CODIFI (Concordance in Diabetic Foot Ulcer Infection): a cross-sectional study of wound swab versus tissue sampling in infected diabetic foot ulcers in England

Objective: To determine the extent of agreement and patterns of disagreement between wound swab and tissue samples in patients with an infected diabetic foot ulcer (DFU). Design: Multi-centre, prospective, cross-sectional study. Setting: Primary and secondary care foot ulcer/diabetic outpatient clinics and hospital wards across England.

Participants: Inclusion criteria: Consenting patients aged ≥18 years; diabetes mellitus; suspected infected DFU. Exclusion criteria: Clinically inappropriate to take either sample. Interventions: Wound swab obtained using Levine’s technique; tissue samples collected using a sterile dermal curette or scalpel. Outcome measures: Co-primary: Reported presence, and number, of pathogens per sample; prevalence of resistance to antimicrobials among likely pathogens. Secondary: Recommended change in antibiotic therapy based on blinded clinical review; adverse events; and, sampling costs. Results: 400 consenting patients (79% male) from 25 centres. Most prevalent reported pathogens were Staphylococcus aureus (43.8%), Streptococcus (16.7%), and other aerobic gram-positive cocci (70.6%). At least one potential pathogen was reported from 70.1% of wound swab and 86.1% of tissue samples. Pathogen results differed between sampling method in 58% of patients, with more pathogens and fewer contaminants reported from tissues.

The majority of pathogens were reported significantly more frequently in tissue than wound swab samples (p<0.01); with equal disagreement for Staphylococcus aureus and Pseudomonas aeruginosa. Blinded clinicians more often recommended a change in antibiotic regimen based on tissue compared with wound swab results (increase of 8.9%, 95% CI:2.65,15.3%). Ulcer pain and bleeding occurred more often after tissue collection versus wound swabs (pain: 9.3%,1.3%; bleeding: 6.8%,1.5%, respectively). Conclusion: Reports of tissue samples more frequently identified pathogens, and less frequently identified non-pathogens compared with wound swab samples. Blinded clinicians more often recommended changes in antibiotic therapy based on tissue compared with wound swab specimens. Further research is needed to determine the effect of the additional information provided by tissue samples. Trial registration number ISRCTN52608451.