Information for Clinical Laboratory Directors
On This Page
- Table 1: Influenza Virus Testing Methods
- Table 2: Rapid Influenza Diagnostic Tests (RIDTs)
- Table 3: Nucleic Acid Detection Based Test
Rapid influenza diagnostic tests (RIDTs) detect influenza viral antigens in respiratory tract specimens. Available RIDTS detect and differentiate between type influenza A and B viruses, but do not specifically identify or differentiate subtypes of influenza A viruses. RIDTs are not recommended for use in hospitalized patients with suspected influenza. Molecular assays, including RT-PCR, are recommended for testing respiratory tract specimens from hospitalized patients because of their high sensitivity and high specificity. RIDTs can provide results within approximately 15 minutes.
Some tests are waived from requirements under the Clinical Laboratory Improvement Amendments of 1988 (CLIA) and cleared for point-of-care use.
Most RIDTs are immunoassays that utilize antibodies against the nucleoproteins of influenza A and B viruses to detect viral antigens.
One RIDT is an immunofluorescence assay
Some RIDTs utilize a digital analyzer reader device to standardize result interpretation. RIDTs that utilize an analyzer reader device have higher sensitivities compared to RT-PCR than RIDTs without reader devices.
RIDTs without analyzer reader devices have low to moderate sensitivities compared to RT-PCR.
All RIDTs have high specificities compared to RT-PCR.
RIDTs are approved for specific kinds of respiratory specimens Specimens to be used with RIDTs should be collected as close as is possible to the start of symptoms (e.g., less than 4 days after illness onset). 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 detect influenza viruses. Immunosuppressed persons may have detectable influenza viruses in respiratory specimens for prolonged periods (weeks to months).
For a list of currently available RIDTs and the respiratory specimens approved for the tests by the U.S. Food and Drug Administration (FDA), see Table 2: Rapid Influenza Diagnostic Tests (RIDTs).
Recent Developments
In 2017, the FDA re-classified RIDTs from class I to class II devices, and now requires RIDTs to meet specific minimum criteria for sensitivity and specificity.
Sensitivity
Compared to RT-PCR, FDA-cleared RIDTs must achieve 80% sensitivity for detection of influenza A and influenza B viruses.
Compared to viral culture, FDA-cleared RIDTs must achieve 90% sensitivity for detection of influenza A and 80% sensitivity to detect influenza B viruses.
Specificity
Compared to RT-PCR, FDA-cleared RIDTs must achieve 95% specificity for detection of influenza A and influenza B viruses.
Compared to viral culture, FDA-cleared RIDTs must achieve 95% specificity for detection of influenza A and influenza B viruses.
Interpretation of Testing Results
Proper interpretation of testing results, particularly negative testing results is very important.
Predictive Value Depends Upon Prevalence
The positive and negative predictive values vary considerably depending upon the prevalence of influenza (level of influenza activity) in the patient population being tested.
- False-positive (and true-negative) influenza test results are more likely to occur when disease prevalence is low, which is generally 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 is typically at the height of the influenza season.
| If Influenza Prevalence is… | And Specificity is… | Then PPV is… | False Pos. rate1 is… |
|---|---|---|---|
| VERY LOW (2.5%) | MODERATE (80%) | VERY LOW (6-12%) | VERY HIGH (88-94%) |
| VERY LOW (2.5%) | HIGH (98%) | LOW (39-56%) | HIGH (44-61%) |
| MODERATE (20%) | MODERATE (80%) | LOW (38-56%) | HIGH (44-62%) |
| MODERATE (20%) | HIGH (98%) | HIGH (86-93%) | LOW (7-14%) |
The false positive rate is the number of false positives divided by the number of total positives, or 1-PPV.
The interpretation of positive results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population being tested (e.g., level of influenza activity in the community). If an important clinical decision is affected by the test result, the RIDT result should be confirmed by a molecular assay, such as reverse transcription polymerase chain reaction (RT-PCR).
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| If Influenza Prevalence is… | And Sensitivity is… | Then NPV is… | False Neg. rate2 is… |
|---|---|---|---|
| MODERATE (20%) | LOW (50%) | MODERATE (86-89%) | MODERATE (11-14%) |
| MODERATE (20%) | HIGH (90%) | HIGH(97-99%) | LOW (2-3%) |
| HIGH (40%) | LOW (50%) | MODERATE (70-75%) | MODERATE (25-30%) |
| HIGH (40%) | HIGH (90%) | HIGH (93-94%) | LOW (6-7%) |
The false negative rate is the number of false negatives/number of total positives, or 1-NPV.
