Diagnosing diabetic foot infection – wound swab or tissue sample?

A longstanding debate in the field of diabetic foot infection concerns the best method for microbiology assessment in patients with a suspected foot infection: a wound swab or a tissue sample? A wound swab is relatively easy, cheap, and has fewer adverse events such as bleeding or pain. A tissue sample is more likely to contain the pathogens causing the infection, and as such is thought to lead to better treatment. This is also the recommended option in the IWGDF Guidance. A new, very elaborate, and extensive, report has been published, trying to answer this question.

Published in the highly acclaimed UK Health Technology Assessment, professor Nelson and her colleagues report the outcomes of 400 participants with a suspected diabetic foot infection. In short, they found at least one pathogen in 70% of the wound swabs, and 86% of the tissue samples. In a virtual treatment assessment based on these microbiological results, doctors reported significantly more (virtual) treatment changes after tissue sampling.

However, finding more pathogens after sampling and changing treatment more often does not simply lead to the conclusion of choosing tissue sampling over wound swabbing. The devil is, as always, in the details. For the lovers of details, please read the full report, as it provides some excellent discussions. Some of these discussions concern the fact that in some patients swabs resulted in more (and different) pathogens, and that no differences were found in prevalence of the widespread S. aureus, MRSA and Pseudomonas.

Furthermore, a very small sub-study on 12 samples that also underwent molecular analysis showed that this technique reported additional or different pathogens in 50% of the wounds. As this was only a small sub-study, these results should be interpreted with caution. They are, however, in line with findings of DFA’s Matthew Malone.

Apart from the virtual assessment by an experienced (and blinded) team, no clinical differences between either swab or sampling could be investigated in the current study. This might be the reason why the authors are very cautious in their recommendations to clinical practice. They provide four potential scenarios, but do not recommend one over another. It is hoped that guideline authors, who will undoubtly use this report in future guidelines, will be more outspoken, otherwise the results may be of limited use to daily clinical practice.

The four scenarios described are as follows:

  • Swab sampling during the first assessment, followed by tissue sampling for any subsequent assessment; for example when clinical assessment determines an inadequate antimicrobial regime
  • Both swab and tissue sampling at any assessment
  • Only tissue sampling
  • Only swab sampling

Combining both methods may give the most information, but is also most costly and time-consuming. Choosing one method over another leads to the known and above-described disadvantages in pathogen discovery (more pathogens are found, but not all, in tissue sampling) and risk of pain or bleeding (higher in tissue sampling). The first scenario therefore seems a good combination of both worlds (swab and sample), while also combining these methods with clinical assessment. Without definite proof, and with all the minor differences between patients, clinical assessment still holds a crucial position in daily care.

As a clinician, each foot infection deserves consistent attention to the different assessment and treatment options available. But if you would be forced to choose one option, this report underlines the IWGDF recommendation of doing a tissue sample, rather than a wound swab, when possible.