Introduction: Staphylococci are a well recognized cause of orthopaedic implant infections, due to their ability to produce biofilms, that limit antibiotic and host defence capabilities. To detect serum IgM levels against staphylococcal slime polysaccharide antigens (SSPA), an original immunoenzymatic assay has been previously developed and tested in patients affected by staphylococcal vascular graft infections [The Lancet • 359:2166–2168, 2002]. This is the first report on the efficacy of this serum ELISA testing of anti-SSPA IgM in Staphylococcal Orthopaedic Implant-Related Infections (SOIRI).

Methods: SSPA is extracted and purified using a suitable original patented bacterial strain and method. The immunoenzymatic assay to detect anti-SSPA IgM was performed on sera collected from 53 patients with joint prosthesis (24 hips, 17 knees, 2 shoulders). Each serum sample was tested in triplicate in two different assays and values are expressed as means. Main exclusion criteria were: time from implant < 6 months, rheumatoid arthritis, concurrent known infections. The study was approved by the local ethical committee and all patients gave their informed consent.

28 sera were collected from controls (patients with uninfected joint prosthesis, as judged from clinical, laboratory and radiological data) and 25 sera were obtained from patients with known SOIRI, sustained by S. aureus or coagulase-negative Staphilococci (CNS) (positive joint aspiration and/or intra-operative cultural examination).

Results: 25 patients were classified as true negative, 21 true positive, 4 false negative, 3 false positive. Test efficacy calculation provided the following Results: sensitivity: 84 %, specificity: 89.3 %, positive predictive value: 87.5 %, negative predictive value: 86.2 %.

Discussion and Conclusion: Anti-SSPA IgM immunoenzymatic assay showed interesting sensitivity and specificity in this preliminary prospective study. The test appear as an innovative, user-friendly, cheap and non-invasive diagnostic tool for orthopaedic implant-related infections. Further studies should be devoted to better define the cut-off value, testing reproducibility and standardization for large scale application.