Comparison of microbiology cultures from deep tissue biopsies compared to superficial swabs from infected diabetic ulcers

S. Brookes-Fazakerley, D. Harvey, G. Jackson, S. Platt

Introduction: Diabetic ulcer superficial swab cultures have a low specificity for guiding antibiotic treatment. Some studies have recently re-assessed and advocated the role of superficial swabs. We have performed an analysis of microbiology results in patients with infected diabetic ulcers to further appraise the need for using deep tissue cultures as a guide for antimicrobial treatment.

Methods: We reviewed 23 consecutive diabetic patients in 2013. All patients underwent investigation and treatment by the Orthopaedic department for deep, intractable diabetic ulceration. Microbiology culture results from superficial swabs were compared to deep tissue and bone biopsies.

Results: The mean numbers of isolates from soft tissue and bone biopsies were 2.1 and 1.8 respectively (range 1-4). The most prevalent organisms seen in deep samples were anaerobes (9 patients), Staphylococcus aureus (8 patients) and entrococci (4 patients). In superficial swabs, 74% cultured non-specific, mixed skin flora and enteric species. The remaining 6 patients cultured Staphylococcus aureus alone (1), with Steptococcus (2), Pseudomonas (2) and MRSA (1). All 23 soft tissue biopsies were culture positive, 19 bone biopsies were positive of which 14 grew the same organisms of soft tissue cultures. In deep tissue/ bone biopsies, 13/23 patients cultured specifically organisms that were seen non-specifically in superficial swab cultures. However, in 10 patients deep tissue specimens, grew organisms that were not cultured from superficial swabs with 6 of these being anaerobes.

Conclusion: We have shown that in 43% of cases, deep tissue cultures isolated organisms that were not grown by superficial swab cultures. In 26% of these cases the organism was an anaerobe favouring deep, low oxygen tension environments. We refute recent evidence claiming the value of superficial swabs. We implore physicians treating patients with these ulcers to refer to an Orthopaedic surgeon to perform deep tissue biopsies and treat according to their culture results.