Influenza laboratory findings: Difference between revisions
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*False-positive (and true-negative) results are more likely to occur when disease prevalence in the community is low, which is generally at the beginning and end of the influenza seasons. | *False-positive (and true-negative) results are more likely to occur when disease prevalence in the community is low, which is generally at the beginning and end of the influenza seasons. | ||
*False-negative (and true-positive) results are more likely to occur when disease prevalence is high in the community, which is typically at the height of the influenza season. | *False-negative (and true-positive) results are more likely to occur when disease prevalence is high in the community, which is typically at the height of the influenza season. | ||
*Some tests are approved for use in any outpatient setting, whereas others must be used in a moderately complex clinical laboratory. | |||
*Specimens to be used with rapid tests generally should be collected as close as is possible to the start of symptoms and usually no more than 4-5 days later in adults. | |||
*In very young children, influenza viruses can be shed for longer periods; therefore, in some instances, testing for a few days after this period may still be useful. | |||
===Serologic Testing=== | ===Serologic Testing=== | ||
Revision as of 19:40, 23 October 2014
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]
Overview
Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, polymerase chain reaction (PCR), immunofluorescence assays, and rapid molecular assays. Sensitivity and specificity of any test for influenza might vary by the laboratory that performs the test, the type of test used, and the type of specimen tested. Among respiratory specimens for viral isolation or rapid detection, nasopharyngeal specimens are typically more effective than throat swab specimens. As with any diagnostic test, results should be evaluated in the context of other clinical and epidemiologic information available to health-care providers.
Laboratory Findings
- A number of tests can help in the diagnosis of influenza, however tests do not need to be performed in all patients.
- For individual patients, tests are most useful when they are likely to give results that will help with diagnosis and treatment decisions.
- During a respiratory illness outbreak in a closed setting, such as hospitals, nursing home, cruise ship testing for influenza can be very helpful in determining if influenza is the cause of the outbreak.
- Preferred respiratory samples for influenza testing include nasopharyngeal or nasal swab, and nasal wash or aspirate, depending on which type of test is used.
- Samples should be collected within the first 4 days of illness.
- Rapid influenza diagnostic tests provide results within 15 minutes or less; viral culture provides results in 3-10 days.
- Most of the rapid influenza diagnostic tests that can be done in a physician’s office are approximately 50-70% sensitive for detecting influenza and approximately greater than 90% specific.
Influenza Virus Testing Methods
| Method | Types Detected | Acceptable Specimens | Test Time |
|---|---|---|---|
| Viral cell culture (conventional) | A and B |
|
3-10 days |
| Rapid cell culture (shell vials; cell mixtures) | A and B |
|
1-3 days |
| Immunofluorescence, Direct (DFA) or Indirect (IFA) Antibody Staining | A and B |
|
1-4 hours |
| RT-PCR (singleplex and multiplex; real-time and other RNA-based) and other molecular assays | A and B |
|
Varied (Generally 1-6 hours) |
| Rapid Influenza Diagnostic Tests | A and B |
|
<30 min. |
| Table adapted from CDC Guidance for Clinicians on the Use of Rapid Influenza Diagnostic Tests [1] | |||
Rapid Diagnostic Testing
- In general, rapid diagnostic testing for influenza should be done when the results will affect clinical decision making.
- The reliability of rapid diagnostic tests depends largely on the conditions under which they are used.
- Sensitivities of rapid diagnostic tests are approximately 50-70% when compared with viral culture or reverse transcription polymerase chain reaction (RT-PCR), and specificities of rapid diagnostic tests for influenza are approximately 90-95%.
- False-positive (and true-negative) results are more likely to occur when disease prevalence in the community is low, which is generally at the beginning and end of the influenza seasons.
- False-negative (and true-positive) results are more likely to occur when disease prevalence is high in the community, which is typically at the height of the influenza season.
- Some tests are approved for use in any outpatient setting, whereas others must be used in a moderately complex clinical laboratory.
- Specimens to be used with rapid tests generally should be collected as close as is possible to the start of symptoms and usually no more than 4-5 days later in adults.
- In very young children, influenza viruses can be shed for longer periods; therefore, in some instances, testing for a few days after this period may still be useful.
Serologic Testing
- Routine serological testing for influenza requires paired acute and convalescent sera, does not provide results to help with clinical decision-making, is only available at a limited number of public health or research laboratories and is not generally recommended, except for research and public health investigations.
- Serological testing results for human influenza on a single serum specimen is not interpretable and is not recommended.