Diabetic Foot Infections: Culture Results from Bone Biopsy and Swab Specimens

Abstract and Commentary

By Dean L. Winslow, MD, FACP, Chief, Division of AIDS Medicine, Santa Clara Valley Medical Center, Clinical Professor of Medicine, Stanford University School of Medicine. Dr. Winslow is a consultant for Bayer Diagnostics and Pfizer/Agouron, and is on the speaker's bureau for Pfizer/Agouron.

This article originally appeared in the February 2006 issue of
Infectious Disease Alert. It was originally reviewed by the physician editor, Stan Deresinski, MD, FACP, and peer reviewed by Connie Price, MD. Dr. Deresinski is Clinical Professor of Medicine, Stanford University, and Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center. He serves on the speaker's bureau of Merck, Pharmacia, GlaxoSmithKline, Pfizer, Bayer, and Wyeth, and does research for Merck. Dr. Price is Assistant Professor of Medicine, University of Colorado School of Medicine. She reports no relevant financial relationship related to this field of study.

Synopsis: Seventy-six patients with diabetic foot osteomyelitis underwent surgical bone biopsy for culture had bone culture results compared to swab culture results. The results of bone and swab cultures were identical in only 17% of patients and bone bacteria were isolated from swab cultures only 30% of the time.

Source: Senneville E, et al. Culture of Percutaneous Bone Biopsy Specimens for Diagnosis of Diabetic Foot Osteomyelitis: Concordance with Ulcer Swab Cultures. Clin Infect Dis. 2006;42:57-62.

This study from a single diabetic foot clinic in france involved a retrospective chart review of patients who underwent surgical percutaneous bone biopsy with culture for microbiologic diagnosis of osteomyelitis. Patients included for study were those who had not received either local or systemic antibiotics for at least 4 weeks prior to cultures being obtained. Osteomyelitis was defined by a variety of reasonable criteria, which are detailed in the article. Swab specimens were obtained from foot ulcers after brief cleansing of the ulcer with sterile physiologic glucose solution applied with a sterile compress. Percutaneous bone biopsies were performed in the operating room using sterile technique with an 11-gauge biopsy needle inserted through a 5-10 mm skin incision made at least 20 mm from the periphery of the ulcer. When debridement was required, the biopsy was obtained prior to the foot being opened. Standard microbiological methods were employed to isolate and identify bacterial pathogens.

Eighty-one bone biopsy samples and 69 swab samples were obtained from 76 patients. A mean of approximately 1.5-1.6 bacterial species were isolated from both culture sources. Interestingly, staphylococci were isolated much more frequently from bone samples (52%) than from swab samples (38%), but the isolation rate for Staphylococcus aureus was fairly similar (26% bone vs 33% swab). Somewhat counterintuitively, the difference is largely explained by the fact that coagulase negative staphylococci were isolated much more often from bone than from wound swabs (26% vs 5%). Streptococci were isolated from only 12% of bone biopsy specimens and 20% of ulcers. Gram negative bacilli were obtained from 18% of bone and 26% of swab samples. Anaerobes were isolated from 5% of bone and 3% of swab specimens. When looking at the proportion of pathogens isolated from cultures of bone biopsy and/or swab samples obtained from the 69 patients who had cultures from both sources, the concordance was poor. The percent concordance for S. aureus was 43%, for gram negative bacilli 29%, streptococci 26%, enterococci 7%, coagulase negative staphylococci 3%, and there was no concordance for corynebacteria and anaerobes.

Commentary

This study serves as a good reminder of the historically poor reliability of superficial swab cultures in diagnosis of the etiologic agents causing diabetic foot infections in individual patients. Even the old saw I have been repeating for 30 years to fellows, residents, and students that, "only when one isolates S. aureus in pure culture from an ulcer can one assume that is the cause of the osteomyelitis," is proved false by this study. While surgical debridement is necessary in many cases, virtually all patients with osteomyelitis complicating diabetic foot infections will also require prolonged, often parenterally administered antibiotics. Because of the toxicities, expense, and potential inactivity of empirically chosen and excessively broad-spectrum antibiotics, it behooves us as hospital medicine clinicians to push our medical and surgical colleagues to obtain bone biopsies for culture so we can properly treat these serious infections.