Our study found that the QuickVue Influenza A+B test performed with a moderate sensitivity (68.5%) and high specificity (98.1%) in comparison to RT-PCR in a developing country primary care setting. Laboratory technicians found the test easy to use and needed minimal training to adequately perform the test.
In the US and other developed countries, the QuickVue Influenza A+B rapid test has been found to have a sensitivity of 67%–85% and specificity of 97%–100% 
. However, one recent study conducted in three sites in the US found the test performed with a significantly lower sensitivity (19%–32%)
. One study performed in a developing country reported the QuickVue test to have a sensitivity and specificity of 71% and 98%, respectively, in patients aged one month to 85 years in hospital outpatient clinics in Thailand 
. As most of these prior studies included only patients meeting the CDC/WHO definition of ILI, it is more appropriate to compare the results of our sub-analysis limited to children with ILI to these studies. Thus, the sensitivity of 72.1% (95% CI 66.3, 77.5) and specificity of 98.1% (95% CI 95.9, 99.3) we observed are well within the ranges of the sensitivities and specificities previously reported.
Importantly, a substantial number of participants (23%) presented on the first day of illness, and the sensitivity of the rapid test was found to be lower among children who presented and were tested on the first day. Few studies of rapid tests have captured data on these early cases. In a study in Hong Kong, researchers examined the performance of the QuickVue rapid test and did not find a significant difference in the first 24 hours following symptom onset 
. One possible explanation for our differing results is that the Hong Kong study population was very different from ours. Subjects were predominantly adults and were recruited because they were seeking medical care at an outpatient clinic, and therefore might have been sicker and hence have a higher viral load than our study subjects at the time of testing. In a recent meta-analysis of influenza volunteer challenge studies, symptoms were reported in the first 24 hours following virus inoculation, a day before peak viral load and two days before the peak of symptoms 
. It is possible that early in illness, children are symptomatic at relatively low viral loads, leading to the lower sensitivity we report on day 1. More studies are needed to elucidate the natural history of influenza infection in children.
The large number of children presenting so early resulted from the cohort study protocol, in which families of participants were asked to bring in their children at the first sign of illness. However, persons with influenza typically do not present until several days into the illness. Therefore, it is likely that the sensitivity and resulting positive predictive values we observed are underestimates of the test's performance in the clinical setting, since it is less common for patients to present on the day of symptom onset. However, during an influenza pandemic, patients may present very early following symptom onset; thus, it is imperative to evaluate the performance of any rapid test in detecting influenza very early after onset of illness.
One of the limitations of this study is the age range of the patients. Because the study was restricted to children aged 2–14 years, the sensitivities and specificities we found apply only to this group. However, because the rapid test performed at approximately the same level in this study as in studies performed in children in developed country settings, the rapid test may well perform at the same level across all age ranges in developing country settings as observed in studies conducted in developed countries. Another limitation of this study is the use of RT-PCR as the gold standard. However, considering the very high (~98%) specificity of the rapid test in this study and the fact that RT-PCR is substantially more sensitive than viral isolation, it is unlikely that use of RT-PCR alone, instead of a combination of RT-PCR and viral isolation, significantly affected the results.
In conclusion, the rapid influenza test we used performed well in a primary healthcare setting in Managua, Nicaragua. Clinical laboratory staff needed minimal training to correctly perform the test and found the test easy to perform. Further evaluation of rapid tests in developing countries is needed to determine the feasibility of using these tests to augment existing surveillance systems and for quick deployment in the event of an influenza pandemic, as well as for use in clinical decision-making.