Accurate diagnosis of influenza is critical for clinical management, infection control, and public health actions to minimize the burden of disease. Commercially available rapid influenza diagnostic tests (RIDTs) that detect the influenza virus nucleoprotein (NP) antigen are widely used in clinical practice for diagnosing influenza because they are simple to use and provide results within 15 minutes. However, there has not been a recent comprehensive analytical evaluation of available RIDTs using a standard method with a panel of representative seasonal influenza viruses.
This report describes an evaluation of 11 FDA-cleared RIDTs using 23 recently circulating influenza viruses under identical conditions in a laboratory setting to assess analytical performance. Most RIDTs detected viral antigens in samples with the highest influenza virus concentrations, but detection varied by virus type and subtype at lower concentrations. Clinicians should be aware of the variability of RIDTs when interpreting negative results and should collect test samples using methods that can maximize the concentration of virus antigen in the sample, such as collecting adequate specimens using appropriate methods in the first 24–72 hours after illness onset. The study design described in this report can be used to evaluate the performance of RIDTs available in the United States now and in the future.
As part of a collaboration between CDC, the Biological Advanced Research and Development Authority, and the Medical College of Wisconsin (MCW), CDC provided 16 influenza A and seven influenza B viruses to MCW to evaluate RIDTs commercially available during the 2011–12 influenza season. Stock viruses were representative of viruses circulating in the United States since 2006. In addition, the concentration of influenza virus NP antigen (the antigen detected by RIDTs) was measured. All samples, either prepared swabs or liquid, were added to RIDTs and incubated, with results interpreted as described in the instructions for use. Three separate tests were performed for each combination of virus and RIDT.
The numbers of RIDTs that were positive (defined as at least two positive results of the three tests performed) at each dilution for each of the 23 influenza viruses were compared. RIDTs overall had fewer positive results with viruses that had the lowest stock NP concentrations. Each influenza virus had variable levels of positivity with RIDTs, suggesting that several viruses of each type and subtype should be evaluated with each RIDT on a regular basis.
The numbers of positive test results for each of the 11 RIDTs by influenza virus type and influenza A subgroup were compared. One RIDT (SAS FluAlert Influenza A [SA Scientific]) did not uniformly detect influenza A (H1N1)pdm09 (pH1N1) viruses or other influenza A viruses at high concentrations. Four RIDTs detected the majority of influenza B viruses in third dilution samples, whereas only one RIDT (BD Directigen EZ Flu A+B [Becton, Dickinson and Co.]) detected at least 50% of all influenza A viruses in third dilution samples.
For the evaluation described in this report, efforts were made to 1) use identical viral concentrations for each kit tested, 2) use all 11 commercially available, FDA-cleared RIDTs for the 2010–11 influenza season, and 3) use a diverse collection of 23 more recent influenza viruses to allow for a more finely detailed characterization of test performance. The analytical sensitivity of the evaluation varied across test kits as well as with different influenza viruses, indicating that test performance for some RIDTs drops significantly with decreasing virus concentration.
The findings in this report further emphasize the importance of collecting respiratory specimens when the amount of influenza virus is at its peak (within 24–72 hours of symptom onset). The high virus concentrations at which the evaluated FDA-cleared RIDTs detected recent circulating viruses might exceed levels expected in clinical specimens, even those collected at the peak of virus load in the specimen. Although all RIDTs were able to detect virus at the highest virus concentrations, some were unable to detect certain viruses at any subsequent dilution. Manufacturers use different antibodies in their RIDTs to capture NP antigen, and this difference in antibody selection might account for some of the variation in performance.
Periodic evaluation of RIDT performance in detecting current or recently circulating influenza viruses might identify needed updates in antibodies used in commercial RIDTs. In addition, given the narrow range of virus concentrations that can be detected by the majority of RIDTs, clinicians should follow best practices for specimen collection and timing to maximize the number of influenza viruses per specimen and improve the clinical utility of the test. Ideally, RIDT performance should be evaluated using respiratory specimens from patients with influenza-like illnesses.
These findings highlight the need for clinicians and laboratorians to use RIDTs cautiously for diagnostic, treatment, and infection control decisions in clinical settings. Because of variability in RIDT performance, especially at lower viral concentrations, negative RIDT test results might not exclude influenza virus infection in patients with signs and symptoms suggestive of influenza. Therefore, antiviral treatment, if indicated, should not be withheld from patients with suspected influenza because they have a negative RIDT test result. Clinicians and laboratorians can take measures to improve detection of influenza, such as 1) collecting specimens early in the course of illness, 2) ensuring that the appropriate type and highest quality of respiratory specimen is collected, and 3) using the current local prevalence of influenza activity to raise or lower the suspicion of influenza and to assess the benefit of testing.
See the CDC Report