From the Guidelines
Rapid antigen influenza tests are most sensitive when used within the first 3-4 days after symptoms begin, with optimal results achieved when testing is performed as soon as possible after symptom onset, ideally within 48-72 hours 1. The sensitivity of these tests decreases significantly after this window because viral shedding typically peaks early in the illness and then gradually declines. This timing is important because the tests detect viral proteins (antigens) that are present in higher concentrations during peak viral replication. Some key points to consider when interpreting the results of rapid antigen influenza tests include:
- The performance of influenza assays, including NAAT and rapid antigen tests, varies depending on the assay and the circulating strains 1.
- Rapid diagnostic tests for influenza have high specificity (>90%) but have low to moderate sensitivity (20%–70%) compared with other influenza tests 1.
- The sensitivity of rapid antigen tests is lower than for viral culture or RT-PCR and varies by test 1.
- RT-PCR is the most accurate and sensitive test for detecting influenza viruses, including the 2009 H1N1 virus 1. After the initial few days of illness, false negative results become more common as the amount of virus in respiratory secretions decreases, even though the person may still be experiencing symptoms. If testing is delayed beyond this optimal window, PCR testing may be more appropriate as it can detect smaller amounts of viral genetic material for a longer period after symptom onset. It is also important to note that the types of specimens acceptable for use (i.e., nasopharyngeal or nasal aspirates, swabs, and washes or throat swabs) also vary by test, but all perform best when collected as close to illness onset as possible 1.
From the Research
Sensitivity of Rapid Antigen Influenza Tests
- The sensitivity of rapid antigen influenza tests can vary depending on the time of testing after symptom onset.
- A study published in 2020 found that the sensitivity of a rapid influenza diagnostic test was very high in adults and elderly individuals within 48 hours of the onset of illness, with a sensitivity of 97.1% 2.
- Another study from 2011 found that in children aged 1-3 years, influenza could be accurately diagnosed within 24 hours of symptom onset, with a sensitivity of 90% for influenza A viruses, but only 25% for influenza B viruses 3.
- The sensitivity of rapid antigen tests can also depend on the type of test used and the viral load, with one study finding that the sensitivity of a rapid antigen test for influenza A was 85% when the cycle threshold (Ct) value was <25, but dropped to 69.23% when the Ct value was <30 4.
- It is generally recommended that rapid antigen tests be used within a short time frame after symptom onset, such as within 24-48 hours, to ensure optimal sensitivity 2, 3, 5.
Factors Affecting Test Sensitivity
- The sensitivity of rapid antigen tests can be affected by various factors, including the type of test used, the viral load, and the time of testing after symptom onset.
- The specificity of rapid antigen tests is generally high, but the sensitivity can be lower, especially for certain types of influenza viruses, such as influenza B 3, 4.
- The performance of rapid antigen tests can also be affected by viral genetic and antigenic changes, and should be re-assessed annually 5.
Clinical Use of Rapid Antigen Tests
- Rapid antigen tests can be a useful tool for diagnosing influenza, especially in clinical settings where rapid results are needed.
- However, the clinical use of rapid antigen tests should be carefully considered due to the possibility of insufficient sensitivity, and the results should be interpreted in conjunction with clinical symptoms and other diagnostic tests 4, 5.