The present systematic review was conceived to collect the available data from the four major diagnostic tests, namely 75SeHCAT, C4, fecal BA assay and FGF19, proved to have clinical validity in identifying BAM in patients with chronic diarrhea due to various causes, mainly including functional (i.e., IBS-D, functional diarrhea) and inflammatory (Crohn’s disease) disorders. This evidence concerns the gene FGF19 and Chronic diarrhea.