Multi-site and nasal swabbing for carriage of Staphylococcus aureus: what does a single nose swab predict?

Abstract

Background: Carriage of Staphylococcus aureus is a risk for infections. Targeted decolonization reduces postoperative infections but depends on accurate screening.

Aim: To compare detection of S. aureus carriage in healthy individuals between anatomical sites and nurse- versus self-swabbing; also to determine whether a single nasal swab predicted carriage over four weeks.

Methods: Healthy individuals were recruited via general practices. After consent, nurses performed multi-site swabbing (nose, throat, and axilla). Participants performed nasal swabbing twice-weekly for four weeks. Swabs were returned by mail and cultured for S. aureus. All S. aureus isolates underwent spa typing. Persistent carriage in individuals returning more than three self-swabs was defined as culture of S. aureus from all or all but one self-swabs.

Findings: In all, 102 individuals underwent multi-site swabbing; S. aureus carriage was detected from at least one site from 40 individuals (39%). There was no difference between nose (29/102, 28%) and throat (28/102, 27%) isolation rates: the combination increased total detection rate by 10%. Ninety-nine patients returned any self-swab, and 96 returned more than three. Nasal carriage detection was not significantly different on nurse or self-swab [28/99 (74%) vs 26/99 (72%); χ²: P=0.75]. Twenty-two out of 25 participants with first self-swab positive were persistent carriers and 69/71 with first self-swab negative were not, giving high positive predictive value (88%), and very high negative predictive value (97%).

Conclusion: Nasal swabs detected the majority of carriage; throat swabs increased detection by 10%. Self-taken nasal swabs were equivalent to nurse-taken swabs and predicted persistent nasal carriage over four weeks.

Keywords: Carriage; Multi-site screening; Staphylococcus aureus; spa typing.

Figure 1

Flow chart of patient participation from recruitment, through nurse swabbing, first self-swab, and repeated self-swabs. Numbers of patients included and excluded at each stage are shown, along with the patterns detected on multiple self-swabs.

Figure 2

(A) A total of 102 patients underwent swabbing of three sites by a study nurse. Numbers in circles are the numbers of participants with S. aureus cultured from nose only (green), throat only (red) or axilla only (blue), and number of people with multiple S. aureus cultured from each combination of anatomical sites. (B) Number of patients with S. aureus cultured from a swab from each anatomical site with a single spa type (solid colours) or a mixture of spa types (hatched).

Figure 3

Ninety-nine patients returned self-swabs from the nose over a four-week period. Four participants were found to have carriage on multiple swabs without meeting the definition of persistent carriage and two showed changing spa type. spa types recovered are plotted against swab number; related spa types are enclosed in a dotted-line box.