Reliability of a rapid test for the clinical diagnosis of influenza A/H1N1 2009

Background: The rapid diagnosis of a pandemic influenza A/H1N1 2009 (H1N1pdm) virus infection is required in ambulatory care settings, since early identification can prevent further transmission. However, the sensitivity of rapid influenza diagnostic tests (RIDTs) is still questionable, and specific indicators for H1N1pdm and/or false-negative results by RIDTs have not been clearly determined.Methods: From June to December 2009, nasal swabs from 324 patients at Kochi Health Science Center were used for the diagnosis of infection by RIDT and reverse transcription polymerase chain reaction. Results: The sensitivity of the RIDT was determined to be 80.0% and the specificity 97.1%. Multivariate analysis revealed that the frequencies of contagiousness and headache were significant in patients with H1N1pdm infection, in addition to common symptoms of respiratory infection. These data indicated that the H1N1pdm virus had high infectivity and was harmful to the endocranial environment. In the false-negative group, the time interval between onset and consultation was 5.5±6.5 h (median ± interquartile range), which was significantly shorter than the 11.5±7.0 h in the true-positive group. The sensitivity of the RIDT was significantly low during the time-period within 3 h from onset (56.0%); however after 4 h the sensitivity was determined to be >80%. These data indicated that the concentration of the virus in nasal swabs was elevated over the course of the disease. Conclusions: We have demonstrated that the RIDT is reliable for the diagnosis of H1N1pdm infection. Taking into consideration the time interval between onset and consultation and other features of H1N1pdm, such as contagiousness and headache, it may be necessary to re-test RIDT-negative cases later.