Comparison of Nasal and Nasopharyngeal Swabs for Influenza Detection in Adults

Abstract

OBJECTIVE: Examine differences in the detection of influenza by specimen and test type using paired nasal and nasopharyngeal swabs. DESIGN: Prospective study SETTING: Enrollment took place between January and March 2007 in a central Wisconsin population. PARTICIPANTS: Adult patients were screened and enrolled by trained research coordinators following medical encounters for acute respiratory illnesses of <10 days duration. METHODS: Paired nasal and nasopharyngeal swabs were collected from consenting patients and tested by both real-time reverse transcriptase polymerase chain reaction (rRT-PCR) and viral culture. A composite measure of positivity was used as the gold standard; cases included any positive result by rRT-PCR or viral culture from either specimen type. RESULTS: Paired samples were collected from 240 adults; 33 (14%) individuals tested positive for influenza by rRT-PCR. Using rRT-PCR, the sensitivity of the nasal swab was 89% (95% CI, 78%-99%) and the sensitivity of the nasopharyngeal swab was 94% (95% CI, 87%-100%), compared to a composite gold standard. CONCLUSION: Test sensitivity did not vary significantly by swab type when using a highly sensitive molecular diagnostic test, but power was limited to detect modest differences.