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Rapid influenza diagnostic tests (RIDTs) are immunoassays that detect the core nucleoproteins of influenza viruses A and B in respiratory specimens and provide qualitative (positive or negative) results. Some of these tests are easy to perform (CLIA-waived), and most take 15 min or less. Although their specificity is approximately 90% to 95%, they have suboptimal sensitivity, generally in the range of 40% to 70%, and results must be interpreted within the context of influenza activity. That is, false-negative results are most likely to occur when disease prevalence is high. To minimize the chance of false-negative results, the Centers for Disease Control and Prevention recommends the following: collecting test specimens less than 4 days from the onset of illness, following the manufacturer’s instructions for specimens, and using confirmatory tests for negative results if laboratory confirmation is desired. The laboratory reference standards are viral culture or analysis using reverse-transcription polymerase chain reaction (RT-PCR).
Use of the RIDT for the Diagnosis of Influenza
Most cases of human influenza are diagnosed clinically, and testing a patient with the signs and symptoms of influenza is not needed when influenza activity has already been documented in the community or geographic region. RIDTs can be used to help with treatment decisions, but because of their limited sensitivity, negative results in patients with signs and symptoms of influenza should not preclude antiviral treatment.
Use of the RIDT for Influenza Surveillance
Although some RIDTs differentiate between influenza A and B virus, they do not provide subtype or strain information, and therefore cannot be used to provide data essential for developing seasonal vaccines. However, they can be used for surveillance during seasonal and pandemic outbreaks. Because they can be used at the point of care and in remote areas, they are useful for providing timely information that enables rapid initiation of preventive measures. In addition, RIDTs are useful for the early detection of outbreaks of influenza in institutions, cruise ships, summer camps, etc.
Features of RIDTs
RIDTs are membrane-based immunochromatographic lateral-flow immunoassays. The types of analyzable specimens vary according to assay; most tests require a nasal, oral, or nasopharyngeal swab or aspirate, and some also can evaluate a nasal wash specimen. To maximize sensitivity, it is very important to follow the instructions of the manufacturer. Many assays have been CLIA-waived, but some tests are of moderate complexity and require specific laboratory certification. There are tests than detect only influenza A or B, others that detect both, and others that distinguish between the two types.
Some CLIA-waived, qualitative tests:
- Alere Inc. (Waltham, MA) offers the Alere BinaxNOW® Influenza A&B Card, which distinguishes between influenza A and B. It can be used with specimens from nasal washes/aspirates, nasopharyngeal swabs, or nasal swabs; includes internal controls; and accommodates many different transport media. The time to result is 15 min. Each kit contains 22 test units.
- Quidel Corporation (San Diego, CA) offers the QuickVue® Influenza A+B test, which distinguishes between influenza A and B. It can be used with specimens from nasal washes/aspirates, nasopharyngeal swabs, or nasal swabs; and the time to result is 10 minutes. Each kit contains 25 tests.
- SA Scientific, Ltd. (San Antonio, TX) offers the SASTM FluAlert Test, which contains separate assays for detecting influenza A and B in nasal aspirates and washes. The time to result is 15 min. Each kit contains 15 tests for A and 15 tests for B.
Some moderately complex, qualitative tests:
- BDTM Diagnostics (Sparks, MD) offers the DirectigenTM EZ Flu A+B test kit, which distinguishes between influenza A and B. It can be used with specimens from nasopharyngeal washes/aspirates, nasopharyngeal swabs, or throat swabs. The time to result is 15 min. Each kit contains control swabs and 30 test devices.
- Sekisui Diagnostics (Framingham, MA) offers the OSOM® Influenza A&B test, which distinguishes between influenza A and B. It can be used with nasal swab specimens. The time to result is 10 min. Each kit contains 25 tests.
- Meridian Bioscience, Inc. (Cincinnati, OH) offers the Tru Flu® Test, which distinguishes between influenza A and B. It can be used with specimens from nasopharyngeal aspirates, nasopharyngeal swabs, nasal swabs/washes. The time to result is 15 min. Each kit contains 32 tests.
Please check out our Clinical Diagnostics Assay section for more information or to find manufacturers that sell these products