Findings of standard laboratory studies, such as a complete blood count (CBC) and electrolyte levels, are nonspecific but helpful in the workup of influenza. Leukopenia and relative lymphopenia are typical findings. Thrombocytopenia may be present. In severe cases of influenza, the patient is likely to have hypoxemia, and the alveolar-arterial (A-a) gradient may be increased (>35 mm Hg). Patients with physical examination findings compatible with meningitis should undergo lumbar puncture.
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.
The criterion standard for confirming influenza virus infection remains reverse transcription-polymerase chain reaction (RT-PCR) 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.
Some laboratories offer direct immunofluorescence tests on fresh specimens, but these tests are labor- and personnel-intensive and are 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 are required to perform the test's multiple steps. Test sensitivities generally range from 60% to 70%.
For more on the workup of influenza, read here.
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