Microbiological Evaluation of Diabetic Foot Osteomyelitis

Received 7 September 2005; accepted 7 September 2005; electronically published 21 November 2005. Reprints or correspondence: Dr. John M. Embil, Infection Prevention and Control Unit, Health Sciences Centre, MS 673820 Sherbrook St., Winnipeg, Canada R3A 1R9 (jembil @hsc.mb.ca). Clinical Infectious Diseases 2006; 42:63–5 2005 by the Infectious Diseases Society of America. All rights reserved. 1058-4838/2006/4201-0010$15.00 Diabetic foot ulcers and osteomyelitis may limit health-related quality of life and contribute to mobility impairment, treatment morbidity, amputation, and death [1–6]. The ulcer may be caused by minor or repetitive trauma to the neuropathic foot, and bacterial seeding may result in a chronic draining sinus, osteomyelitis, and secondary bony deformity. Diagnosis of osteomyelitis in the diabetic foot is based on history, physical examination findings, radiographic and other imaging studies, probing the ulcer to bone, wound swab culture results, and deep-bone culture results. However, these studies have varied sensitivity and specificity [7–11]. Therefore, it would be helpful to have simple, reliable, and cost-effective techniques to establish the diagnosis and identify the causative microorganisms. Theoretically, this should facilitate appropriate treatment, including the type and duration of targeted antimicrobial therapy with or without surgical debridement, and should potentially minimize complications that may arise from treatment with broadspectrum antibiotics. Clinicians frequently obtain a wound swab for culture either from the ulcer base or the draining sinus and select antimicrobial therapy according to the microorganisms recovered. Although such wound swabs for culture may be reliable in determining the pathogens responsible for superficial infection [12], cultures of sinus tract swab specimens may be unreliable for chronic osteomyelitis [13]. Cultures of superficial swab samples from diabetic ulcers and sinus tracts may not adequately identify the true bacteriological characteristics of diabetic foot osteomyelitis because of bacterial colonization of the wound surfaces with microorganisms that are typically not considered to be pathogenic (such as the enterococci and coagulase-negative staphylococci). However, practitioners often accept swab cultures as an alternative to bone debridement or biopsy specimens because of the ease with which swab specimens can be obtained. Cultures of bone debrided from the base of the wound may also yield colonizing organisms that are usually considered to be nonpathogenic. In theory, cultures of bone specimens from deep within the osteomyelitic focus should be the most accurate method for identification of bacteria that are pathogenic and that can be eradicated by directed antimicrobial therapy; however, deep-bone cultures may be difficult to obtain because of limited technical expertise, time, and availability of surgical facilities. In the study that appears in the current issue of Clinical Infectious Diseases, Senneville et al. [14] attempt to define the true concordance between cultures of swab samples and cultures of bone biopsy specimens obtained from areas of osteomyelitis in the diabetic foot. This wellconducted study nicely summarizes the complexity of establishing an appropriate microbiologic diagnosis in cases of diabetic foot osteomyelitis. It is important that patients who had received previous antimicrobial therapy in the antecedent 4 weeks were excluded from the study, so that culture specimens would be unaltered by recent antimicrobial therapy. It required almost 8 years to amass 76 patients with 81 episodes of diabetic foot osteomyelitis from a single diabetic foot clinic, presumably because many patients whose cases were initially evaluated had already received empirical antimicrobial therapy from their primary care physician or emergency medicine physician. The strength of this study is that the authors accrued this large cohort of patients with supporting microbiological data to correlate between cultures of superficial swab samples and of deep-bone samples. The study shows that bone biopsy may clarify the causative organisms in cases of chronic diabetic foot osteomyelitis [14]. Pathogens identified from cultures of bone samples were identified from only 30% of the corresponding cultures of superficial swab specimens, and concordance varied