Validation of self-administered nasal swabs and postage for the isolation of $\textit{Staphylococcus aureus}$

Abstract

$\textit{Staphylococcus aureus}$ carriers are at higher risk of $\textit{S. aureus}$ infection and are a reservoir for transmission to others. Detection of nasal $\textit{S. aureus}$ carriage is important for both targeted decolonization and epidemiological studies. Self-administered nasal swabbing has been reported previously, but the effects of posting swabs prior to culture on $\textit{S. aureus}$ yield have not been investigated. A longitudinal cohort study was performed in which healthy volunteers were recruited, trained in the swabbing procedure and asked to take weekly nasal swabs for 6 weeks (median: 3 weeks, range 1-6 weeks). Two swabs were taken at each sampling episode and randomly assigned for immediate processing on arrival to the laboratory (Swab A) or second class postage prior to processing (Swab B). $\textit{S. aureus}$ was isolated using standard methods. A total of 95 participants were recruited, who took 944 swabs (472 pairs) over a median of 5 weeks. Of these, 459 swabs were positive for $\textit{S. aureus}$. We found no significant difference (P=0.25) between 472 pairs of nasal self-swabs processed immediately or following standard postage from 95 study participants (51.4 % vs. 48.6 %, respectively). We also provide further evidence that persistent carriers can be detected by two weekly swabs with high degrees of sensitivity [92.3 % (95 % CI 74.8-98.8 %)] and specificity [95.6 % (95 % CI 84.8-99.3 %)] compared with a gold standard of five weekly swabs. Self-swabbing and postage of nasal swabs prior to processing has no effect on yield of $\textit{S. aureus}$, and could facilitate large community-based carriage studies.