real-time RT-PCR. If reactivity of real-time RT-PCR for influenza A is strong (e.g. Ct <30) it is more suggestive of a novel influenza A (H1N1) virus. Confirmation as novel influenza A (H1N1) virus by real-time RT-PCR was originally performed only at CDC, but at this time may be available in your state public health laboratory.

Other influenza tests

Rapid influenza antigen test

Some commercially available rapid tests can distinguish between influenza A and B viruses. A patient with a positive rapid test for influenza A may meet criteria for a suspected case of novel influenza A (H1N1) virus infection. However, it is not possible to differentiate from seasonal influenza A viruses. Also, these tests have unknown sensitivity and specificity to detect human infection with novel influenza A (H1N1) virus in clinical specimens, and have suboptimal sensitivity to detect seasonal influenza viruses. Therefore, a negative rapid test could be a false negative and should not be assumed a final diagnostic test for novel influenza A (H1N1) virus infection. 

Immunofluorescence (DFA or IFA)

These tests can distinguish between influenza A and B viruses.  A patient with a positive for influenza A by immunofluorescence may meet criteria for a suspected case.  However, it is not possible to differentiate from seasonal influenza A viruses.   Immunofluorescence depends upon the quality of a clinical specimen, operator skills, and has unknown sensitivity and specificity to detect human infection with novel influenza A (H1N1) virus in clinical specimens. Therefore, a negative immunofluorescence could be a false negative and should not be assumed a final diagnostic test for novel influenza A (H1N1) virus infection. 

Viral culture

Isolation of novel influenza A (H1N1) virus is diagnostic of infection, but may not yield timely results for clinical management. A negative viral culture does not exclude infection with novel influenza A (H1N1) virus.

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