Novel Diagnostic for Orthopaedic Staphylococcus aureus Infections Using Media Enriched for Newly Synthesized Antibodies (MENSA).

Background: Acute orthopedic Staphylococcus aureus (S. aureus) infections are difficult to diagnose. Pathogen-specific, antibody-secreting cells (ASC) emerge into the blood only during active immune responses making them highly specific biomarkers for acute infection. We hypothesized that harvested ASCs will secrete IgG in vitro creating a novel analytic fluid, MENSA (medium enriched for newly synthesized antibody) that will enable the antibody-based diagnosis of acute S. aureus infections.

Methods: Subjects with infection-related orthopaedic complaints were enrolled in the Emergency Department (n = 37), and 9 cc blood samples were taken prior to culture-based diagnosis and within 48 hours of initiation of antibiotic therapy. Twenty healthy control subjects were enrolled. Serum and MENSA were prepared from each subject. Each fluid was then analyzed for the abundance of IgG specific for as many as 14 S. aureus antigens, including ClfA, coagulase, IsdA, IsdB, SCIN, Hla, and CHIPS, using a multiplex immunoassay.

Results: Among the S. aureus-infected patients (n = 31), 27 had positive MENSA tests for at least one S. aureus antigen, and 24 patients were positive for at least two antigens. MENSA tests from the blood of most control subjects were negative for all S. aureus antigens (n = 20). Patients with non-S. aureus infections yielded negative MENSA tests for S. aureus (n = 6). Positive MENSA tests were measured in patients with prosthetic joint infections, implant-associated infections, deep tissue abscesses and diabetic foot ulcers. ROC analyses of the IgG increases for S. aureus in serum (AUC = 0.61) and MENSA (AUC = 0.83) for culture-proven S. aureus infections showed that MENSA is the better predictor of S. aureus infections. The S. aureus antigens that were the strongest predictors include IsdA, IsdB, SCIN, Hla and CHIPS.

Conclusion: We have demonstrated a novel diagnostic for S. aureus using MENSA made from circulating ASC during acute orthopedic infections. MENSA tests for antibodies against recombinant S. aureus antigens were superior to serum and demonstrated high sensitivity and specificity for difficult to diagnose orthopaedic conditions.