Defining Normal Bowel Function Relative to GI Disease: A Systematic Review of the Literature

Abstract

Purpose: Gastrointestinal diseases are often studied and compared to healthy control subjects. While textbook definitions of normal bowel habits range between 3 bowel movements per week to 3 bowel movements per day, this definition is uni-dimensional since there is no determination of change in function and no accounting for stool form. Thus there are various perceptions of normal in the clinic and in research. However, it is nearly impossible to define clinical disease in the absence of a clear definition of normal. This is especially true in conditions such as irritable bowel syndrome where the diagnosis is purely based on symptoms. In this study, we aim to systematically review the terms used to define normal in the study of gastrointestinal disease. Methods: A systematic review of the literature was conducted between 1950 and April, 2010 using search terms that sought case control citations in gastrointestinal disease. Title, abstract and full paper reviews were focused on identifying the papers for final inclusion based on confirming a control group with >10 subjects, a comparator group of confirmed disease (true case control study) and not a review article. The final papers were then reviewed for the definition of normal control used for the study. Results: Using the initial search terms, 340 abstract titles were identified. After applying the a priori defined inclusion and exclusion criteria, 58 papers met criteria for the systematic review. Of the 58 papers, 44 compared controls to functional bowel disease. The remaining abstracts compared controls to organic disease. In both instances, the most common definition for normal was that they did not meet criteria for the disease to be studied. The second most common definition was just the term “healthy” with no real explanation of what that term meant. Thus in over 80% of studies, the definition of normal was not provided in detail. In a minority of studies was there a definition used which detailed stool form and frequency. However, even in these instances, there was heterogeneity among different studies on the definition of normal. Conclusion: There is no uniform definition of healthy controls in the study of gastrointestinal disease. In the absence of a uniform definition of normal bowel function, the interpretation of the GI literature in case-control studies is suspect.Table