Clinical meaning of unexpected positive cultures (UPC) in revision shoulder arthroplasty

Purpose: This study analyzed the prevalence and clinical meaning of unexpected positive cultures (UPCs) in revision shoulder arthroplasty for causes different than infection. Methods: Between 1976 and 2007, 107 consecutive patients with UPCs, no previous suspicion of infection, and fulfilling inclusion criteria were identified. Forty-five partial (PSA) and 62 total shoulder arthroplasties (TSA) with different preoperative diagnoses were reviewed. Cases were classified as true infections, possible infections, contaminants, and undetermined. Mean follow-up was 5.6 +/- 5.3 years. Results: The prevalence of UPC was 15%. Male sex was a risk factor for UPC. Results of preoperative blood tests and intraoperative pathology were negative in 94 and 97 cases, respectively. Most prevalent bacteria were Propionibacterium acnes (n = 68) and Staphylococcus epidermidis (n = 21). Fifty-three patients received treatment with antibiotics and 54 did not. In 11 patients, a postoperative arthroplasty infection during follow-up was diagnosed by culture and was produced by the same microorganism as the one isolated on the UPC (true infection). Risk factors for true infection vs contamination included revision of a TSA vs a PSA and the number of previous surgeries. Antibiotic use and number of positive cultures did not influence the rate of true infections. Conclusions: UPCs are a prevalent condition in revision shoulder arthroplasty for causes different than infection. In at least 25% of cases, UPC had no clinical relevance. In 10% of cases, a persistent infection was demonstrated. (C) 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.