The criterion standard for confirming influenza virus infection is reverse transcription-polymerase chain reaction or viral culture of nasopharyngeal or throat secretions. Rapid diagnostic tests for influenza are available and are becoming more widely used. These tests have high specificity but only moderate sensitivity.
The US Food and Drug Administration has waived federal Clinical Laboratories Improvement Amendments (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. These tests can produce results within 30 minutes. A meta-analysis examining the accuracy of rapid influenza diagnostic tests found a pooled sensitivity of 62% and specificity of 98%. The tests tended to be more sensitive in children (67%) than in adults (54%) and better at detecting influenza A (65%) than influenza B (52%).
Some laboratories offer direct immunofluorescent tests on fresh specimens, but these tests are labor- and personnel-intensive and less sensitive than culture methods. In order to overcome the expensive and time-consuming obstacle of culturing, several serologic tests have become available. In reality, many of these are not bedside tests; generally, 30-60 minutes is required to perform the tests' multiple steps. Test sensitivities generally range from 60% to 70%.
In elderly or high-risk patients with pulmonary symptoms, chest radiography is indicated to exclude pneumonia. Early radiographic findings include no or minimal bilateral symmetrical interstitial infiltrates. Later, bilateral symmetrical patch infiltrates become visible. Focal infiltrates indicate superimposed bacterial pneumonia.
For more on the workup of influenza, read here.
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Cite this: Michael Stuart Bronze. Fast Five Quiz: Influenza - Medscape - Nov 06, 2018.