Focus On The Body Of Evidence…Not Just Individual Studies

JAMA published an important article last week. This is an important article because it reminds us to review the totality of the evidence and not just an individual study. There are several reasons to focus on the whole body of evidence and not just individual studies:
So as not to be misled by an individual publication. Individual studies are subject to publication bias- the disproportionate publication of positive studies to the exclusion of negative studies. Individual studies are also just one of a distribution of findings.
Evaluate the consistency of the effect. Is the magnitude of the effect similar across studies? If so this is reassuring. If not you will have to figure out why and what it means.
Individual studies can be flawed/biased and a body of evidence is less likely to be flawed (though it could be).
– Multiple small underpowered studies can become more useful when combined (ie metaanlysis). If you only read the one underpowered study you might conclude that the intervention is useless.

A colleague of mine has recently changed his prescribing habits for diuretics in the treatment of hypertension. He based this decision on the recent publication of a single study comparing hydrochlorothiazide to chlorthalidone. When I examined the totality of the evidence (2 meta-analyses I come to a different conclusion than he did (I will be posting a blog on this in the near future).

So what does this mean as you try to answer a clinical question? What about when you are just reading through your weekly journal?
1) Use synthesized evidence first to answer questions. These are higher up in the Haynes’ hierarchy (the figure below) and included evidence based textbooks like Dynamed. Second line would be synopses of systematic reviews (called syntheses in the Haynes’ hierarchy). Next would be a systematic review (synopsis).
Haynes' 6S Hierarchy

2) After reading an individual study look for systematic reviews on the same intervention. Authors will sometimes refer to them in the intro and discussion sections of their paper. Look to see how the current paper compares to the rest of the evidence so you can put the new findings in perspective.

The limitation of this approach is time (although reading an individual study actually takes more time than reading a synthesis) and access to resources. There is no reason for a practicing clinician not to have access to an evidence based resource like Dynamed (or UpToDate- though I hesitantly put this here for my own reasons). Costs of these resources is reasonable and is money better spent than journal subscriptions.

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