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Evidence Based Medicine: Module 2: Acquire

Acquiring the Best Available Evidence

Finding evidence to answer clinical questions can be challenging for several reasons, chief among them is the gut of available information. The PubMed database contains over 33 million citations, with over 800,000 citations being added annually. These citations include letters, commentaries, narrative reviews, errata and other types of information that are not appropriate for use in clinical decision making.

Next, the enormous variation in human language can make searching challenging. Consider the number of results in PubMed for a variety of search terms one might reasonably use to search for evidence on whether it is safe and effective to treat a pulmonary embolism patient as an outpatient.

Search Terms # of Results
pulmonary embolism oral anticoagulants outpatient 180
pulmonary embolism outpatient 1,314
pulmonary embolism outpatient treatment 803
pulmonary embolism treatment setting 2,037

In addition, though the evidence hierarchy is straightforward, the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more compelling evidence than the findings from a weaker RCT. And, of course, the strongest research evidence does not exist for every question (e.g. it is difficult to blind studies of surgical treatments and ethical standards prohibit RCTs that would expose subjects to harm).

Because searching the primary literature is challenging and time consuming, a number of resources exist to help clinicians identify the most relevant evidence to answer their clinical questions. These include clinical practice guidelines from professional organizations and commercial resources which aim to collect and synthesize all available literature on a topic (referred to in this guide as evidence summary resources).