The interpretation of negative results should take into account the clinical characteristics of the patient and the prevalence of influenza in the patient population being tested (e.g., level of influenza activity in the community). If an important clinical decision is affected by the test result and influenza is still suspected, then the RIDT result should be confirmed by a molecular assay, such as RT-PCR.
Selecting Tests
Many factors should be considered when selecting a test, including the following:
- Tests with high sensitivity and high specificity will provide higher positive and negative predictive values, respectively.
- Tests that are CLIA-waived can be used at the point-of-care; tests that are classified as moderately complex are required to be performed in a clinical laboratory.
Information about these characteristics can be found in product inserts and scientific articles, and by contacting the manufacturers.
Changes in Recommended Procedures Can Affect Test Results
Modification by the user can affect test performances and increase false-positive and/or false-negative rates. Such modifications include:
- Using specimens for which the test is not optimized
- Using swabs that did not come with the rapid influenza diagnostic test kits [unless recommended (see package insert for specific instructions
- Improper storage or prolonged storage before specimens are tested
When Is Use of Rapid Diagnostic Tests Beneficial?
- Testing during an outbreak of acute respiratory disease can determine if influenza is the cause and to guide prompt implementation of infectionprevention and control measures.
- During influenza season, testing of selected patients presenting with acute respiratory illnesses compatible with influenza can help establish whether influenza is present in a specific outpatient population and help health-care providers determine how to use their clinical judgment for diagnosing and treating respiratory illness. (Testing need not be done for all patients.)
- For outpatients with suspected influenza, rapid molecular assays are recommended over RIDTs because of higher sensitivities of rapid molecular assays to detect influenza A and B viruses in respiratory specimens.
- For hospitalized patients with suspected influenza, molecular assays are recommended because of their high sensitivities, and RIDTs are not recommended.
- RIDTs do not address the public health need for influenza virus isolates that can only be obtained through the collection of respiratory specimens for viral culture. Influenza virus isolates are essential for determining the match between circulating influenza virus strains and those virus strains contained in the vaccine and for aiding in the selection of new influenza vaccine virus strains.
- A positive RIDT result for influenza A virus cannot identify or differentiate between seasonal influenza A viruses circulating among humans and influenza A viruses circulating among animals (e.g. avian influenza A viruses or swine influenza A viruses). If human infection with a novel influenza A virus of animal origin is suspected on the basis of recent exposures to poultry or pigs, the state public health department should be consulted to arrange specific RT-PCR testing for novel influenza A viruses.
Table 1:Influenza Virus Testing Methods
Table 2:Rapid Influenza Diagnostic Tests (RIDTs)
Table 3: Nucleic Acid Detection Based Tests
Additional Information
- State Health Departments
FAQs
What information does rapid influenza testing gather? ›
Rapid influenza diagnostic tests (RIDTs) detect influenza viral antigens in respiratory tract specimens. Available RIDTS detect and differentiate between type influenza A and B viruses, but do not specifically identify or differentiate subtypes of influenza A viruses.
How do rapid influenza diagnostic tests work? ›Rapid influenza diagnostic tests (RIDTs)
They can identify the presence of influenza A and B, and they work by detecting the parts of the flu virus called antigens that trigger an immune response. RIDTs usually involve inserting a swab into your nostril to get a sample.
Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, reverse transcription polymerase chain reaction (RT-PCR), immunofluorescence assays, and rapid molecular assays.
How accurate are rapid influenza diagnostic tests? ›Most of the rapid influenza diagnostic tests that can be done in a physician's office are approximately 50-70% sensitive for detecting influenza virus antigens and greater than 90% specific. Recently the FDA has reclassified the RIDTs and published requirements for improved accuracy, including higher sensitivity.
What is the purpose of rapid testing? ›Types of tests
When used regularly, rapid tests may help identify individuals who may be infectious early on.
Rapid tests used in point-of-care settings can be NAAT, antigen, or antibody tests. These tests can be used to diagnose current or detect past SARS-CoV-2 infections in various point-of-care settings, including but not limited to: Physician offices. Urgent care facilities.
What is the best way to diagnose influenza? ›The most common are called “rapid influenza diagnostic tests (RIDTs).” RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response.
How do you read a rapid flu test? ›You get easy-to-read digital results for COVID-19 and Flu A+B in just 15 minutes: FLU A Results: "FLU A: +" for positive, "Flu A: -" for negative. FLU B Results: ”FLU B:+” for positive, “FLU B:-“ for negative. SARS-CoV-2 Results: "CoV2: +" for positive, "CoV2: -" for negative.
