To Floss or Not to Floss? That’s not the question

Bioethics-in-the-News-logoThis post is a part of our Bioethics in the News series

By Robyn Bluhm, PhD

Should we stop flossing?
Earlier this month, the Associated Press published a report showing that the evidence in support of flossing is “‘weak, very unreliable,’ of ‘very low’ quality, and carries ‘a moderate to large potential for bias.’” So, should we stop flossing? The American Dental Association says no: “interdental cleaners such as floss are an essential part of taking care of your teeth and gums.”

The AP report and the ADA’s response illustrate interesting questions about evidence-based medicine (EBM), an approach to medical decision-making that has become central to medicine since its introduction in the early 1990s. EBM is often defined as “the conscientious explicit and judicious use of current best evidence in making decisions about the care of individual patients.” The “current best evidence” referred to in this definition is (at least in the case of studies examining treatments) supplied by well-designed randomized controlled trials (RCTs). Although RCTs have been conducted to investigate the effectiveness of flossing, the quality of the studies – and therefore of the evidence they provide – has been poor; this much the AP report and the ADA agree on.

What counts as good evidence? The ADA and EBM
In their response to the report, the ADA appeals to other sources of evidence, ones that EBM considers inferior to RCTs. EBM provides a hierarchy of evidence that ranks research methods according to the quality of evidence they are expected to generate (see, for example, Guyatt et al. 2014, p. 11). At the top of the hierarchy are RCTs, followed by controlled trials that are not randomized. These studies use epidemiological methods to assess treatment outcomes in large groups of patients. Below these study designs comes (1) evidence from physiological research that aims to elucidate the details of biological processes, including the effects of an intervention, and (2) evidence from clinicians’ unsystematic clinical experience, described, for example, in case reports. According to EBM, the first of these lower sources of evidence is problematic because knowledge of physiology does not accurately predict clinical outcomes, while the second source is too limited and small in scale to give generalizable information.

dental floss embroidered in green
Image description: a dental floss package has been embroidered with green thread onto a plain piece of fabric. Image source: Flickr Creative Commons

In fact, the ADA’s response to the report on flossing appeals to both of these “lower” forms of evidence. They begin with an appeal to clinical experience, saying that “As doctors of oral hygiene, dentists are in the best position to advise their patients on oral hygiene practices because they know their patient’s oral health status and history.” The report later endorses the traditional rationale for flossing, which is exactly the kind of physiological reasoning criticized by EBM: “Cleaning between the teeth removes plaque that can lead to cavities or gum disease from the areas where a toothbrush can’t reach. Interdental cleaning is proven to help remove debris between teeth that can contribute to plaque buildup.”

Is the ADA ignoring EBM’s recommended approach to evidence? Not necessarily. Early accounts of EBM seem to recommend strict adherence to the hierarchy of evidence, advising clinicians that “they should look for the highest available evidence from the hierarchy” (Guyatt et al., 2002, p. 8). More recently, this advice has changed to “they should seek the highest-quality evidence available” (Guyatt et al., 2014, p. 11). The problem is that, in the absence of the “best” evidence, well-conducted RCTs, EBM does not provide clear guidance as to how to assess – and act on – the evidence that’s actually available.

What EBM misses…
Despite all of these problems with assessing the evidence, the ADA says that we should just keep flossing. For that matter, (poor evidence alert!) everyone I’ve told about this blog post has said that they don’t plan to stop. And it’s here that a dimension of evidence becomes apparent that is missing in EBM, but is central to bioethics: in deciding whether to use a treatment, we need to consider the balance of benefits and harms. We also need to set our standards of evidence accordingly. Flossing is unlikely to be harmful, and (despite the lack of evidence) may have significant benefits. It is also easy to do and inexpensive, both factors that count on the “benefits” side of the scale. We could try to acquire better evidence to support the ADA’s recommendations, but given how unlikely it is that flossing is actually harmful, we already have enough evidence to justify flossing.

…and why it matters
I suggest that the best way to understand the recent discussion over the evidence for flossing is not as an invitation to rethink our oral hygiene practices, but as an invitation to think more broadly about what kind of evidence, and how much, we should require before we change our health care practices – whether to begin to use a new therapy or to stop using an old one. The “low stakes” in this case may help to illuminate the issues and give us a simple case from which we can begin to think about more complicated questions about values and evidence. And there are plenty of these complicated questions, many of which have also appeared in recent media stories. Consider:

My first response to each of these stories is that I’d want better evidence than I would in the flossing case, in each case because the stakes are higher – both the risks and the potential benefits are greater. But I suspect that there’s no straightforward way to set evidentiary standards based on possible benefits and harms in these cases. I’d welcome readers’ comments on how we might think about the evidence required to decide about these and other complicated cases!

robyn-bluhm-photoRobyn Bluhm, PhD, is an Associate Professor in the Department of Philosophy and Lyman Briggs College at Michigan State University.

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