Diagnostic Influenza Tests 

Diagnostic influenza tests aid with identification of influenza types A and B and influenza A subtypes 2009 H1N1, H1, H3, H5, N1, and N2. Variation exists among diagnostic methods for identification of types and subtypes of influenza (see Tables 1, 2, and 3).

These tests can be used for any age or sex.

Qualitative: Positive or negative.

If the value is positive, the patient may have influenza.

• False-positive and true-negative test results occur more frequently when disease prevalence is low.

• False-negative and true-positive test results occur more frequently when disease prevalence is high.

Specimen: nasopharyngeal swab/aspirate, nasal swab/aspirate/wash, throat swab, bronchoalveolar lavage, sputum.

• Acceptable specimens vary amongst diagnostic tests

• Specimens from adults should be collected at the start of symptoms and usually no more than 4 or 5 days later. Very young children can shed influenza virus for more than 5 days.

• Place specimen in viral transport medium.

• Transport specimen on wet ice.

• Specimens can be stored at 2-8º C and tested within 48 hours.

• Specimens can be frozen at -80º for prolonged periods of time. Avoid multiple freeze-thaws since this results in a decrease in viral titer.

Table 1. Diagnostic Methods for Influenza* (Open Table in a new window)

Table 2. Rapid Antigen Tests for Influenza* (Open Table in a new window)

Table 3. FDA-Approved Molecular Tests for Influenza ^{[1, 2]} * (Open Table in a new window)

^* List may not include all test kits approved by the FDA.

Description

In recent years, companies have introduced a number of rapid antigen assays, immunofluorescent reagents, rapid viral culture methods, and molecular tests for influenza virus. Some of the molecular methods can rapidly determine influenza A subtypes, which can be useful in determining the potential for resistance to some of the antiviral agents used in therapy. ^[3]

Indications/Applications

Diagnostic tests for influenza are useful in establishing a diagnosis and in the management of patients who have signs and symptoms compatible with influenza. They are also used to determine whether outbreaks of respiratory disease are due to influenza.

The susceptibility of influenza virus types and subtypes varies by antiviral (Table 4). Some of the molecular tests can rapidly provide the influenza type and subtype data.

Table 4. Susceptibility Profile of Influenza Types and Subtypes to Antiviral Agents* ^[4] (Open Table in a new window)

Rapid antigen tests, immunofluorescent tests, and virus culture have been reported to often lack in assay sensitivity, specificity, testing time, and ability to subtype for influenza compared to molecular methods. ^[5]  In a study of the nucleic acid amplification tests ID Now (Abbott), Cobas Influenza A/B Assay (Roche Molecular Diagnostics), and Xpert Xpress Flu (Cepheid), Kanwar et al found the three products to have comparable sensitivities for influenza A (93.2%, 100%, 100%, respectively) and B (97.2%, 94.4%, 91.7%, respectively) detection. In addition, each product had greater than 97% specificity for influenza A and B detection. ^[6]

Rapid antigen tests generally have a sensitivity of 50-70% and a specificity of 90-95%. Limited studies have demonstrated very low sensitivity for detection of 2009 H1N1 with some commercial brands. ^{[7, 8, 9, 10, 11, 12]}  A study by Fowlkes et al found that of 3681 patients who tested positive for influenza on real-time reverse transcription polymerase chain reaction (rRT-PCR) assay, 40% displayed negative results on rapid influenza diagnostic testing. PCR assays were run for influenza A (H1N1, H1N1pdm09, H3N2) and influenza B. ^[13]

Immunofluorescent tests, with nasopharyngeal specimens, have been reported to have high sensitivity and specificity for identification of influenza A and B. ^[14]

Very early and very late in respiratory season, and when disease prevalence in the community is low, positive diagnostic test results should be confirmed by an alternate method, such as viral culture or a molecular test. ^[15]

Hospitalized patients with a serious respiratory condition should preferably be tested by a molecular method for influenza. ^[15]