Abstract
Evidence-based medicine (EBM) integrates individual clinical expertise with the best available external evidence in the care of individual patients. By enabling clinicians to directly appraise and apply current clinical research, EBM deals with the problems of deterioration in clinical performance, information overload, and lag in application of research findings to clinical practice. Thus, EBM is a useful tool to address the problems faced by clinicians attempting to provide optimum, current care for their patients. The rationale for EBM, its principles and application, as well as some limitations, are described here.
Why evidence-based medicine?
Clinicians caring for patients are faced with numerous questions that impact on clinical decisions. One study showed that an average half-day of clinical practice generates about 16 clinically important questions, which would alter clinical management at least four times per day (1). More disturbing is the fact that few of these relevant information needs are met. In the study of Covell et al., only about 30% of clinicians' information needs were met (mostly through asking colleagues), and the most frequently cited reasons for this were lack of time and lack of access to relevant, updated, organized information (1). This is particularly relevant because it has been demonstrated that clinicians' knowledge becomes rapidly outdated. Studies have consistently shown a significant negative correlation between clinicians' up-to-date knowledge and the time elapsed since graduation from medical school (2, 3).
Another important problem is the explosion of biomedical literature. It has been said that ‘the central job of doctors is to meet the needs of patients by drawing on the knowledge accumulated by medicine over 5000 years’ (4). Medical knowledge increases fourfold during a medical professional lifetime, making it very difficult for clinicians to keep updated (5). It has been estimated that one would have to read 19 journal articles a day, 365 days a year in order to keep abreast of relevant knowledge in one's own area (6).
In their fight against clinical entropy, clinicians need a method that helps them deal with these problems in a systematic fashion. One such method is evidence-based medicine (EBM), defined by Sackett et al. as ‘the conscientious, explicit use of the best external evidence in making decisions about the care of individual patients’ (7).
What is evidence-based medicine?
EBM as a discipline is relatively new. However, its principles can be traced back several centuries. This occurred most explicitly in the work of 18th century French clinicians such as Pierre Louis, to whom we owe the demise of the millennium-old practice of bloodletting (8). The distinguishing feature of these clinicians was their emphasis on the need for external evidence, as opposed to pathophysiological inference, when making decisions about diagnosis, therapy or prognosis of individual patients. More recently, this emphasis has been formalized in the field of clinical epidemiology (9) and has been given prominence by individuals such as Sackett and Feinstein.
The purpose of EBM is to help clinicians accomplish what every good clinician wants to do, that is, to integrate individual clinical expertise with the best available external evidence in the care of their patients. Best external evidence refers to patient-centred research into the usefulness of diagnostic and therapeutic interventions and prognostic indicators. Therefore, EBM is neither prescriptive nor ‘cookbook’ medicine. Rather, by focusing on an individual patient's needs, it informs, but does not replace, clinical expertise. EBM does not make decisions. It is the experienced clinician who decides whether and how the external evidence applies to a particular patient.
Practising EBM
To incorporate best external evidence with clinical expertise, clinicians need to learn to use tools that allow them to find, critically appraise and apply the evidence to their patients. A prerequisite is the recognition that there are knowledge gaps that need to be filled. Potentially, each clinical encounter can generate a plethora of clinical questions. Deciding which questions to address requires prioritization based on clinical importance, the feasibility of getting an answer within time constraints, and the frequency of the problem. Once a question is chosen, EBM principles can be used to answer it. The application of EBM to a question of managing a patient with acute migraine is illustrated elsewhere in this supplement. Here, we describe the general process of EBM, which consists of at least five steps.
Turn information needs into focused, answerable clinical questions
A focused, answerable question is patient-centred, directly relevant to the problem at hand, and contains at least three parts:
The patient or problem, for example, a 35-year-old woman suffering migraine with aura.
The intervention, exposure or finding: this can be a particular therapy, a diagnostic test, or even the passage of time if what we are interested in is prognosis. For example, we could focus on the comparison of two triptans for acute migraine.
The outcome: this refers to clinically relevant events that the intervention is expected to modify. In the case of therapy, events may be prevented, improved or altogether abolished. In diagnostic interventions, outcomes are the correct diagnoses obtained by the new test. For prognostic studies, outcomes are events that occur with the passage of time or as predicted by a number of variables. In a patient with chronic migraine treated with triptans, outcomes could be time to pain relief, 24-h pain-free, or adverse events.
