Use of Saliva Swab for Detection of Influenza Virus in Patients Admitted to an Emergency Department

Nasopharyngeal (NP) specimens are commonly used for the detection of influenza, but saliva swabs are easier to obtain and cause less discomfort to the patients. The objective of this study was to evaluate the usefulness of saliva swab specimens for the diagnosis of influenza compared with NP specimens. Influenza virus detection rate in saliva and NP swabs was compared in adult patients admitted to an emergency department from January to March 2020, using the Xpert Xpress Flu/respiratory syncytial virus (RSV) test. Cycle threshold (C(T)) values were evaluated in all the cases. Among the 82 patients recruited, 19 had an influenza-positive diagnostic test result (11 influenza A and 8 influenza B). Overall, the agreement between saliva and NP swabs results was 97.6% (80/82; κ = 0.929; 95% confidence interval [CI], 0.832 to 1.0). There was no significant difference in the influenza detection rate between saliva swab and NP specimens (20.7% [17/82] versus 23.2% [19/82]; P = 0.5). There were only two discordant results (influenza B in an NP and false negative in a saliva sample). Manual inspection of the amplification curves showed that influenza RNA had been amplified in saliva with high C(T)s (C(T) of 40) that the test reported as a negative result. The overall sensitivity and specificity for saliva was 89.5% (73.0% to 100%) and 100% (99.2% to 100%), respectively. In all the cases, the same influenza virus (A/B) was detected. Median C(T) values were significantly lower in NP (31; interquartile range [IQR], 21.0 to 32.0) than in saliva (33; IQR, 23.0 to 38.0) (P = 0.001) specimens. Saliva swabs have high sensitivity and specificity for the detection of influenza virus by the Xpert Xpress Flu/RSV test and a high overall agreement and C(T) correlation with NP specimens. Saliva swab is a feasible specimen type for influenza testing that might be easily self-collected with minimal equipment and discomfort. IMPORTANCE Early detection of influenza virus is important for guiding antiviral and antibacterial treatment for infection control and public health measures. We have observed that saliva swab specimens have high sensitivity and specificity for the detection of influenza by the Xpert Xpress Flu/respiratory syncytial virus (RSV) test and high overall agreement and C(T) correlation with nasopharyngeal specimens. Saliva swab may therefore be a feasible specimen type for influenza testing that can be easily self-collected with minimal equipment and discomfort.