Comparison of Combined Nose-Throat Swabs with Nasopharyngeal Aspirates for Detection of Pandemic Influenza A/H1N1 2009 Virus by Real-Time Reverse Transcriptase PCR

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Abstract
Data assessing the diagnostic accuracies of use of different respiratory samples for the detection of the novel influenza A/H1N1 2009 virus by molecular methods are lacking. The objective of this study was to compare the sensitivity of combined nose and throat swabs (CNTS) with that of nasopharyngeal aspirates (NPA). This was a prospective study of adults and children with suspected influenza. Real-time reverse transcriptase PCR testing was used for the virological diagnosis. Of the 2,473 patients included, 264 with paired CNTS and NPA were randomly selected. Novel influenza A/H1N1 virus was identified in at least one sample for 115 (43.6%) patients, the majority of them young adults. In 109 patients (94.8%) the virus was identified in the CNTS, and in 98 (85.2%) it was identified in the NPA (P = 0.02). In 93 patients (80.1%), the virus was identified in both specimens. Spearman's rho correlation coefficient between the two methods was 0.82 (P < 0.001). There were no significant differences in accuracy between the specimens when patients were stratified according to demographic or clinical characteristics except in the case of women, in whom the sensitivity of CNTS was higher (P = 0.01). The combination of CNTS and NPA had a significantly higher sensitivity in identifying the virus than did each method alone (P = 0.02 for the comparison of the combination of both sampling methods with CNTS, and P < 0.001 for the comparison with NPA). We conclude that in patients with the novel influenza A/H1N1 virus, the diagnostic yield of CNTS is higher than that of NPA. The combination of both sampling methods increases the likelihood of diagnosing the virus.

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