Abstract
BackgroundThe presentation of new influenza A(H1N1) is broad and evolving as it continues to affect different geographic locations and populations. To improve the accuracy of predicting influenza infection in an outpatient setting, we undertook a comparative analysis of H1N1(2009), seasonal influenza, and persons with acute respiratory illness (ARI) in an outpatient setting.Methodology/Principal FindingsComparative analyses of one hundred non-matched cases each of PCR confirmed H1N1(2009), seasonal influenza, and ARI cases. Multivariate analysis was performed to look for predictors of influenza infection. Receiver operating characteristic curves were constructed for various combinations of clinical and laboratory case definitions. The initial clinical and laboratory features of H1N1(2009) and seasonal influenza were similar. Among ARI cases, fever, cough, headache, rhinorrhea, the absence of leukocytosis, and a normal chest radiograph positively predict for both PCR-confirmed H1N1-2009 and seasonal influenza infection. The sensitivity and specificity of current WHO and CDC influenza-like illness (ILI) criteria were modest in predicting influenza infection. However, the combination of WHO ILI criteria with the absence of leukocytosis greatly improved the accuracy of diagnosing H1N1(2009) and seasonal influenza (positive LR of 7.8 (95%CI 3.5–17.5) and 9.2 (95%CI 4.1–20.3) respectively).Conclusions/SignificanceThe clinical presentation of H1N1(2009) infection is largely indistinguishable from that of seasonal influenza. Among patients with acute respiratory illness, features such as a temperature greater than 38°C, rhinorrhea, a normal chest radiograph, and the absence of leukocytosis or significant gastrointestinal symptoms were all positively associated with H1N1(2009) and seasonal influenza infection. An enhanced ILI criteria that combines both a symptom complex with the absence of leukocytosis on testing can improve the accuracy of predicting both seasonal and H1N1-2009 influenza infection.
Highlights
A novel influenza of swine origin, designated as new influenza A (H1N1), emerged in late April 2009
From a health perspective, differentiating the new influenza A(H1N1) ( referred to as H1N1(2009)) and influenza viruses from other pathogens presenting with symptoms of acute respiratory illness (ARI) is valuable
Between 27 April and 11 June, a total of 983 patients presented to the Emergency Department to be screened for influenzaThis included 137 adults with PCR-confirmed seasonal influenza strains presenting between the above dates (Figure 1), from which 100 patients (83 with influenza A(H3N2), 7 seasonal influenza A(H1N1) and 10 cases of influenza B were randomly selected
Summary
A novel influenza of swine origin, designated as new influenza A (H1N1), emerged in late April 2009. From a health perspective, differentiating the new influenza A(H1N1) ( referred to as H1N1(2009)) and influenza viruses from other pathogens presenting with symptoms of acute respiratory illness (ARI) is valuable. Making such a distinction serves to both improve individual case management—given the availability of safe and effective anti-influenza drugs to which H1N1(2009) remains presently largely susceptible to—and to augment wider public health surveillance and mitigation measures [1,2,3]. The presentation of new influenza A(H1N1) is broad and evolving as it continues to affect different geographic locations and populations. To improve the accuracy of predicting influenza infection in an outpatient setting, we undertook a comparative analysis of H1N1(2009), seasonal influenza, and persons with acute respiratory illness (ARI) in an outpatient setting
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