A Comparison of Tissue versus Swab Culturing of Infected Diabetic Foot Wounds

Objective. To compare the efficacy of swabbing versus tissue biopsy for microbiological diagnosis of diabetic foot infection. Methods. This was a prospective trial. Fifty-six patients with diabetic foot infection were divided into the following 3 groups according to the PEDIS grading system: grade 2 (n = 10), grade 3 (n = 29), and grade 4 (n = 17). Two specimens were collected from each wound for microbial culturing after debridement, including a superficial swab and a deep tissue punch biopsy specimen. Results. Swab culturing identified all of the microorganisms isolated from the corresponding deep tissue specimens in 9/10 of grade 2 wounds (90.0%), and this proportion decreased to 12/29 (41.4%) and 7/17 (41.2%) for grades 3 and 4 wounds, respectively (p = 0.02). Moreover, the sensitivity for identifying Gram-negative bacteria, such as E. coli and Citrobacter, by swabbing was low (33.3%). In addition, some Gram-negative bacteria, such as Serratia and Ralstonia pickettii, were isolated from deep tissues but not from swabs. Conclusions. Swab culturing may be reliable for identification of pathogens in diabetic foot wounds classified as grade 2. However, it is advisable to culture deep tissue specimens for wounds of grade ≥3 because swab culturing is associated with a high risk of missing pathogens, especially Gram-negative bacteria.