How do you swab for rapid flu test? ›1 Tilt patient's head back 70 degrees. 2 While gently rotating the swab, insert swab less than one inch into nostril (until resistance is met at turbinates). 3 Rotate the swab several times against nasal wall and repeat in other nostril using the same swab.
Can the rapid test be incorrect? ›Be aware that at-home COVID-19 antigen tests are less accurate than molecular tests. COVID-19 antigen tests may not detect the SARS-CoV-2 virus early in an infection, meaning testing soon after you were exposed to someone with COVID-19 could lead to a false-negative result, especially if you don't have symptoms.
What is a disadvantage of flu rapid test? ›
Disadvantages. Sensitivity of RIDTs to detect influenza B viral antigens is lower than for detection of influenza A viral antigens. Although specificity is high, false positive results can also occur, especially during times when influenza activity is low.
Why is it important to diagnose influenza? ›Accurate and early diagnosis of influenza viral infections are critical for rapid initiation of antiviral therapy to reduce influenza related morbidity and mortality both during seasonal epidemics and pandemics.
Is influenza A clinical diagnosis? ›Abstract. Accurate diagnosis of influenza is critical for clinical management, infectious control, and public health actions to minimize the burden of disease. Rapid influenza diagnostic tests can detect circulating influenza viruses within 10-15 min and are widely used in clinical practice for diagnosing influenza.
What is the gold standard for diagnosing influenza? ›The gold standard for confirming influenza virus infection is reverse transcription-polymerase chain reaction (RT-PCR) or viral culture of nasopharyngeal or throat secretions. Rapid diagnostic tests for influenza are available and are becoming more widely used.
How long test positive on rapid test? ›After a positive test result, you may continue to test positive for some time after. You may continue to test positive on antigen tests for a few weeks after your initial positive. You may continue to test positive on NAATs for up to 90 days.
What is the difference between PCR and rapid test? ›The rapid test offers a faster turnaround time—often providing results within 15 minutes. Patients who receive the PCR test will likely wait at least two days for results, and during periods of high demand, results may not be available for up to a week.
How is the rapid test different? ›Rapid Antigen Tests
This test involves collecting nose and throat secretions via nasopharyngeal swab and then examining them for protein fragments specific to the COVID-19 virus. While these tests provide quick results—within 15 minutes—they are generally considered to be less accurate than PCR tests.
“Unlike the PCR test, the antigen test can only determine if you have an active virus in your body. The rapid test can't detect small amounts of the virus or asymptomatic cases as accurately as the PCR test can,” Heather said. The rapid test is less accurate and there is a greater chance for a false negative.
How is influenza diagnosed and treated? ›Usually, you'll need nothing more than rest and plenty of fluids to treat the flu. But if you have a severe infection or are at higher risk of complications, your health care provider may prescribe an antiviral medication to treat the flu.
What are the main clinical symptoms of influenza? ›Uncomplicated influenza illness is typically characterized by the abrupt onset of constitutional and upper respiratory tract signs and symptoms (e.g., fever, chills, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis).
Can rapid flu test be false positive? ›
A positive result means that signs of the virus were found and that you may have the flu, especially if other cases of the flu have been reported in your area at the time you are tested. It's possible to get a false-positive result, meaning that you aren't really infected. You may need more tests to confirm this.
Can you test positive for flu? ›Yes, it's possible to test positive for the flu and COVID-19 at the same time, but experts say the chances of having a coinfection are very, very low. If you find yourself experiencing any of the symptoms discussed in this article, here's a game plan for how to quickly determine a diagnosis. Talk to your doctor.
How do you do a rapid test step by step? ›Take the test device out of the package. Remove the white cap from the bottom of the tube and squeeze 5 drops of the fluid into the specimen well of the test device. Set a timer for 15 minutes. Read results after 15 minutes and no later than 20 minutes.
What kind of swab is the rapid test? ›Antigen tests, sometimes called rapid tests, look for specific proteins on the surface of the virus. Antigen tests produce results more quickly than other tests. They are given with a nasal swab.
Should you swab mouth and nose for rapid test? ›To collect a sample for a rapid antigen test (RATs), users should follow the instructions described in the kit insert. In addition to the collection method option approved by Health Canada (as described in the kit insert), users may choose to perform combined oral and nasal sampling as it may increase test sensitivity.
What causes invalid rapid tests? ›An invalid result is likely caused by user not following the procedure properly. Read instruc on for use carefully before restart with a new test device. If the problem persists, contact technical support.
