Influenza Diagnosis
There are numerous diagnostic tests for detecting influenza virus, but testing is not necessary for every patient with influenza infection. Individual patients who are more likely to achieve clinically feasible results, which will aid in treatment decisions, benefit more from diagnostic tests. When there is an outbreak of a respiratory ailment in a closed facility, such as a long-term care facility, boarding school, hospital, cruise ship, or summer camp, testing for influenza can help determine the cause of the outbreak. Serology, viral culture, reverse transcription-polymerase chain reaction (RT-PCR), rapid molecular assays, rapid antigen testing, and immunofluorescence assays are all useful influenza diagnostic tests. The specificity and sensitivity of each influenza test may vary depending on the laboratory performing the test, the duration between the onset of illness and the collection of specimens, the type of test used, and the specimen’s form.
The use of nasal or throat swabs for the collection of respiratory specimens to promptly detect human influenza viruses or viral isolation is not as effective as the use of nasopharyngeal specimens. The nasal-pharyngeal specimens have the highest yield of all specimen types. As is the case with all diagnostic tests, the medical staff must evaluate the test results in the context of other epidemiologic and clinical information they possess. Nasal aspirate, nasal wash, and nasopharyngeal or nasal biopsy are the most common respiratory samples used to diagnose the influenza virus. The use of respiratory samples for diagnosing influenza infection largely depends on the type of test utilized. Additionally, it is crucial to obtain samples three to four days after the onset of influenza symptoms. Rapid influenza diagnostic test (RIDTS) results are available in approximately 15 minutes, compared to 3 to 10 days for viral culture testing.
The majority of tests that can be performed in a physician’s office for rapid influenza diagnosis are over 90% specific and between 50 and 70% sensitive for detecting influenza virus. Consequently, the likelihood of obtaining false-negative results is greater than that of obtaining false-positive results, particularly during the zenith of influenza activity in the community. The implementation of rapid molecular assays can yield precise and sensitive results in under 20 minutes. As a result of advances in molecular assays, it is now more accessible and can generate results in 60-80 minutes with extremely high precision and sensitivity. Immediate collection of respiratory specimens after the advent of influenza symptoms is essential. Ideally, specimens should be collected less than three days after the onset of influenza symptoms.
Using RT-PCR, specimens from the lower respiratory tract can be used to detect influenza viruses in patients residing in hospitals or other healthcare facilities who exhibit symptoms of influenza and respiratory tract disease. As influenza virus dissemination is more detectable and persists longer in the lower respiratory tract than in the upper respiratory tract, the specimens must come from the lower respiratory tract. If a patient is critically ill or receiving mechanical ventilation, bronchioalveolar lavage fluid or endotracheal aspirate can be collected for influenza testing. Following is a description of some diagnostic testing methodologies used in practice for the influenza virus:
Viral ecology
As an outbreak of respiratory disorders occurs and individuals become suspicious of influenza infection, some respiratory specimens are viable for molecular testing. After evaluating specimens with molecular assays, it is possible to identify additional diseases. For the determination of influenza A virus subtypes, it is essential to obtain respiratory tract samples. The viral culture aids in the detection of influenza A and B virus subtypes and disease-causing strains. In addition, viral culture aids in the surveillance of emerging virus strains that may need to be included in a future vaccine. Viral culture also reveals the precise cause of respiratory infections.
