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    Is it the flu?

    When and how to use rapid testing for influenza


    Available testing methods

    Depending on the testing modality, an influenza diagnosis depends on the identification of virus or detection of viral protein or viral RNA in respiratory tract secretions.6 Negative antigen tests do not rule out an influenza infection.

    The pediatrician and office staff need to be cognizant of appropriate specimen collection and its impact on influenza diagnosis. An inadequate specimen (right site but too few cells), inappropriate specimen (generally nasopharyngeal specimens are preferred), incorrect transport medium or dilution, and improper storage or transport of specimens all can lead to an inaccurate diagnostic test.10

    RT-PCR assays

    Reverse transcriptase polymerase chain reaction (RT-PCR) assays are molecular assays that have good sensitivity (86%-100%)2and make a diagnosis by identifying influenza viral RNA in respiratory specimens. Currently, RT-PCR is considered the gold standard test, but these samples are usually required to be sent to specialized laboratories, and processing can take 1 to 8 hours. Results are not generally available at the conclusion of an outpatient encounter.

    Consequently, rapid influenza diagnostic tests (RIDTs) utilizing viral antigen detection are much more commonly performed in the outpatient setting because results are provided more quickly and before the end of the visit. Additionally, RT-PCR testing is generally more expensive than other testing methods.6,11

    False-positive and false-negative RT-PCR assays are relatively rare. Other advantages of RT-PCR tests include the ability to detect influenza viral RNA for longer periods of time compared with other tests. However, a positive RT-PCR test does not necessarily indicate viable virus or ongoing influenza viral replication. Some RT-PCR assays can detect influenza A or B viruses, influenza A subtypes, and novel influenza A virus infections.11

    Rapid molecular assays

    Two rapid molecular assays (Alere Influenza A & B Test [Abbott; Abbott Park, Illinois] and cobas Liat PCR System [Roche Diagnostics; Indianapolis, Indiana]) are Clinical Laboratory Improvement Amendments (CLIA)-waived and can be used at point of care in a pediatrician’s office or emergency department (ED). Sensitivity is slightly lower than RT-PCR (66%-100%),11 but a recent meta-analysis found sensitivity of 92% and 87% for influenza and B, respectively.6 Sensitivities also were generally higher in children compared with adults as well.6,11 The main advantage of these assays over RT-PCR is the ability to perform testing at the point of care, and results are available within 20 minutes.

    More: 3 steps to boost health literacy

    It is important to obtain a patient’s history of vaccination because live attenuated vaccine virus strains (not recommended for the 2017-2018 influenza season) may give a positive PCR test from 3 to 10 days after administration.


    Rapid influenza diagnostic tests are immunoassays that detect influenza A and B viral nucleoprotein antigens in respiratory specimens. Many of these tests are CLIA-waived and can yield a result in less than 15 minutes, but they also have a lower sensitivity (50%-70%).6,12 However, in studies in which only children are tested, the sensitivity appears to be at the higher end of reported sensitivity. It is thought that higher viral loads and increased viral shedding in pediatric patients account for this.13

    The risk of a false-negative test is significantly greater with this test and may occur even when incidence of influenza in the community is high. If signs, symptoms, and clinical examination all point to influenza, the pediatrician should not be dissuaded by a negative RIDT.6,12

    There is also a risk of false-positives, especially when incidence of influenza is low in the community. As a result, the pediatrician cannot exclude influenza with a negative test, only confirm with a positive test. In addition, RIDT testing is limited by onset of symptoms, with false-negatives more likely to occur as time increases from symptom onset to testing.6,12

    NEXT: More testing choices

    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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