Rapid identification of influenza cases is essential, especially among hospitalized patients, not only for the initiation of antiviral treatment and avoidance of the unnecessary use of antibiotic therapy but also for the prevention of nosocomial transmission of influenza virus to patients at high risk of influenza-related complications. During the flu season, the combination of fever and cough was reported to be the best clinical definition of influenza, with sensitivities of 64 to 78%, specificities of 55 to 67%, and PPVs of 77 to 87% (3
). Some investigators have suggested that physicians could use this association to rapidly enact specific antiviral treatment without using rapid testing for influenza virus antigen detection, which could be limited to patients with atypical presentations and cases occurring at the beginning of the flu season (3
). However, most of those studies were done in the setting of randomized clinical trials evaluating the efficacies of neuraminidase inhibitors and might have overestimated the PPV of the clinical definition of influenza due to the selection of relatively young, healthy people with defined symptoms of influenza-like illness. Therefore, the pertinence of these results is questionable when they are applied to the target population served by practicing physicians in the community (patients with high-risk conditions who consult their physician for influenza-related complications) (16
). Other, earlier studies reported that no clinical symptoms or signs are specific for influenza virus infections (5
). Our study confirms the earlier findings and suggests that the poor specificity and PPV of the clinical case definition for influenza preclude its use for predicting influenza virus infections, especially when infection control decision making in the hospital setting is considered.
The 2000-2001 flu season allowed us to test the devices with both influenza virus A and influenza virus B, with an excess of influenza virus B. Only nasopharyngeal aspirates were considered because they usually provide the greatest sensitivity (11
), although this may not be true for children (17
). When the results of both rapid tests were compared with those of RT-PCR, both rapid tests appeared to be more sensitive for the detection of influenza virus A than for the detection of influenza virus B (for Directigen, 84 versus 78%; for QuickVue, 90 versus 84% [data not shown]). In our study, the performance of Directigen was equivalent or superior to what has previously been described for Directigen Flu A (12
) and consistent with the performance of Directigen in two previous studies with nasopharyngeal aspirates from a pediatric population (6
). Our results are also consistent with those from previous studies of QuickVue, which reported sensitivities and specificities varying between 75 and 95% and between 76 and 93%, respectively (21
). Our use of QuickVue within 7 days of storage at 4°C rather than within 1 h, as suggested by the manufacturer, might have affected the performance of the test but reflects the limitations of a hospital laboratory to perform tests “stat.”
Globally, the LRPs for Directigen were two to three times higher than those for QuickVue when the results of viral culture were used and three to eight times higher than those for QuickVue when the results of RT-PCR were used. The LRNs for Directigen and QuickVue were comparable. We were unable to demonstrate a statistical difference between the LRPs for the two tests due to the low power of the study and the absence of statistical adjustment for the fact that both tests were performed with samples from the same patients. Nevertheless, we believe that the difference between the LRPs for the two tests is clinically significant and favors the use of Directigen as a rapid test for the diagnosis of influenza.
There were significantly more invalid results by Directigen than by QuickVue (16 and 1 samples, respectively). Most of these samples were very mucoid, even after they were diluted 1:4 in saline. Of note, the results for two samples which were culture positive for parainfluenza virus were invalid when they were tested by Directigen. More tests would be necessary to evaluate if this was only a coincidental finding or if the parainfluenza virus antigens inhibit the internal control reaction. No cross-reaction with other viruses was observed by Chan et al. (6
) in their extensive evaluation. Finally, the interpretation of QuickVue results was often complicated by a difficulty in distinguishing negative samples from faintly positive samples, a problem also noted by Quach et al. (31
). We have considered all faintly positive samples as true positive, as recommended by the manufacturer.
We, as others (2
), noted that RT-PCR identified more samples as positive than the other methods did. Viral culture may miss between 3 and 46% of influenza virus-positive samples, most often for patients whose clinical course of disease is more advanced (2
). Thus, some investigators now consider that RT-PCR should be the gold standard for the diagnosis of influenza (46
). In the present study, 36% of clinical samples were influenza virus positive by culture; the diagnostic yield increased to 43% by RT-PCR. Specificity was also shown to be excellent with samples from children; among 43 samples from children, we found only 1 that had a positive RT-PCR result but that was not also positive by culture or by both rapid antigen tests. This RT-PCR-positive sample was positive by one of the two rapid tests and indeterminate by the other (and, thus, probably also had a true-positive result).
Although RT-PCR can detect nonviable organisms in minute quantities that might be present in respiratory secretions after symptoms resolve, our observations with samples from children do not show that this occurs frequently. Among the samples from adults, eight were RT-PCR positive but negative by culture and the rapid tests. When a second aliquot was extracted and analyzed by RT-PCR, these samples were again positive for the same influenza virus type. As it is not obvious why RT-PCR could be falsely positive with samples from adults but not samples from children, we presumed that these samples were falsely negative by the other tests due to the lower level of excretion of virus by adults compared to the level of excretion by children (23
). Our study was limited by the absence of an internal control for RT-PCR to better identify possible inhibition or sample loss. However, among 71 samples cultured, only 1 culture-positive sample was not also RT-PCR positive, indicating inhibition or sample loss in, at most, only 1.4% of culture-positive samples.
Age was a critical factor in the diagnostic yields of the rapid tests. The sensitivities of both devices were dramatically higher with samples from children than with samples from adults, which probably reflects higher levels of viral excretion by children (23
). The high false-negative rate of the rapid antigenic tests with samples from adults has important repercussions on the isolation and management of hospitalized adults with influenza, which argue in favor of the use of RT-PCR instead of viral culture as a rapid test for confirmation of influenza. In our hospital, we compared the efficacy of a surveillance program for influenza using Directigen and viral culture during the first year and Directigen and RT-PCR during the second year. The replacement of viral culture by RT-PCR as the reference standard allowed rapid identification of samples from patients with false-negative results by Directigen, which resulted in a 1.5-fold reduction in the rates of nosocomial influenza infection among adults by decreasing the median delay of isolation of adults from 60 to 24 h and by allowing prompt administration of antiviral prophylaxis to the room contacts of these patients (unpublished data).
In conclusion, our study suggests that the clinical case definition of influenza based on the presence of fever and cough is inaccurate for prediction of influenza virus infections, especially when infection control decision making in a hospital setting is considered. Directigen has a higher diagnostic yield than QuickVue because of its higher specificity and LRP and its ability to differentiate between influenza virus A and influenza virus B. The higher number of indeterminate results by Directigen than by QuickVue is, however, a drawback. The sensitivities of the rapid diagnostic tests are significantly lower with samples from adults than with those from children, with false-negative rates reaching up to 29%. The high false-negative rates of the rapid diagnostic tests with samples from adults have important implications for the management of hospitalized patients and on the infection control measures used for hospitalized patients. Finally, RT-PCR detects more influenza cases than viral culture, and this greater accuracy makes it a more useful reference standard.