Wednesday, November 17, 2010

Evidence based medicine ≠ science based medicine

David Gorski explains in a recent post.

The apologists for science based medicine have for some time been pointing out a shortcoming of EBM: its utter failure in the evaluation of certain claims of complementary and alternative medicine. The reason for this failure, the apologists point out, is that EBM ignores biologic plausibility.

The science based medicine (SBM) movement has been criticized by some EBM proponents. The criticism (and this may be an oversimplification) is that there's no need for SBM as a new discipline to address plausibility because EBM already does that. Those proponents might say it's inherent in David Sackett's original description of EBM:

It's about integrating individual clinical expertise and the best external evidence...

Evidence based medicine is the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients. 

I say that because as a proponent of EBM myself I thought so, until fairly recently. I was aware of EBM's multiple failures to evaluate quackery but I just dismissed them as occasional unintended consequences. I naively believed that EBM in its true notion, as advanced by those who really understood it, appropriately took scientific plausibility into account.

I began to wonder, though, as I saw more and more failures, even instances in which EBM appeared to promote quackery. Then yesterday I ran across the above referenced post by Gorski. That did it for me. It explained in a way I hadn't realized before that this failure of EBM is more than just one of occasional unintended consequences. While it may have been unintended in the minds of the original founders, now it's systematic.

The problem is illustrated by the evidence pyramids, which are at the very core of the teaching about EBM. There are numerous versions of these pyramids, and Gorski links to a few examples, but what they have in common is that they put biologic plausibility (variously termed basic science, physiologic rationale, etc) at the bottom if they include it at all.

A casual observer might think that putting basic scientific rationale at the bottom means it's intended to be foundational (remember the old stepped care diagrams for treatment of hypertension and rheumatoid arthritis?). But that's not the way the EBM hierarchies of evidence are designed. Instead, higher levels on the pyramid trump those below. Students of EBM are taught to start at the top of the pyramid and “drill down,” level-by-level, until they find the evidence they want, and stop there.

Gorski includes an interesting quote from SBM blogger Kimball Atwood. Atwood uses homeopathy as an absurd example of EBM's pyramid approach, in which occasional equivocal results of clinical trials favoring homeopathy, easily explained by chance variation, are valued above the overwhelming basic science case against it (my italics):

When this sort of evidence is weighed against the equivocal clinical trial literature, it is abundantly clear that homeopathic “remedies” have no specific, biological effects. Yet EBM relegates such evidence to “Level 5”: the lowest in the scheme. How persuasive is the evidence that EBM dismisses? The “infinitesimals” claim alone is the equivalent of a proposal for a perpetual motion machine. The same medical academics who call for more studies of homeopathy would be embarrassed, one hopes, to be found insisting upon “studies” of perpetual motion machines. Basic chemistry is still a prerequisite for medical school, as far as I’m aware.

As to that last sentence, well, maybe not for long.

No comments: