Point-of-care rapid tests for influenza are now available to physicians in both hospital and ambulatory settings. Previous reports have suggested that rapid tests are associated with sufficient sensitivity and specificity to be used, albeit with caution, in the routine detection of influenza virus among ILI patients (7
). The results obtained from our study indicate that while the specificity of the test was nearly perfect (99%), there was suboptimal field performance of the sensitivity of the rapid test, even among patients for whom a high index of suspicion for influenza exists. Our results show that less than 1 in 5 influenza patients with clinical ILI were correctly identified by the rapid test performed in the physician's office. This is lower than the rate in previous studies. Since most studies assessing the sensitivity of the rapid test compared it to viral culture, lower sensitivity relative to the more-sensitive PCR assay is reasonable, although it is even lower than we had anticipated (13
Our results are consistent with other reports suggesting lower sensitivity among adults (16
). Stein et al. conducted a study among 258 adults presenting to the emergency room with a new illness associated with cough, sinus pain, nasal congestion, rhinorrhea, sore throat, or fever. In this setting, the Quidel QuickVue test had an overall sensitivity of 33%, which was comparable to the performance of clinical judgment (22
). In a 2005-to-2006 study of 555 United Kingdom pilgrims attending the Hajj, the overall sensitivity of the test was 22%. Both studies were carried out with patients who presented within 7 days after onset of a respiratory illness associated with cough, sore throat, rhinorrhea, or fever (18
). Our study has the advantage of testing based on a consistent clinical case definition within a discrete population of ambulatory patients visiting general practitioners' offices.
Other factors may contribute to the low sensitivity found in our study. While the majority of our patients were above the age of 14 years (81%), most previous studies of the field performance of rapid tests used specimens from pediatric patients; since children shed considerably more virus than adults, the sensitivity for the latter group may be lower. We used NPA from all patients, irrespective of age, which is the type of specimen expected to yield the largest number of virally infected epithelial cells (28
). Our procedure required the instillation of 2 ml of sterile solution into one nostril, which may have diluted the final sample and contributed to lower sensitivity. However, our procedures were carried out in accordance with the manufacturer's instructions included in the package insert, which suggested the instillation of 2.5 ml of saline. To date, few studies have attempted to specifically compare the effect of clinical specimen type on rapid test performance. The manufacturer's insert presents higher test sensitivity results for nasal swabs (94% and 70% for influenza A and B viruses, respectively) than for nasal washes and aspirates (77% and 82% for influenza A and B viruses, respectively), but our results remain much lower (5
Results from other studies suggest that the delay since the onset of symptoms is an important factor driving test sensitivity, and our results also support this hypothesis. In a study conducted with adults, the overall sensitivity of the QuickVue test was 74%, ranging from 86% on day 1 of illness to 50% on the second day and 0% on the fourth day (2
). In our study, most patients diagnosed with influenza virus infections presented 3 to 4 days after symptom onset. For this group of patients, sensitivity was only 24%. In another study of three distinct populations of children and adults, the QuickVue test had an overall sensitivity of 27% relative to PCR (24
Our results are consistent with those previously published suggesting lower sensitivity when patients were tested 48 h or more following the onset of symptoms (2
). Previous reports suggesting that rapid tests have sufficient sensitivity to be used in ambulatory settings were typically based on tests conducted with pediatric patients presenting within 48 h of illness onset. Because antiviral treatment is most effective when administered within 48 h of symptom onset, that threshold is of great clinical relevance. In the real-world context, few patients actually seek outpatient medical care within such a short time, further limiting the clinical usefulness of the Quidel QuickVue Influenza A+B test in ambulatory settings.
Point-of care rapid tests are designed and marketed for use in the ambulatory setting, in order to guide physicians in making the best possible clinical decisions. Our sentinel physicians considered the Quidel QuickVue Influenza A+B test insufficient for the needs of ambulatory adult patients due to its poor sensitivity. Negative results for patients with classic ILI created uneasiness, because physicians felt that they had to justify their diagnoses to their patients. Because the Quidel QuickVue Influenza A+B test is easy to perform and has good specificity, it could nevertheless be useful in clinical or epidemiological situations where test sensitivity is not a critical issue. For example, it may be acceptable for community surveillance, because despite its low sensitivity, it may appropriately detect epidemiological trends. It may also be useful in facility outbreaks where multiple specimens are collected so as to identify the causative organism rapidly. The collection of multiple samples per outbreak could compensate for the low sensitivity. In any case, negative results must be interpreted with caution.
In conclusion, the low sensitivity of the Quidel QuickVue Influenza A+B test for ambulatory adult patients with ILI suggests that its use should probably be limited to situations where sensitivity is not an issue.