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Abstract & Commentary
Synopsis: The results of underlying bone culture were not accurately predicted by those of overlying open wounds. While bone culture affected physician’s choice of antimicrobial therapy, whether it affected outcomes was not studied.
Source: Khatri G, et al. Effect of bone biopsy in guiding antimicrobial therapy of osteomyelitis complicating open wounds. Am J Med Sci. 2001;321:367-371.
The microbial ecology of infected human bone is complex, yet bone biopsy well performed can often guide what is otherwise purely empiric therapy. In this article, Khatri and colleagues attempt to address the microbial ecology of bone infections associated with open wounds.
Their approach was to review charts of patients who had bone cultures done during debridement. Khatri et al wanted to determine the usefulness of bone cultures in directing antimicrobial therapy. No attempt was made to study outcome.
Cases from 1994-1998 were reviewed. Osteomyelitis was determined to be present clinically if the physicians caring for the patient and/or the infectious diseases consultants made that diagnosis. In all, 44 biopsies were done: 22 in paraplegics, 5 in peripheral vascular disease, 1 in diabetes, and 18 had no reported diagnosis. Quantitative cultures were not performed but the bone to be biopsied was carefully cleaned to avoid contamination.
Biopsies yielded 53 isolates, 36 Gram-positive aerobes, 12 Gram-negative aerobes, and 5 anaerobes. A total of 31 isolates were found in bone but not in wound cultures. Only 13 isolates were found in bone that werealso isolated in wounds. Staphylococcus aureus was the most common isolate in bone and its presence in bone tended to reflect wound cultures. Yet in 6 cases, S aureus was present in bone and not in wound, 3 with MSSA and 3 with MRSA. Other species found in bone and not in wound included diphtheroids (8), Pseudomonas species (4), enterococci (3), Bacterioides species (3), Acinetobacter species (2), coagulase-negative staphylococci (2), and the MSSA and MRSA isolates mentioned.
In the 44 patients reviewed, bone biopsy cultures prompted a change in antibiotic therapy in 14 patients, antibiotics were discontinued in 6, coverage was in place to cover bone organisms in 10, and bone culture results were ignored in 14.
Bone cultures were negative in 11 cases, 5 of whom had antibiotics discontinued, 2 of whom had additional antibiotics added, and 3 of whom had the same agents continued. One patient was not on antibiotics at the time of the negative culture.
COMMENT by Joseph F. John, MD
Osteomyelitis is a difficult disease to study. Many of the published studies are flawed due to methodologic confounders. Yet due to the scarcity of data over the years, articles on osteomyelitis—though flawed—are readily published and often in good journals. Khatri et al have given us a focused study of an important subset of patients prone to osteomyelitis, albeit with some flaws. This study would have been strengthened if quantitative bone cultures had been performed. Bacteria that grew from thioglycollate broth—that we would assume were present in small quantities—were included in the study although were are not told how many organisms grew from "thio" and were considered pathogens. Nevertheless, we may assume that organisms enumerated from bone were likely real since there was care taken during the biopsy to avoid contamination.
How does this study help us practitioners caring for these difficult patients? First it raises an old issue of the lack of value of overlying wound cultures. At the best, we can infer that if S aureus is part of the overlying flora, we probably should cover for that organism, paying attention to whether it is an MSSA and/or MRSA. However, up to 9-17% of S aureus isolates in wound will not be found in bone! Other organisms in wound culture matched bone isolates even more poorly, therein the strength of the study’s findings. Disturbingly, there was an unduly large percentage of diphtheroids isolated from bone. Future studies will need to be done to determine if these Gram-positive rods of low virulence are emerging pathogens in contiguous osteomyelitis (the focus of this study) or only an aberration of the study population in question and culture techniques.
Many of the practitioners (we are not told the total of prescribers) in this study apparently believe the bone cultures as 20 of 44 resulted in discontinued or altered therapy and 10 of 44 in continuing the same therapy. Khatri et al point out that the bone cultures in this study were usually obtained at the time of a therapeutic debridement. For that reason, Khatri et al suggest encouraging the surgeon to perform cultures, as that may alter therapy in many cases.
Some ID physicians favor biopsying bone through unaffected tissue. A simple procedure popularized by Dr. Ben Lipsky in Seattle allows the physician to consider doing the biopsy using a small trephene biopsy needle.1 Many a time in the last several years I wish I had skill of this procedure and could proceed to biopsy certain patients, particularly those with diabetic foot infections. On the other hand, many surgeons still are reluctant to biopsy bone through intact skin so patients with contiguous osteomyelitis still have to be managed in an individualized fashion. The work by Mader and colleagues emphasizes that nothing ensures success more than a good working relationship with a surgeon who will work closely with the infectious disease consultants.2 Even more important than a correlation of wound bone cultures and its effect on antimicrobial prescribing would be a study that determined the effect of bone biopsy culture and clinical outcome.
1. Lipsky BA. Osteomyelitis of the foot in diabetic patients. Clin Infect Dis. 1997;25:1318-1326.
2. Mader JT, et al. A practical guide to the diagnosis and management of bone and joint infections. Drugs. 1997; 54:253-264.