Observational studies versus randomized controlled trials: avenues to causal inference in nephrology

A common frustration for practicing Nephrologists is the adage that the lack of randomized controlled trials (RCTs) does not allow us to establish causality, but merely associations. The field of nephrology, like many other disciplines, has been suffering from a lack of RCTs. The view that without RCTs, there is no reliable evidence has hampered our ability to ascertain the best course of action for our patients. However, many clinically important questions in medicine and public health, such as the association of smoking and lung cancer, are not amenable to RCTs owing to ethical or other considerations. Whereas RCTs unquestionably hold many advantages over observational studies, it should be recognized that they also have many flaws that render them fallible under certain circumstances. We provide a description of the various pros and cons of RCTs and of observational studies using examples from the nephrology literature, and argue that it is simplistic to rank them solely based on preconceived notions about the superiority of one over the other. We also discuss methods whereby observational studies can become acceptable tools for causal inferences. Such approaches are especially important in a field like nephrology where there are myriads of potential interventions based on complex pathophysiologic states, but where properly designed and conducted RCTs for all of these will probably never materialize.