From the Guidelines
The sensitivity and specificity of influenza testing vary significantly by test type, with rapid molecular assays offering the best balance of high sensitivity (90-95%) and high specificity (95-99%), making them the preferred diagnostic option when available. The choice of test depends on various factors including the clinical context, the need for rapid results, and the availability of resources. According to the most recent and highest quality study 1, the performance of different types of influenza diagnostic tests is as follows:
- Rapid molecular assays: high sensitivity and high specificity
- Rapid influenza diagnostic tests: low to moderate sensitivity and high specificity
- Direct and indirect immunofluorescence assays: moderate sensitivity and high specificity
- Molecular assays (including RT-PCR): high sensitivity and high specificity
- Multiplex molecular assays: high sensitivity and high specificity
- Rapid cell culture: high sensitivity and high specificity
- Viral culture: high sensitivity and high specificity, but results take 3-10 days. It is essential to consider the timing of sample collection, with best results obtained when samples are collected within 3-4 days of symptom onset, and the type of specimen, with nasopharyngeal samples generally yielding better results than throat swabs 1. During peak influenza season, clinical diagnosis based on symptoms may be sufficient for treatment decisions when testing is unavailable, as the positive predictive value of symptoms increases when influenza is prevalent in the community. For optimal management, clinicians should consider both test characteristics and clinical context when interpreting results. In hospitalized patients, molecular assays are preferred due to their higher sensitivity compared to antigen detection, and early detection, prompt antiviral treatment, and infection-control interventions can lead to improved individual patient outcomes and allow for effective cohorting and disease containment 1. The Centers for Disease Control and Prevention (CDC) provides updated guidance on the use of rapid influenza diagnostic tests and molecular assays, emphasizing the importance of selecting the most appropriate test based on the clinical scenario and the need for timely results 1. Overall, the choice of influenza test should be guided by the need for accurate and timely diagnosis, as well as the availability of resources and the clinical context.
From the Research
Sensitivity and Specificity of Influenza Testing
- The sensitivity and specificity of influenza testing can vary depending on the type of test used.
- A study comparing the ID Now Influenza A & B 2, Cobas Influenza A/B, and Xpert Xpress Flu Point-of-Care Nucleic Acid Amplification Tests for Influenza A/B Virus Detection in Children found that the overall sensitivities for the ID Now assay, LIAT, and Xpert assay for Flu A virus detection were 93.2%, 100%, and 100%, respectively, and for Flu B virus detection were 97.2%, 94.4%, and 91.7%, respectively 2.
- The specificity for Flu A and B virus detection by all methods was >97% 2.
- Another study evaluating molecular point-of-care testing and digital immunoassays for influenza virus A/B and respiratory syncytial virus found that the positive rate for influenza virus with molecular POC test was significantly higher than that with DIAs (51.6% versus 40.7%, p = 0.046) 3.
- The study also found that a molecular POC test could contribute to the accurate diagnosis of influenza in patients with influenza-like illness, especially those who visited a hospital immediately after the onset of symptoms 3.
- A retrospective comparison of rapid influenza PCR and multiplex respiratory virus PCR tests found that commencement of antibiotics was less frequent in the RPCR than MPCR cohorts (51% vs 67%; p < 0.01, OR 0.52; 95% CI 0.34-0.79) 4.
- People at high risk of complications from influenza who were tested with the RPCR were more likely to be treated with oseltamivir compared to those tested with the MPCR (76% vs 63%; p = 0.03, OR 1.81; 95% CI 1.07-3.08) 4.
Factors Affecting Sensitivity and Specificity
- The timing of testing can affect the sensitivity and specificity of influenza testing.
- A study found that treatment is more effective if initiated within 30 hours of symptom onset in febrile individuals 5.
- The type of test used can also affect the sensitivity and specificity of influenza testing.
- A study comparing different types of tests found that molecular assays had higher sensitivity than did a historical standard-of-care test from the BD Veritor antigen test (Flu A virus, 79.5%; Flu B virus, 66.7%) 2.