Introduction
With its ability to spread rapidly, especially during the flu season, timely and accurate diagnosis becomes important for effective management and containment. Influenza virus testing is not needed to make a clinical diagnosis of influenza in patients with suspected influenza, especially during increased influenza activity when seasonal influenza A and B viruses are circulating in the local community. However, influenza virus testing helps in clinical management when the results influence clinical decisions, like initiating antiviral treatment, performing other diagnostic testing, or implementing infection prevention and control measures for influenza. Influenza virus testing is advised for all patients with suspected influenza admitted to the hospital. This article discusses various influenza diagnostic tests, their significance, and technological advancements that have improved the ability to detect and combat this infectious disease.
Why Is the Influenza Diagnostic Test Required?
Influenza is known for its ability to mimic other respiratory infections, making clinical diagnosis challenging. The symptoms, such as fever, cough, and fatigue, overlap with other respiratory illnesses, highlighting the importance of specific diagnostic tests. Rapid and adequate identification of influenza not only aids in early treatment but also plays an essential role in public health strategies to control outbreaks.
What Are the Different Types of Influenza Diagnostic Tests?
The different types of influenza diagnostic tests include:
1. Molecular Tests: Molecular tests, including polymerase chain reaction (PCR) and nucleic acid amplification tests (NAATs), are sensitive and specific methods for detecting influenza viruses. These tests identify the viral RNA and can differentiate between influenza A and B strains. While molecular tests are considered the gold standard, they are usually time-consuming and require specialized. Rapid molecular assays are a molecular influenza diagnostic test to identify influenza virus nucleic acids in upper respiratory tract specimens with high specificity and sensitivity (90 to 95%). FDA-cleared rapid molecular assays that provide results within 15 to 30 minutes (approximately) are available.
2. Reverse Transcription-Polymerase Chain Reaction (RT-PCR): It can detect the presence of influenza viral RNA or nucleic acids in respiratory samples, which is highly sensitive and specific. Certain molecular assays can help detect and discriminate between infections with influenza A and B viruses; other tests can help identify specific seasonal influenza A viruses subtypes. These assays can give results in approximately 45 minutes to a number of hours, based on the type of assay. In particular, detecting influenza viral RNA or nucleic acids by these assays does not primarily specify the detection of viable infectious viruses or ongoing viral replication.
It is essential to understand that not all assays are approved by the FDA for diagnostic use. Some multiplex molecular assays are available that help identify influenza viral nucleic acids and differentiate influenza virus infection from other respiratory pathogens and can also be helpful for the management of immunosuppressed severe patients or for use in detecting the cause of an institutional spread of respiratory illness.
3. Immunofluorescence: Immunofluorescence assays involve antigen detection that usually requires the use of a fluorescent microscope to provide results in approximately two to four hours with moderate sensitivity and high specificity. Both direct fluorescent antibody (DFA) and indirect fluorescent antibody (IFA) staining assays are there to detect influenza A and B viral antigens in respiratory tract samples. Subtyping or further identification of influenza A viruses is not possible by immunofluorescence assays. One rapid immunofluorescence assay includes RIDT which involves an analyzer device to give results in approximately 15 minutes.
4. Rapid Influenza Diagnostic Tests (RIDTs): RIDTs are commonly used in clinical settings due to their quick turnaround time. These tests detect viral antigens in respiratory specimens and provide results within 15 to 30 minutes. Although quick, RIDTs are less sensitive than molecular tests and may yield false negatives, particularly in low viral loads or mutated strains.
5. Viral Culture: Viral culture involves isolating and growing influenza viruses in a laboratory setting. While this method provides valuable information about viral characteristics and mutations, it is not commonly used for routine clinical diagnosis due to its time-consuming nature and the availability of more rapid alternatives.
6. Serological Tests: Serological tests detect influenza antibodies in patient serum, such as enzyme-linked immunosorbent assay (ELISA) and immunofluorescence assays. These tests are helpful for retrospective diagnosis and surveillance. However, they are less commonly used for rapid clinical diagnosis.
What Are the Advancements in Influenza Diagnostics?
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Point-Of-Care Testing (POCT): The demand for rapid results has given way to the development of point-of-care testing for influenza. These tests, conducted near the patient, provide rapid and actionable results. POCT devices, including lateral flow assays, have become famous for their simplicity and accessibility, making them valuable tools in urgent care settings.
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Multiplex PCR Panels: Advances in molecular technology have paved the route for the development of multiplex PCR panels that can simultaneously identify various respiratory pathogens, including influenza viruses. This hastens the diagnostic process and helps identify co-infections, improving patient management.
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Next-Generation Sequencing (NGS): NGS technology helps in the rapid sequencing of entire viral genomes. In the context of influenza, NGS provides insights into viral evolution, antigenic drift (genetic variation in viruses), and emerging strains. This information is essential for vaccine development and enhancing the understanding of influenza dynamics.
What Are the Challenges Being Faced in the Diagnosis?
While influenza diagnostic tests have come a long way, challenges persist. The variability of influenza viruses, the emergence of new strains, and the potential for mutations complicate diagnostic efforts. False negatives and positives, particularly in the case of RIDTs, can impact patient care and public health strategies. Further, the cost and accessibility of advanced diagnostic technologies remain barriers in certain sections of society.
Conclusion
Influenza diagnostic tests play a pivotal role in identifying and managing influenza cases, contributing to public health efforts to control outbreaks. From traditional methods like viral culture to cutting-edge technologies like NGS, the landscape of influenza diagnostics continues to evolve. Despite challenges, ongoing research and technological advancements promise more accurate, rapid, and accessible diagnostic tools, enhancing our ability to combat this perennial public health threat. As one navigates future flu seasons, integrating diverse diagnostic approaches will be essential in staying one step ahead of the ever-adapting influenza viruses.
