Influenza World Health Care Professionals

Laboratory methods

Several diagnostic tests are available for influenza. Ideally, a diagnostic test should have high specificity and sensitivity and results should be available within a short period of time. [1]

Diagnostic tests for the clinical management of influenza infection include: enzyme-linked immunosorbant assay (ELISA), direct immunofluorescence (DIF), viral culture, polymerase chain reaction (PCR) and serologic testing. The utility of these different diagnostic methods for influenza clinical management are summarised in the table below. [1, 2]
 

Characteristic	ELISA	DIF	Viral culture	PCR
Specificity (%)	100	90	100	100
Sensitivity (%)	85	70	100	100
Assay time	Minutes to hours	2–4 hours	2–7 days	4–6 hours
Complexity	Relatively simple	Skilled persons needed	Skilled persons needed	Specialist laboratories only

ELISA, enzyme-linked immunosorbant assay; DIF, direct immunofluorescence; PCR, polymerase chain reaction

ELISA

ELISA kits are the most frequently used diagnostic assay for influenza. [1] This method detects free antigen in nasopharangeal secretions using free conjugated antibodies. The tests are simple and can be performed reasonably rapidly; however, they do not provide any information about the virus strain. [2]

DIF

DIF is used to detect viral antigen from nasopharangeal aspirates. It is no longer used routinely as specimens require special processing. [1] Although detection of influenza using this technique is relatively inexpensive, it has lower specificity and sensitivity than ELISA and skilled laboratory technicians are required to interpret the results.

Viral culture

Viral culture is of limited use when a rapid treatment decision is required, due to the time it takes to make a diagnosis (up to 7 days). However, this technique has the advantage of high specificity and sensitivity. New techniques aimed at shortening the time to detection are promising.

PCR

PCR can be used to confirm a diagnosis of influenza within a few hours. However, this technique has the disadvantage of being relatively expensive and can only be performed in specialist laboratories.

Serologic testing

Serologic testing is mainly used for epidemiological evaluation of influenza and has limited clinical value because it takes at least 20 days to make a diagnosis. [1]

Rapid diagnostic kits

There are many commercially available rapid diagnostic and screening tests for influenza A and B virus infections, which can provide results within 30 minutes.
The tests either:

• detect or distinguish between influenza A and influenza B infections
• detect but do not distinguish between influenza A and B infections
• detect influenza A infections only.

The complexity, type of specimens suitable for testing and the time needed to produce results differ between the tests. The majority of these tests are immunoassays which detect influenza viral antigen; one, however, detects viral neuraminidase activity.
The table below shows some commercially available influenza rapid diagnostic tests. [3]

      

Influenza surveillance

The presence of a local outbreak can be regarded as one of the most useful criteria for establishing a differential diagnosis of influenza. [4]

More information about influenza surveillance systems can be found here.