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    Is it the flu?

    When and how to use rapid testing for influenza


    Direct and indirect IF assays

    Immunofluorescence (IF) studies rely on the use of antibodies to apply fluorescent dye to a target antigen. Cells are affixed to glass slides and stained with antibodies. Slides are then reviewed under a fluorescence microscope to identify viral protein in infected cells. In direct IF assays, a single antibody is used against the antigen of interest whereas indirect IF assay utilizes multiple antibodies. Additional testing is then needed to identify the influenza strain. These tests require significant technical expertise and 1 to 2 hours of processing time.10 Immunofluorescence studies have a sensitivity in the 70% to 100% range, good specificity, and they perform better than RIDT studies.2,7,12


    Digital immunoassays (DIAs), also known as rapid immunochromatographic antigen detection assays, improve upon IF technology using digital analysis by performing an automated scan of a test strip and processing the result. Digital immunoassays eliminate the need for a lab technician to review and interpret the result.14

    Two DIAs have been commercialized and granted CLIA waivers: BD Veritor (BD Diagnostics; Sparks Glencoe, Maryland) and Sofia (Quidel Corporation; San Diego, California). Similar to rapid molecular assays, these DIAs are able to be used at the point of care in a pediatrician’s office or ED. These tests produce results in less than 15 minutes but are slightly less sensitive than the rapid molecular assays (80% and 77% for influenza A and B, respectively). As with the rapid molecular assays, sensitivity appears to be higher in children compared with adults.6,11

    Viral culture

    The RT-PCR assay supplanted viral culture as the gold standard test for influenza, but viral culture is still commonly used as a comparator in studies of new commercial tests. Because results are not available for 10 to 14 days, clinical utility for influenza is limited. Although rapid culture assays are now available, results are still not available for 24 to 48 hours. One potential clinical use of viral culture is the confirmation of a negative RIDT.10

    Serologic testing

    Serologic testing looks for a minimum fourfold rise in antibody titers when comparing acute and convalescent samples to make a diagnosis of influenza. As such, it is not helpful in the clinical decision-making process related to influenza acutely. However, if making a diagnosis serves a particular clinical purpose, serologic testing may be of benefit.

    Interpretation of test results

    If the influenza test is negative, the pediatrician cannot rule out an influenza infection if the test has a low sensitivity or if the specimen was collected a significant amount of time after onset of symptoms. Signs, symptoms, and risk of complications should be considered to determine if antiviral treatment needs to be initiated.15

    If the influenza test is positive, the pediatrician should begin antiviral treatment as soon as possible. Of note, dual infections with influenza A and B rarely occur together. If testing is positive for both strains, the specimen should be referred to the public health department for testing. Additionally, referral to the health department may be needed if the clinical situation requires subtyping of influenza A virus, differentiation between influenza A and B, or other analyses are needed.15

    If there is a concern for coexisting bacterial infection, the pediatrician needs to consider coverage for Streptococcus pneumoniae, Staphylococcus aureus (including MRSA), and group A Streptococcus.15

    Next: How to help parents cut healthcare costs

    If the influenza test is negative, the pediatrician cannot rule out an influenza infection, especially if the pediatrician is utilizing RIDT or another test with low sensitivity. Similarly, if the patient is presenting 4 or more days after symptom onset, the negative test could be a false negative. The pediatrician should use clinical signs and symptoms along with local influenza activity to decide if antiviral treatment is indicated. If the patient is being admitted to the hospital, it is advisable to begin therapy without waiting for testing results if the pediatrician suspects influenza. Similarly, the high-risk patient or any patient with progressive disease also is a candidate for empiric treatment.15


    1. Centers for Disease Control and Prevention. 12/27/2017: Lab advisory: Seasonal influenza A (H3N2) activity and antiviral treatment of patients with influenza. Available at: https://www.cdc.gov/ophss/csels/dls/locs/2017/H3N2-antiviral-treatment.html. Updated January 29, 2018. Accessed February 5, 2018.

    2. Committee on Infectious Diseases. Recommendations for prevention and control of influenza in children, 2017-2018. Pediatrics. 2017;140(4):e20172550.

    3. Ison MG. Contemporary influenza diagnostics: renewed focus on testing patients. Ann Intern Med. 2017;167(6):438-439.

    4. Ison MG. Editorial commentary: failing our patients by suboptimally treating influenza infections. Clin Infect Dis. 2014;59(6):783-786.

    5. Campbell AP, McGowan C, Rha B, et al. Influenza clinical diagnostic testing and antiviral treatment among children hospitalized with acute respiratory illness during the 2015-16 influenza season. Open Forum Infect Dis. 2017;4(suppl 1):S356.

    6. Merckx J, Wali R, Schiller I, et al. Diagnostic accuracy of novel and traditional rapid tests for influenza infection compared with reverse transcriptase polymerase chain reaction: a systematic review and meta-analysis. Ann Intern Med. 2017;167(6):394-409.

    7. Harper SA, Bradley JS, Englund JA, et al; Expert Panel of the Infectious Diseases Society of America. Seasonal influenza in adults and children—diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America. Clin Infect Dis. 2009;48(8):1003-1032.

    8. Centers for Disease Control and Prevention. Influenza (Flu). Guide for considering influenza testing when influenza viruses are circulating in the community. Available at: https://www.cdc.gov/flu/professionals/diagnosis/consider-influenza-testing.htm. Updated November 2, 2016. Accessed February 5, 2018.

    9. Baron EJ, Miller JM, Weinstein MP, et al. A guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2013 recommendations by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM)(a). Clin Infect Dis. 2013;57(4):e22-e121.

    10. Peaper DR, Landry ML. Rapid diagnosis of influenza: state of the art. Clin Lab Med. 2014;34(2):365-385.

    11. Centers for Disease Control and Prevention. Guidance for clinicians on the use of RT-PCR and other molecular assays for diagnosis of influenza virus infection. Available at: https://www.cdc.gov/flu/professionals/diagnosis/molecular-assays.htm. Updated October 25, 2016. Accessed February 5, 2018.

    12. Centers for Disease Control and Prevention. Guidance for clinicians on the use of rapid influenza diagnostic tests. Available at: https://www.cdc.gov/flu/pdf/professionals/diagnosis/clinician_guidance_ridt.pdf. Accessed February 5, 2018.

    13. Kondrich J, Rosenthal M. Influenza in children. Curr Opin Pediatr. 2017;29(3):297-302..

    14. Dunn JJ, Ginocchio CC. Can newly developed, rapid immunochromatographic antigen detection tests be reliably used for the laboratory diagnosis of influenza virus infections? J Clin Microbiol. 2015;53(6):1790-1796.

    15. Centers for Disease Control and Prevention. Algorithm to assist in the interpretation of influenza testing results and clinical decision making during periods when influenza viruses are circulating in the community. https://www.cdc.gov/flu/professionals/diagnosis/algorithm-results-circulating.htm. Updated November 2, 2016. Accessed February 5, 2018.

    Pat F Bass III, MD, MS, MPH
    Dr Bass is Chief Medical Information officer and professor of Medicine and of Pediatrics, Louisiana State University Health Sciences ...


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