Diagnostic Approach for Diarrhea-Predominant Irritable Bowel Syndrome

Initial Evaluation

The American Gastroenterological Association recommends beginning with targeted baseline laboratory testing, including complete blood count, C-reactive protein or erythrocyte sedimentation rate, celiac serology, and fecal calprotectin, while taking a detailed history to establish a positive diagnosis of IBS-D 1
Determine the duration and onset of symptoms, and whether they started after an acute gastroenteritis, antibiotic use, or psychological stress 1
Confirm the relationship between abdominal pain and altered bowel habit, specifically whether pain is relieved or worsened by defecation, or temporally associates with changes in stool frequency or consistency 1
Use the Bristol stool chart to classify predominant stool type on abnormal days, with loose/watery stools >25% of the time suggesting IBS-D 2, 1
Actively screen for alarm features, including rectal bleeding, unintentional weight loss, nocturnal symptoms, fever, family history of inflammatory bowel disease or colorectal cancer, and age >50 years at symptom onset 2, 1, 3

All patients under 45 years with these symptoms require initial tests, including complete blood count, C-reactive protein or erythrocyte sedimentation rate, celiac serology, and fecal calprotectin 1
The American Gastroenterological Association recommends stool examination for ova and parasites if travel history, endemic area exposure, or persistent diarrhea 2, 4
Stool occult blood testing and serum chemistries and albumin may also be considered, particularly if malabsorption is suspected 2, 4

If fecal calprotectin is ≥250 μg/g, there is a high suspicion for inflammatory bowel disease, and colonoscopy should be performed 2
If fecal calprotectin is 100-249 μg/g, the test should be repeated off NSAIDs and proton pump inhibitors, and colonoscopy should be considered if the repeat test remains indeterminate or abnormal 2
If fecal calprotectin is <100 μg/g, it supports a functional diagnosis 2

If baseline tests are normal but symptoms persist, consider lactose hydrogen breath test for carbohydrate malabsorption, fructose breath testing, small bowel biopsies, or colonic biopsies via flexible sigmoidoscopy based on clinical judgment 2, 4, 5, 6, 7

If baseline investigations are normal and no alarm features exist, make a positive diagnosis of IBS-D rather than continuing exhaustive testing, using the Rome IV criteria 1, 2
Supportive features that strengthen the diagnosis include bloating and visible abdominal distension, abnormal stool frequency, passage of mucus, and urgency or feeling of incomplete evacuation 2, 6

If the diagnosis remains uncertain after initial testing, a therapeutic trial can be both diagnostic and therapeutic, with a trial of loperamide for diarrhea-predominant symptoms with normal testing 2, 4