Investigations

1st investigations to order

Test
Result
Test

There are no pathognomonic features of influenza, and further testing is indicated only when the results are likely to affect diagnosis and treatment decisions and to provide community disease surveillance.[78]

Result

febrile respiratory illness during a known seasonal influenza outbreak

Investigations to consider

Test
Result
Test

Definitive test for laboratory diagnosis, but takes 3-10 days for results to be reported. It is not often used for initial clinical management as results may take up to 72 hours to be reported. It is used for confirming screening tests and for public health surveillance.

Shell vial (centrifuge-enhanced) culture, if available, may reduce time for results to 1-3 days.[78]

Acceptable specimens include nasopharyngeal swab, nasal wash, bronchial wash, nasal aspirate, and sputum.[94]

Result

detection of influenza virus or viral antigen

Test
Result
Test

Detects influenza A and B.

Acceptable specimens include nasopharyngeal swab, nasal wash, bronchial wash, nasal aspirate, and sputum.

Results reported in 2-4 hours.

Result

detection of influenza virus

Test
Result
Test

Detects influenza A and B.

Acceptable specimens include nasopharyngeal swab, nasal wash, bronchial wash, nasal aspirate, and sputum.

Results reported in 2-4 hours.

Result

detection of influenza virus

Test
Result
Test

Detects influenza A and B.

Assesses paired acute (collected within the first week of illness) and convalescent (collected 2-4 weeks after the acute sample) serum samples.

Positive test result is indicative of recent infection.

May take 2 or more weeks to receive results.

Result

fourfold or greater rise in antibody titre from acute to convalescent sample

Test
Result
Test

Detects influenza A and B.

Acceptable specimens include nasopharyngeal swab, throat swab, nasal wash, and bronchial wash.

Results reported in 2 hours.

Result

detection of influenza virus

Test
Result
Test

Multiple tests are available that are designed to detect one or other, or both, of influenza A and influenza B in less than 30 minutes.

Can be useful in sporadic cases of influenza that cannot be differentiated from infections caused by other respiratory viruses on clinical grounds alone.[95][96][82][97]

Detect low quantities of viral RNA. More sensitive than culture.

Tests should be done within 24-48 hours of symptoms due to peak in viral shedding. Sensitivities and specificities of rapid diagnostic tests are approximately 70% to 75% and 90% to 95%, respectively.

Relatively costly to perform.

The CDC provides a list of available tests. CDC: influenza symptoms and the role of laboratory diagnostics external link opens in a new window

Result

positive for influenza A and/or influenza B, depending on particular test used

Test
Result
Test

Should be done to exclude either a primary viral or secondary bacterial pneumonia.

The radiographic appearances of community-acquired pneumonia include lobar consolidation, interstitial infiltrates, and cavitations.

It is commonly thought that lobar consolidation is suggestive of bacterial pneumonia and interstitial infiltrates are suggestive of pneumonia due to Pneumocystis jirovecii (formerly P carinii) and viruses. However, radiologists cannot reliably differentiate bacterial from non-bacterial pneumonia on the basis of the radiographic appearance alone.[98]

Result

normal in uncomplicated cases; may show infiltrates consistent with pneumonia in complicated cases

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