Rapid diagnostic tests for influenza: information for clinical laboratory managers (2023)

Rapid influenza diagnostic tests (RIDTs) detect influenza viral antigens in respiratory specimens. Available RIDTS recognize and discriminate between influenza A and B virus types, but do not specifically identify or discriminate influenza A virus subtypes. RIDTs are not recommended for use in hospitalized patients with suspected influenza. Molecular assays, including RT-PCR, are recommended for testing respiratory tract samples from hospitalized patients due to their high sensitivity and high specificity. RIDTs can provide results in approximately 15 minutes.

Some tests are exempt from the requirements of the Clinical Laboratory Improvement Amendments of 1988 (CLIA) and approved for point-of-care use.

Most RIDTs are immunoassays that use antibodies against the nucleoproteins of influenza A and B viruses to detect viral antigens.

A RIDT is an immunofluorescence assay

Some RIDTs use a digital analyzer reader to standardize the interpretation of results. RIDTs using an assay reader have higher sensitivity compared to RT-PCR than RIDTs without readers.

RIDTs without analyzer readers have low to moderate sensitivity compared to RT-PCR.

All RIDTs have high specificities compared to RT-PCR.

RIDTs are approved for specific types of respiratory specimens. Specimens to be used with RIDT should be collected as close to symptom onset as possible (eg, less than 4 days after illness onset). In very young children, influenza viruses can be shed longer; Therefore, in some cases, the tests can still detect the influenza virus for a few days after this period. Immunocompromised individuals may have detectable influenza virus in respiratory specimens for a longer period of time (weeks to months).

For a list of currently available RIDTs and respiratory specimens approved by the US Food and Drug Administration (FDA) for testing, see Table 2: Rapid Influenza Diagnostic Tests (RIDTs).

recent developments

In 2017, the FDA reclassified RIDTs from Class I to Class II devices and now requires RIDTs to meet certain minimum sensitivity and specificity criteria.

sensitivity

Compared to RT-PCR, FDA-approved RIDTs must achieve 80% sensitivity for detection of influenza A and B viruses.

Compared to virus culture, FDA-approved RIDTs must achieve a sensitivity of 90% for detection of influenza A virus and 80% for detection of influenza B virus.

specificity

Compared to RT-PCR, FDA-approved RIDTs must achieve 95% specificity for the detection of influenza A and influenza B viruses.

Compared to virus culture, FDA-approved RIDTs must achieve 95% specificity for detection of influenza A and B viruses.

Interpretation of test results

Correct interpretation of test results, especially negative results, is very important.

The predictive value depends on the prevalence

Positive and negative predictive values ​​vary considerably depending on the prevalence of influenza (level of influenza activity) in the patient population tested.

• False-positive (and true-negative) influenza test results are more likely to occur when disease prevalence is low, which typically occurs at the beginning and end of the influenza season.
• False-negative (and true-positive) influenza test results are more likely to occur when disease prevalence is high, which typically occurs at the peak of the influenza season.

The false positive rate is the number of false positives divided by the number of total positives or 1-PPV.

Interpretation of positive results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population tested (eg, level of influenza activity in the community). If the test result influences an important clinical decision, the RIDT result should be supported by a molecular assay such as B. reverse transcriptase-polymerase chain reaction (RT-PCR).

The false negative rate is the number of false negatives/number of total positives or 1-NPV.

Interpretation of negative results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population tested (eg, the level of influenza activity in the community). If the test result influences an important clinical decision and influenza is still suspected, the RIDT result should be confirmed by a molecular assay such as RT-PCR.

test selection

Many factors should be considered when choosing a test, including the following:

• Tests with high sensitivity and high specificity provide higher positive and negative predictive values, respectively.
• Tests that are CLIA waived can be used at the point of care; Tests classified as moderately complex must be performed in a clinical laboratory.

Information on these properties can be found in product inserts and scientific articles, and by contacting manufacturers.

Changes to recommended procedures may affect test results

User modifications may affect test performance and increase the false positive and/or false negative rate. Such modifications include:

• Using samples for which the test is not optimized
• Use of swabs not supplied with influenza rapid diagnostic test kits [unless recommended (see package insert for specific instructions).
• Improper storage or prolonged storage before testing samples

When does the use of rapid diagnostic tests make sense?

• Testing during an acute respiratory illness outbreak can determine if influenza is the cause and guide immediate implementation of infection prevention and control measures.
• During the influenza season, testing in selected patients with acute respiratory illness compatible with influenza can help determine if influenza is present in a specific outpatient population and help health care providers determine how to use clinical judgment for treatment. diagnosis and treatment of respiratory diseases. (It is not necessary to perform the test in all patients).
• For outpatients with suspected influenza, rapid molecular tests over RIDTs are recommended because rapid molecular tests have higher sensitivity for detecting influenza A and B viruses in respiratory specimens.
• For hospitalized patients with suspected influenza, molecular assays are recommended due to their high sensitivity and RIDTs are not recommended.
• RIDTs do not address the public health needs of influenza virus isolates, which can only be obtained by collecting respiratory specimens for viral culture. Influenza virus isolates are essential to determine the match between circulating influenza virus strains and vaccine virus strains and to aid in the selection of new influenza vaccine virus strains.
• A positive RIDT result for influenza A virus can identify seasonal influenza A viruses circulating among humans and influenza A viruses circulating in animals (eg, avian influenza A viruses or swine influenza A virus), which are not identified or distinguished. If human infection with a novel influenza A virus of animal origin is suspected due to recent exposures in poultry or pigs, the state health department should be consulted to schedule a RT-PCR test specific to novel influenza A viruses. influenza A.

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Table 1: Influenza Virus Test Method

Table 2: Rapid Influenza Diagnostic Tests (RIDTs)

Table 3: Tests based on the detection of nucleic acids