Abstract
Introduction: The correct identification of the infecting micro-organism in prosthetic joint infections is difficult and there is no single method that is wholly reliable. We report a novel method intended to improve accuracy by disrupting the biofilm surrounding the prosthesis and transferring samples rapidly to culture medium.
Method: Explanted prostheses from 20 revision operations were sampled by pressing a microbiology swab or by passing a No.10 surgical blade along it. The sample so obtained was plated immediately in the operating theatre onto horse-agar petri dishes. These were incubated in aerobic conditions in the laboratory. Culture results were compared with those obtained from our standard detection method using multiple tissue samples with are plated or grown in prolonged aerobic and anaerobic culture broth.
Results: The method proved practical to perform in practice. When compared with multiple tissue samples as the standard, the Positive Predictive Value was 90%, Negative Predictive Value 80%, sensitivity 82%, specificity 89%. In 4 of the 10 true positive samples, the theatre-inoculated samples yielded early results within 3 days, while conventional method yielded positives only later on prolonged culture.
Discussion: The above pilot is to continue and has started to alter our practice in sample taking. Blade-scrape does appear to penetrate the biofilm successfully. Growing confidence in interpretation and ease in reading the plates mean that in certain cases, we consider the results to be more reliable than traditional tissue culture. Direct plating also reduces the chance of bacterial overgrowth in broth inhibiting colonies of secondary infective organisms. Further refinement is needed, particularly with regard to anaerobic bacteria. Inaccuracies have resulted when agar plates are allowed to go out of date.
Correspondence should be addressed to The Secretary, BHS, c/o BOA, The Royal College of Surgeons, 35–43 Lincoln’s Inn Fields, London WC2A 3PE.
