Purpose: The Rosenow enriched medium (RW) enables culture of anaerobic germs as well as slow-growing germs sometimes causing chronic infections on implanted material. The purpose of this work was to determine the usefulness of RW for the bacteriological diagnosis of infections on total hip arthroplasty (THA) and total knee arthroplasty (TKA°.
Material and methods: One hundred fifty-four pre- or intraoperative standard and RW samples were obtained in a prospective study of 80 consecutive patients (mean age 67.6 years) with an infected THA (n=56) or TKA (n=24) between January 1998 and June 2000. A bacteria was considered “infecting” if it grew on direct culture on at least two samples after enrichment. A single positive sample after enrichment was considered a contamination except for strict anaerobes for which direct culture could not be achieved. For these germs, growth after enrichment was thus considered significant.
Results: Among the 154 samples, 59 (38%) gave positive direct cultures and 95 (62%) positive enriched cultures. Among the 59 positive direct cultures, the RW culture was concordant in 87% of the cases. For the 13% of discordant cultures, the germs did not grow on standard medium: Staphylococcus (n=6), Pseudomonas (n=1), and enterobacteria (n=1). For the 95 positive enriched cultures, 41 (43.1%) of the RW cultures were condordant with the standard culture; the standard was positive in 13 (13.6%) and the RW negative (11 Staphylococcus including five aureus, one Pseudomonas, and one Corynebacterium), but in 41 cases, the RW was positive while the standard was negative (16 Staphylococcus, including 13 coagulase negative, five Streptococcus, two Entero-coccus, one Corynebacterium, three enterobacteria, and fourteen anaerobes). The infecting nature of the aerobic bacteria was retained because earlier or later samples were positive. The sensitivity and predictive value of a positive RW culture were 86% and 86% respectively.
Discussion: Using RW medium and standard samples is useful if germs grow after enrichment in order to confirm the infecting nature of the isolated germ (double culture). The reliability of RW medium was confirmed since it was concordant in 87% of the cases with a positive “gold standard” culture. For positive cultures after enrichment with a negative standard culture, RW allows correcting the diagnosis of infection in 43% of the cases. Its usefulness lies basically in the isolation of coagulase negative Staphylococcus and anaerobes (especially Propionibacterium spp. and Peptostreptococcus spp.).
