Clinical Epidemiology

Clinical epidemiology is the application of the methods and principles of epidemiology, which is focused on population health, to the practice of clinical medicine, which is focused on the health of particular individuals. Modern medical practice should at all times be predicated on the best available scientific evidence. But at no time is there, in fact, any scientific evidence that pertains directly and reliably to a particular, individual patient. Rather, scientific evidence is derived from the past experience of other patients, or of subjects in clinical trials of various design. The relative correspondence between the one patient now receiving care and the many patients or subjects from whom scientific evidence has been derived is therefore an assessment prerequisite to clinical decisions, and an interface where science in medicine must inescapably collide with judgment. Clinical epidemiology is both a philosophical approach to medical decision making and a collection of tools and techniques that may inform the practice of medicine. This entry reviews the principles of clinical epidemiology, describes the relationship between this field and clinical practice, and discusses the use of clinical epidemiology in determining what diagnostic tests to perform. It also [Page 193]explores fundamental principles of statistics and probability, such as Bayes's theorem, that inescapably underlie medical decisions and can and should be used to inform and fortify them.

Evidence has securely claimed its place among the dominant concepts in modern medical practice. To the extent possible, clinicians are expected to base their decisions (or recommendations) on the best available evidence. The consistent application of evidence to clinical decision making rests on the traditional fault line separating clinical practice from public health. Despite efforts to bridge that gap, the philosophical divide between a discipline devoted to the concerns of populations and another devoted to the advocacy of an individual can seem impassable. But evidence is, in fact, the bridge, because evidencebased practice is population-based practice.

Evidence applied clinically derives from the medical literature, where the standards of evidence, and therefore practice, continuously evolve. But apart from case reports, what is reported in the literature is not the experience of an individual patient and certainly not the experience of the individual patient, but rather the experience of a population of other patients. Therefore, the practice of evidence-based medicine requires the application of populationbased data to the care of an individual patient, different in ways both discernible and not, from the subjects whose experience is reported. All decisions made on behalf of (or, preferably, with) individual patients are extrapolation or interpolation from the prior experience of other patients. Clinical medicine is evidence based only if it is population based.

To base clinical decisions for an individual on the individual alone, the outcome of an intervention would need to be known in advance. In other words, medicine would need to borrow from astrology, or some alternative system of predicting future events. The choice of an initial antihypertensive drug for a hypertensive patient cannot be based, before the drug is prescribed, on the response of the patient in question. Nor can the benefits to the patient be known in advance. The drug is chosen based on the published results of antihypertensive therapy in other patients. The particular drug is selected based on the characteristics of the patient, and how closely they approximate those of others who have benefited from specific therapies. Once the drug is selected, while the therapeutic effect on the surrogate measure (e.g., blood pressure) is detectable, most outcome benefits to the patient (e.g., stroke prevention) remain unknowable. The physician can never identify the stroke that was prevented in an individual but can only surmise that the risk of stroke has been reduced for that individual.

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