Diagnostic Approach to Chronic Diarrhea

Initial Assessment

Chronic diarrhea is defined as abnormal passage of ≥3 loose stools per day for more than 4 weeks, according to the American Gastroenterological Association 2
A detailed history should be taken to establish whether symptoms are organic vs. functional, distinguish malabsorptive from colonic/inflammatory forms, and identify specific causes, as recommended by the European Society of Gastrointestinal Endoscopy 3, 2
Alarm features such as nocturnal diarrhea, weight loss, blood in stool, recent onset (<3 months), and persistent symptoms should be looked for, with a strength of evidence level of high, according to the American College of Gastroenterology 1, 4

Family history of neoplastic, inflammatory bowel, or celiac disease should be assessed, as recommended by the National Institute for Health and Care Excellence 6
Previous surgery, especially involving the ileum, right colon, or gastric bypass, can lead to bile acid malabsorption or bacterial overgrowth, with a moderate level of evidence, according to the European Society of Gastrointestinal Surgery 6

For suspected inflammatory diarrhea, fecal calprotectin should be tested to exclude colonic inflammation, especially in those under 40 with IBS symptoms, with a high level of evidence, according to the British Society of Gastroenterology 9
For suspected malabsorptive/fatty diarrhea, celiac disease serology (anti-tissue transglutaminase IgA) should be tested, with a high level of evidence, according to the American Gastroenterological Association 9, 5
For suspected watery diarrhea, SeHCAT testing or serum bile acid precursor 7α-hydroxy-4-cholesten-3-one should be considered for bile acid diarrhea, with a moderate level of evidence, according to the European Society of Gastrointestinal Endoscopy 9

Failing to distinguish between functional disorders and organic disease can lead to inappropriate treatment, with a high level of evidence, according to the American College of Gastroenterology 2
Overlooking microscopic colitis, which can present similar to IBS but requires different treatment, with a high level of evidence, according to the British Society of Gastroenterology 9
Missing bile acid malabsorption, which is common but often underdiagnosed, with a moderate level of evidence, according to the European Society of Gastrointestinal Endoscopy 9