Pediatric Influenza Workup

Updated: Jul 28, 2023
  • Author: Itzhak Brook, MD, MSc; Chief Editor: Russell W Steele, MD  more...
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Approach Considerations

The criterion standard for diagnosing influenza A and B is a viral culture of nasopharyngeal samples and/or throat samples.

Laboratory Studies

Isolation of influenza viruses or detection of viral antigens in respiratory secretions (eg, throat swabs, nasopharyngeal washes, sputum) can be performed during acute influenza infection. Specimens for culture should be obtained within 3 days of onset of illness.

The type of influenza virus (A or B) may be determined by immunofluorescence or hemagglutination inhibition (HAI) techniques, and the hemagglutinin subtypes of influenza A virus (H1, H2, H3) may be identified using HAI with subtype-specific antisera.

Complement-fixation (CF) and hemagglutination inhibition (HI) tests are the most common methods used to compare sera in persons with acute and convalescent infection, although these tests have low sensitivity and specificity. Rises in immunoglobulin (Ig) titer of at least fourfold are considered diagnostic of infection. Significant rises as measured by enzyme-linked immunosorbent assay (ELISA) are diagnostic of acute infection.

Viral antigens in respiratory secretions can be detected by immunofluorescence (IF) assay, time-resolved immunofluorescence assay (TRIFA), radioenzyme immunoassay, and ELISA. ELISA results can be obtained within 1 hour.


Rapid Diagnostic Tests

The US Food and Drug Administration (FDA) has waived federal Clinical Laboratories Improvement Act (CLIA) requirements and cleared for marketing 7 rapid influenza diagnostic tests that directly detect influenza A or B virus–associated antigens or enzyme in throat swabs, nasal swabs, or nasal washes and can produce results within 30 minutes. [1] The following 3 of these tests are considered low complexity and may be used in physicians’ offices:

  • QuickVue Influenza A+B test (Quidel)

  • ZstatFlu (ZymeTx)

  • QuickVue Influenza test (Quidel)

The QuickVue tests provide results in 10 minutes or less; the ZstatFlu test provides results in 20 minutes. Because of cost, availability, and sensitivity issues, most physicians diagnose influenza based on clinical criteria alone.

A study from the Centers for Disease Control and Prevention (CDC) of 11 rapid influenza diagnostic tests (RIDTs) that were given clearance by the FDA found variations in the tests’ ability to detect flu virus. The researchers determined the following [41, 42] :

  • Most RIDTs detected viral antigens in samples with higher viral concentrations

  • Tests had fewer positive results with lower viral concentrations

  • The tests differed in their ability to detect A and B influenza viruses

Testing for avian influenza

In September 2011 the FDA approved a new kit developed by the CDC for diagnosing human infections with seasonal influenza viruses and novel influenza A viruses with pandemic potential.

The Human Influenza Virus Real-Time RT-PCR Detection and Characterization Panel (rRT-PCR Flu Panel) is an in vitro laboratory diagnostic test that can provide results within 4 hours. It is the only in vitro diagnostic test for influenza that is cleared by the FDA for use with lower respiratory tract specimens and will be given at no cost to qualified international public health laboratories.

Consisting of 3 modules, the kit can:

  • Identify and distinguish between influenza A and B viruses,

  • Classify influenza A viruses by subtype, and

  • Detect highly pathogenic avian influenza A (H5N1) virus infection in human respiratory tract specimens.

Other FDA-approved assays for diagnosing avian influenza include the Influenza A/H5 (Asian Lineage) Virus Real-Time Reverse Transcription–Polymerase Chain Reaction (RT-PCR) Primer and Probe Set and inactivated virus as a positive RNA control for the in vitro detection of highly pathogenic influenza virus A/H5 (Asian lineage). [43]

The CDC recommendation is to test for highly pathogenic (HP) avian influenza A/H5N1 if a patient has severe respiratory symptoms and a risk for exposure (eg, direct contact with ill, dead, or infected poultry in a country with known poultry outbreaks of HP avian influenza A/H5N1), with specific criteria included. Testing must be performed under biosafety level 3. [44, 45] Viral culture of human and animal specimens should not be performed in the same laboratory. [45]

Positive laboratory test findings must be confirmed by the CDC, which is the World Health Organization (WHO) H5 reference laboratory.

If other test findings are negative, paired avian influenza virus serologies can be performed at the CDC. However, reagents may not be widely available, and acute and convalescent serum analysis requires time to allow a 4-fold increase in antibody.

Other laboratory tests include the following:

  • Blood culture

  • Complete blood count (CBC) with differential

  • Electrolyte level measurement

  • Liver enzyme assay

  • Blood urea nitrogen (BUN) and creatinine level measurement

Lumbar puncture is indicated in selected patients. Very rarely, central nervous system involvement is documented by viral isolation from cerebrospinal fluid (CSF) and blood in patients who present with seizure and coma. [46]



Chest radiography may be necessary to exclude the diagnosis of pneumonia. In avian influenza, chest radiography, both posteroanterior and lateral views, may reveal patchy or diffuse infiltrates, an interstitial pattern, and lobar consolidation revealing air bronchogram or an acute respiratory distress syndrome (ARDS)–like picture.


Histologic Findings in Avian Influenza

Histologic findings may include pulmonary changes with alveolar damage similar to seasonal influenza. Fibrinous exudate membrane formation and lymphocyte infiltrates are noted in postmortem analysis of patients who had avian influenza.

Necrosis is noted in organs such as the liver and kidney. The spleen may be depleted of lymphoid cells. Bone marrow demonstrates hemophagocytosis. [17]