This study was designed to investigate the diagnostic accuracy of the QuickVue Influenza A+B rapid test in adults during the 2009 H1N1 pandemic in Austria. Performance of the rapid test was moderately good, with a low sensitivity (26%) but a high specificity (98%) in comparison with a real time RT-PCR assay.
Previous studies on the QuickVue test for pandemic influenza A H1N1 (2009) have found comparable specificities but higher sensitivities 
. In a study on comparative epidemiology of pandemic and seasonal influenza A in households performed in Hong Kong a sensitivity of even 80% was reported 
. The differences in performance of RIDTs in several studies might be due to differences in study design, sample size, and technical factors such as inappropriate specimen collection, transport and storage, and differences between individual patients 
. The sensitivity found in the present study appears particularly low, possibly due to the smaller sample sizes in earlier studies or, more likely, to the different composition of age categories within our study population. In the present investigation most of the patients were young adults, the age group most severely affected during the 2009 H1N1 pandemic. In addition, no children were enrolled in our study. This is an important point as specimens taken from children tend to have higher influenza viral loads than those taken from adults, which results in better overall sensitivity of RIDTs in specimens from children and makes comparison with studies in patients of different age groups difficult 
The high specificity found in our study population is in line with most of the previous studies. In the present study, separate samples were used for the QuickVue test and the RT-PCR, as the swabs provided in the QuickVue kit are unusable for other diagnostic purposes once the swab has been inserted into the kit's testing solution. This could have led to some discordant results and may serve as a possible explanation for the observed test specificity of <100%. However, the recommended sampling procedures were followed for each test, permitting assessment of the diagnostic accuracy of the QuickVue test and the RT-PCR in real-world conditions 
The RT-PCR assay used in this study was specific for pandemic influenza A H1N1 (2009) virus and did not include other influenza A virus subtypes such as seasonal influenza A H1N1 or H3N2. To verify the false positive QuickVue results in the two patients with a positive QuickVue result for influenza A but a negative RT-PCR for pandemic influenza A H1N1 (2009), the samples of these two patients were also investigated for other influenza A viruses by PCR analysis. In both samples no infection with seasonal influenza A virus was found, which is in line with national surveillance data demonstrating that the 2009–2010 influenza pandemic in Austria was driven exclusively by pandemic 2009 (H1N1) virus 
Despite the low sensitivity found in our study population, specificity was high, resulting in an overall high and positive likelihood ratio. This is important because likelihood ratios are good summary metrics that provide sensible estimates of test properties. In addition, likelihood ratios have the advantage of immediate quantitative clinical utility through direct application of Bayes' theorem 
. As shown in our study population, the probability of pandemic influenza A H1N1 (2009) virus infection would be high in a patient with a positive QuickVue test, indicating that a positive result with this test does not necessarily need to be confirmed by RT-PCR during influenza outbreaks and suffices to determine the appropriate course of treatment or other action.
In contrast, a negative likelihood ratio of 0.76 as a consequence of the low sensitivity barely alters the probability of an infection in patients with negative QuickVue results. Applying a test with a poor negative likelihood ratio in a high prevalence population underlines the limitations of using RIDTs alone for management of possible pandemic H1N1 virus infection. Reliance on falsely negative test results could delay the diagnosis of H1N1 infection, resulting in inappropriate exposure of susceptible persons to infected patients and the withholding of appropriate therapy. Thus, our data support current opinion that negative RIDT results do not rule out influenza infection and, if relevant for patient management or public health measures such as isolation, negative RIDT results should be confirmed by PCR.
Several reports have found that the sensitivity of an RIDT declines with decreasing viral load in the specimen 
. Accordingly, we found a higher proportion of positive QuickVue results in patients with higher viral titres (as determined by low Ct values).
Our findings demonstrate that although the QuickVue test is capable of identifying novel influenza H1N1 in respiratory specimens >70% of infections will be missed, particularly in specimens with low viral loads.
The limitations of our study should be noted. The study population in this analysis comprised military personnel and mainly young adults, and the results should not be extrapolated to other age groups. Further, although only patients with influenza-like illness were included, no information was obtained on specific symptoms, severity of symptoms or time elapsed between symptom onset and presentation at the hospital. Thus, differences in performance of the QuickVue test regarding the day of presentation and individual clinical presentation could not be determined.
In conclusion, our data suggest that a positive QuickVue test provides considerable information for the diagnosis of pandemic influenza A 2009 (H1N1) virus infection in young adults, but that negative QuickVue results should, if relevant for patient management or public health measures, be verified with PCR assay.