Efficiently locate the evidence
This is an integral part of EBM. To accomplish it, clinicians need to familiarize themselves with the sources of evidence available to them. Typically, these consist of two types (1): ‘predigested’ evidence that has already been critically appraised (such as the Cochrane® database (available from Canadian Medical Association (http://www.cma.ca) and American College of Physicians (http://www.acponline.org/catalog/electronic) and Best Evidence® (available from the Canadian Medical Association (http://www.cma.ca) and Update Software Inc. (http://www.cochranelibrary.com/); and (2) ‘primary’ evidence from original research. The latter can be accessed through biomedical databases such as MedLine (PubMed®, Ovid®, Grateful Med®, etc.). To use the latter, it is helpful to familiarize oneself with search strategies using parameters such as limits, MeSH terms, Boolean terms, etc. Published strategies for efficient and effective use of MedLine are useful in this regard (10). Using pretested search filters can significantly improve efficiency (for example, http://www.ncbi.nlm.nih.gov/entrez/query/static/clinical.html/).
Critically appraise the evidence for validity and usefulness
EBM is not a sceptical demand for the perfect study. Searching and appraising the
evidence is done with the understanding that there is no perfect study and that
clinical decisions must take into account the best
Two questions should be answered when appraising the evidence: (1) whether it is valid (close to the truth and therefore worth taking into account); and (2) whether it is useful (applicable to our patient).
Based on common sense and on research methodology, critical appraisal guides have been developed to assess the literature dealing with most questions likely to arise in clinical practice.
A total of 19 users' guides to a broad array of medical literature
have been published in
The most important questions to ask when appraising the validity of various types of evidence are presented in Table 1.
Guides for selecting articles that are most likely to provide valid results
Interpret the evidence and apply it to your patient
As clinical encounters involve individual patients, one must move away from attempts at ‘generalizing’ the evidence and towards ‘individualizing’ it to the specific patient or problem. This involves obtaining and applying clinically useful measures of an intervention's effect. Clinically meaningful measures tell us something to which we can relate.
In evidence dealing with therapy, clinicians may find it useful to know how many
more patients one needs to treat with a new drug in order to obtain one
additional good outcome or to prevent one bad outcome. This is also known as the
number needed to treat or NNT (11). Its counterpart, the
number of patients one needs to treat to observe one additional adverse event or
to harm one additional patient, the NNH, is equally useful, and recent
literature provides clinician-friendly information on its application (12). By
contrast, clinically difficult-to-interpret measures include isolated
Equally important in our interpretation of the results is an estimate of their
precision. That is, how much could these results vary by chance alone? This is
often expressed as a 95% confidence interval, and is more meaningful than a
For articles dealing with diagnostic tests, the likelihood ratio is a useful measure that incorporates both sensitivity and specificity and that can explicitly help us adjust our diagnostic probability for an individual patient (14). A complete treatment of diagnostic test result interpretation is provided by Sackett et al. (15).
Finally, good clinicians will always consider their patients' values, preferences, risk perception and socio-economic context when deciding whether valid evidence can be applied to them.
Evaluate your own performance
As clinicians engage in the exercise of EBM principles, it is important that they keep updated and sharp in their ability to appraise and apply the evidence. Sackett et al. recommend rating one's performance by asking questions such as: Am I asking answerable questions? How efficiently do I search the literature? How efficiently do I use critical appraisal guides? Am I able to apply the evidence? (16).
Some limitations of EBM
The EBM process is timeconsuming
Practicing EBM takes time. However, international efforts on many fronts are successfully synthesizing the evidence and presenting it in clinician-friendly formats. Some examples include the Cochrane collaboration and the development of journals and databases of ‘secondary’ predigested literature as stated above. Similarly, clinicians in various fields are developing collections of one-sheet, bottom-line, critically appraised topics to which they can quickly refer for specific clinical questions.
There is no evidence or the evidence is not relevant
Lack of evidence is particularly problematic for highly specialized areas. In
these situations, one has to resort to the next best
Conclusions
Direct examination of the evidence from clinical research is important for several reasons. First, the practice of neurology has shifted from a rich, descriptive discipline, to one of increasing diagnostic and therapeutic interventions. Secondly, clinicians face growing pressure to support their decisions with solid evidence. Thirdly, large variations exist in the way medicine is practised, largely due to suboptimal application of the best available external evidence. Finally, we are inundated with information and we are slow to keep up. There is a lengthy delay (10 years on average) in the incorporation of scientifically sound research results into standard clinical practice (17). EBM allows clinicians to tap directly into clinical research results, assess their validity and usefulness and keep up-to-date.