Is Surgical Site Infection a Unifactorial Problem?

A quick look at the search term “surgical site infection” on Google Trends reveals that has trended positively relative to the Health category  over the past decade. And no doubt a large proportion of the search results have used the term “multifactorial” to describe the problem (see below; blue line = “surgical site infection;” black line = Health category).

Google Trends time ssi

But is SSI really a multifactorial problem? In an era of unfettered access to data, we risk measuring the impact of too many variables–jeopardizing the core assumption of independence central to the statistical techniques employed to analyze the data.  As a result of this “data fatigue,” over-fitting and information gain are real concerns. Furthermore, this data rich environment can make us lazy–after all, why bother collecting your own data if others have already curated a ton of it for your consumption?

What if this focus on collecting more data distracted us from collecting the right data? When we started exploring surgical site infection as Fellows in the Stanford Biodesign Program, we were immediately drawn to the link between wound contamination and SSI. Nothing new here–we recognized that this was well-established in the literature over the years, quantitatively by bacterial concentration risk thresholds of 10^4 CFU/g  (Krizek, 1975), and qualitatively through use of the NRC Wound Classification. But nonetheless, we we struck by the flood of results over the years demonstrating a strong, consistent link between the perceived (“subjective”) degree of wound contamination and the observed SSI rate.

wound class

Alas, the NRC Wound Classification is a bit too subjective and categorical for detailed risk model development, so we dug deeper. A few researchers (Fa-Si-Oen, 2005, Horiuchi, 2010, and Waldron, 1983) went as far as to measure the presence (and sometimes degree) of bacterial contamination present at the wound edge at the time of closure. The result of these studies are pooled and summarized below.

contamination graph

The implications are sobering:

  • 50% of abdominal wounds are contaminated during surgery
  • 20-33% of contaminated wounds result in infection
  • Remarkably, when there was no bacterial contamination present, there is correspondingly no risk of subsequent infection. Perhaps SSI is indeed a unifactorial problem!

Around the same time, the results of the CHIR-Net study were published, representing one of the most comprehensive studies evaluating the effect of wound protection devices on SSI rates. Might bacteriology results be predictive of these SSI results? Fortunately, a study by Mohan et al in 2012 evaluated the rate of bacterial contamination in a similar patient population on the exposed and protected surfaces of the wound protection device. As illustrated below, Mohan’s contamination rates, in view of the contamination studies mentioned above, effectively predicted the results of the CHIR-Net study.

alexis back calculated

To be entirely fair, post-hoc analyses like this can be subject to bias, but the results are highly suggestive, and certainly worthy of follow-up consideration and research. To that end, Prescient Surgical is currently sponsoring one of the most comprehensive studies of wound bacteriology to date, and we look forward to learning how the results expand our scientific knowledge in this arena. I look forward to updating this community with the results this summer.

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