Evidence-Based Medicine

22 Evidence-Based Medicine

22.1 Introduction

Over 20 years ago, Professor David Sackett and his colleagues defined evidence-based medicine (EBM) as the conscientious, explicit and judicious use of current best evidence in making decisions about the care of individual patients. This remains an excellent definition, drawing out all the key aspects of evidence-based practice. It refers to current best evidence and not specifically to evidence from randomised controlled trials (RCTs), or even systematic reviews, despite their premier position in the evidence hierarchy. The critical issues here are that the evidence used should be the best available, and up-to-date.

The process is conscientious, explicit and judicious; each of these is important. Care must be taken in each and every aspect of the use of the evidence, just as an otolaryngologist would with every other element of patient care. The process must be explicit. While not every stage may be outlined explicitly to the patient during a consultation, the practitioner needs to avoid obfuscation and be clear in her or his own mind about how they are weighing the evidence. This must, in particular, include weighing the benefits against any potential harm or cost. ‘Judicious’ implies careful and sensible judgement—the use of common sense. EBM is not, nor ever has been—‘cook book medicine’ involving the slavish following of fixed rules. Rather, the practice involves a careful integration of knowledge of the relevant evidence with the patient’s own individual characteristics.

Amongst the most important of these are the patient’s values and beliefs, and their attitude towards risk and uncertainty. The original definition rightly highlights the care of individual patients. Although data from the trials and reviews that underpin evidence-based practice may come from groups of patients, and aggregated data derived therefrom, the evidence from this data is used, by the clinician, for the individual management of their patient. The evidence can only be used to make good decisions when the clinician has a clear understanding of the patient’s unique characteristics (how pathophysiological processes are affecting them personally, in their specific context) and their wishes, values and beliefs.

In the 21st century, we have learnt the importance of shared decision making. The original definition of Sackett et al may nowadays be seen to be rather paternalistic. It focuses on the practitioner of EBM making the decisions for, or on behalf of, the patient. A more modern version might be as follows:

‘EBM is the conscientious, explicit and judicious use of current best evidence in helping individual patients to make decisions about their care’.

This revised definition emphasises that patients and health professionals work together and share the decision-making process.

22.2 Systematic Reviews and the Evidence Hierarchy

EBM practitioners look for ‘best evidence’. The Oxford Centre for Evidence-Based Medicine has developed a hierarchy of different types of evidence. This can help practitioners when they are looking at different types of evidence into, for example, the effects of treatment, diagnostic test accuracy, prognosis, harms and screening. When considering the effectiveness or otherwise of interventions (treatments), the highest level of evidence (level 1) is provided by systematic reviews (or, much less commonly, by an N-of-1 randomised trial). Level 2 evidence can come from a randomised trial or an observational study where the treatment effect is large. imageTable 22.1 lists the levels of evidence for therapeutic studies.

Systematic reviews locate, appraise and synthesise evidence from scientific studies using a particular, pre-specified, scientific method. This is in contrast to the more traditional, nonsystematic review articles that have been (and continue to be) published in the literature. It is common nowadays to find articles in journals purporting to be systematic reviews; there is evidence that inclusion of such articles can improve a journal’s impact factor. However, the reader should beware. Not all published systematic reviews are of high quality. Many fail to follow a pre-specified plan, or are conducted in such a way that they may be prone to bias.

In order to minimise the chance of bias, and to avoid re-duplication of effort, the authors of systematic reviews should write a protocol setting out, before they start, what they plan to do. That protocol should be published and should ideally be lodged with a database such as PROSPERO, an international prospective register of systematic reviews. Cochrane (www.cochrane.org) is an independent, global, not-for-profit, charitable organisation, originally established in 1993, that publishes high-quality systematic reviews electronically in the Cochrane Library (www.cochranelibrary.com). These reviews have been described as setting the ‘gold standard’ for systematic reviews. All Cochrane reviews are based on protocols that have previously been published in the library.

image Table 22.1 Levels of evidence for therapeutic studies


Type of evidence


Systematic review (with homogeneity) of RCTs


Individual RCT (with narrow confidence intervals)


All or none study


Systematic review (with homogeneity) of cohort studies


Individual Cohort study (including low quality RCT, e.g. < 80% follow-up)


“Outcomes” research; Ecological studies


Systematic review (with homogeneity) of case-control studies


Individual Case-control study


Case series (and poor quality cohort and case-control study


Expert opinion without explicit critical appraisal or based on physiology bench research or “first principles”

Mar 31, 2020 | Posted by in OPHTHALMOLOGY | Comments Off on Evidence-Based Medicine
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