Accurate identification of pathogens is a crucial step for successful treatment of implant-associated infections. Sonication of explanted foreign material and subsequent sonicate-fluid culture is regarded to be more sensitive than conventional tissue culture. However, the duration of incubation of cultures remains controversial. The aim of our study was to evaluate diagnostic yield of prolonged 14-days incubation compared to more classical 7-days incubation.

Consecutive sonicate fluid culture results from a 2-years period (2013–2015) were retrospectively analysed. All sonicate fluids were cultured aerobically, anaerobically and using blood culture system for 14 days and inspected for growth on day 1, 2, 7 and 14 days. Terminal subcultivation was performed on day 7 from broth and blood culture system for additional 7 days aerobically and anaerobically. Time of bacterial isolation was recorded. Microbiological significance was determined based on isolate quantity and concomitant growth in conventional tissue cultures.

A total of 394 sonicate fluid cultures from 304 patients (8–95 years, mean age 62), 53.9% (n=164) women, were analysed. 51.0% (n=201) were from explanted osteosynthetic material, 37.6% (n=148) from hip prosthesis and 11.4% (n=45) from knee prosthesis. Overall, 57.1% (n=225) of cultures were positive. Among them, 71.1% (n=160) were monomicrobial, 21.3% bimicrobial and 7.6% (n=17) polymicrobial. In total, 312 bacterial isolates were isolated. The most frequently isolated bacteria were coagulase-negative staphylococci (CoNS) 34.6% (n=108), Staphylococcus aureus 16.4% (n=51) and Propionibacterium acnes 11.2% (n=35). Gram-negative bacteria and anaerobes represented 18.3% (n=57) and 14.4% (n=45) of isolates, respectively. Among all sonicate fluid cultures, 92.0% (n=207) were positive after 7 days while 8.0% (n=18) were positive only after prolonged 14-days incubation with P. acnes being the predominant bacteria isolated after prolonged incubation. Among all P. acnes isolates 57.1% (n=20) were isolated within 7 days and 42.9% (n=15) within 14 days. Based on microbiologic criteria, 45.7% (n=16) of them were diagnostic; 37.1% (n=13) among early isolates and 8.6% (n=3) among late isolates, difference being statistically significant (p=0.016).

Prolonged 14-days incubation of sonicate fluid culture for the diagnosis of implant-associated infections offers only minor 8.0% improvement with regard to conventional 7-days incubation. The majority of P. acnes isolated after prolonged incubation are non-diagnostic using microbiologic criteria. Caution in an interpretation of significance of P. acnes isolated after 14-days incubation is warranted. However, due to a significant impact on patient management prolonged 14-days incubation is still recommended.