What happens if you test negative on a rapid test? ›If you test negative on a rapid test, this does not always mean you do not have COVID-19. A single negative test result is not completely reliable. If you have symptoms, stay at home until your symptoms have improved and you haven't had a fever for at least 24 hours.
How reliable are rapid antigen tests? ›For rapid antigen tests, this includes a clinical sensitivity of at least 80% (for specimens collected within 7 days of symptom onset) and a clinical specificity of at least 98%.
Is rapid testing less accurate? ›Rapid antigen tests are most accurate when they are used in people who have signs or symptoms of COVID-19, especially during the first week of illness. People who test negative may still be infected.
What is the specificity of the rapid diagnostic influenza test based on the results presented in the table? ›Specificities of RIDTs are approximately 90-95% (range 85-100%). Thus false negative results occur more commonly than false positive results.
What treatment decision is influenced by the result of a rapid influenza test? ›
Rapid influenza diagnostic tests had a critical effect on physician decision-making regarding antiviral treatment for patients with influenza-like illness in the emergency department. However, indications for antiviral therapy were not found to influence the administration of antiviral treatment.
What are the best specimens to be collected for the diagnostic testing of influenza patients? ›Preferred respiratory specimens for influenza virus testing include a nasopharyngeal or nasal swab, or a nasal wash or aspirate. Nasal and throat swabs combined in the same specimen increases detection of influenza viruses than either specimen type alone.
What are the most common tests for diagnosing influenza? ›The most common are called “rapid influenza diagnostic tests (RIDTs).” RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response. These tests can provide results within approximately 10-15 minutes but may not be as accurate as other flu tests.
How do you know if a rapid flu test is positive? ›A positive result means that signs of the virus were found and that you may have the flu, especially if other cases of the flu have been reported in your area at the time you are tested. It's possible to get a false-positive result, meaning that you aren't really infected. You may need more tests to confirm this.
Can you get a false positive on rapid flu test? ›It can be speculated that due to very low prevalence of Influenza viruses, rapid antigen tests might be experiencing some assay interference, producing false positive results. When the prevalence of the disease is low, the positive predictive value of the test is low, and false-positive test results are more likely.
What is the most specific sensitive and timely diagnostic test to confirm your suspicion of influenza? ›The PCR test had a sensitivity of 98.4%, specificity of 99.7%, positive predictive value (PPV) of 99.2% and a negative predictive value (NPV) of 99.5% , whereas the OSOM® Ultra Flu A&B RIDT had a sensitivity of 95.1%, specificity of 98.4%, positive predictive value of 95.1%, and negative predictive value of 98.4%.
What is a disadvantage of flu rapid test technology? ›Disadvantages. Sensitivity of RIDTs to detect influenza B viral antigens is lower than for detection of influenza A viral antigens. Although specificity is high, false positive results can also occur, especially during times when influenza activity is low.
What is the most effective method for preventing influenza? ›Annual vaccination is the most important measure to prevent seasonal influenza infection.
What is the appropriate treatment for the diagnosis of influenza? ›Currently, there are three antiviral drugs recommended for treating the flu: oseltamivir (Tamiflu®), zanamivir (Relenza®), and peramivir (Rapivab®). These drugs work by interrupting the function of neuraminidase on the virus surface and preventing the release of viral particles from infected host cells.
What is the gold standard for diagnosing influenza? ›The gold standard for confirming influenza virus infection is reverse transcription-polymerase chain reaction (RT-PCR) or viral culture of nasopharyngeal or throat secretions. Rapid diagnostic tests for influenza are available and are becoming more widely used.
Why is it important to diagnose influenza? ›
Accurate and early diagnosis of influenza viral infections are critical for rapid initiation of antiviral therapy to reduce influenza related morbidity and mortality both during seasonal epidemics and pandemics.
Is influenza A clinical diagnosis? ›Abstract. Accurate diagnosis of influenza is critical for clinical management, infectious control, and public health actions to minimize the burden of disease. Rapid influenza diagnostic tests can detect circulating influenza viruses within 10-15 min and are widely used in clinical practice for diagnosing influenza.
Which is the most common test used for viral diagnosis? ›The polymerase chain reaction (PCR) is the most sensitive method for revealing the presence of otherwise undetectable quantities of the genome of RNA or DNA of human viruses. PCR is a powerful new technique developed in 1985 and is an alternative to direct hybridization methods.
What is the assessment of influenza? ›The Influenza Risk Assessment Tool (IRAT) is an evaluation tool developed by CDC and external influenza experts that assesses the potential pandemic risk posed by influenza A viruses that currently circulate in animals but not in humans.