Providing guidance in the dark: rare renal diseases and the challenge to improve the quality of evidence.

Among renal diseases, over 100 conditions meet the epidemiological criteria to be defined as rare, including disorders in development, transport and metabolism. Clinical management of rare diseases is likely to be less investigated than that of common disorders and for this reason the scientific evidence to support clinical practice is limited. Furthermore, no specific and validated methods for designing, carrying out or analyzing clinical trials in small populations exist with important consequences for evidence-based medicine. In this paper we aim at discussing the inherent difficulty in finding evidence in rare renal diseases, providing some suggestions on how the quality of evidence and the guidance in these diseases can be improved.