P1726 Direct identification and susceptibility testing of staphylococci from positive BacTec blood cultures with WalkAway system

Abstract

Objectives: Staphylococcus aureus is the leading cause of surgical site infection, and roughly 25−30% of the population are colonised. In general, colonised patients have a 2−14 fold increased risk of infection, but those patients that carry at higher levels are at highest risk. The patient's own nasal flora account for 80% of S. aureus infections. Rapidly identifying colonised patients provides the ability to proactively manage S. aureus carriers for the prevention of infection. The objective of this pilot study was to investigate the clinical performance of a new rapid diagnostic test system (for proposed introduction in 2007 under the trade designation 3M Rapid Detect Staph aureus) to identify high level carriage of S. aureus directly from nasal swabs. Methods: This was a multi-centre, prospective, comparative pilot study. There were a total of four sites, two in Europe and in the United States that screened and sampled healthy subjects. One nasal sample was collected from both nares using a swab. The sample was analysed by the 3M Rapid Detect diagnostic test system, which provides results in approximately 20 minutes and a standard quantitative culture method using a selective medium. Confirmation testing was conducted using the Tube Coagulase Test. Results: A total of 1044 subjects were enrolled across four sites with a total of 999 evaluable subjects (having both culture and detector results). 33.6% of all evaluable subjects were S. aureus carriers, while 12.4% carried 5000 cfu (high level carriers). The mean level of carriage for those who were positive was 3.23 Logs (Range 0.63-6.50 Logs). The sensitivity of the test at 5000 cfu was 87% (95% CI: 80−92%). The overall sensitivity of the test system was 51% (95% CI: 46−57%). The overall specificity was 92% (95% CI: 90−94%). At 5000 cfu the negative predictive value was 98% (95% CI: 97−99%) while the positive predictive value for all levels of carriage was 76% (95% CI: 70−82%). Based on a positive test result 77% of all patients are not treated preemptively. Conclusions:The proposed 3M Rapid Detect diagnostic test system proved to be a sensitive tool to identify subjects carrying high numbers of S. aureus in the nares. Especially, the high negative predictive value provides a screening tool to rapidly and accurately identify patients who are at low risk of developing a S. aureus infection after surgery, thus allowing for more targeted use of antibiotics and other infection control measures.