Diagnostic Approach to Chronic Diarrhea in Adults

Introduction to Diagnostic Guidelines

The American Gastroenterological Association recommends that patients with chronic diarrhea and weight loss undergo colonoscopy with biopsies due to the high risk of colorectal cancer and inflammatory bowel disease 1, 2, 3
The presence of alarm features such as weight loss and age >50 years mandates an aggressive workup, including blood work and stool studies to evaluate for malignancy, inflammatory bowel disease, and malabsorption syndromes 1, 2, 3

Colorectal cancer is a high-priority diagnosis, with a prevalence of 27% in patients with change in bowel habit, and approximately 50% of neoplasia proximal to splenic flexure 3
Microscopic colitis is a common cause of chronic diarrhea in older adults, particularly women, accounting for 15% of cases in this demographic 3
Inflammatory bowel disease, including Crohn's disease and ulcerative colitis, can present at any age and should be considered in the differential diagnosis 1, 3
Celiac disease is the most common small bowel enteropathy in Western populations and should be evaluated with celiac serology 3

Fecal calprotectin is a useful test to distinguish inflammatory from non-inflammatory causes of chronic diarrhea, with elevated levels indicating inflammatory bowel disease or microscopic colitis 2, 5
Stool culture and ova/parasites should be performed to rule out infectious causes, although they are uncommon in immunocompetent patients with chronic symptoms 3
Fecal occult blood testing or FIT testing can increase sensitivity for detecting colorectal neoplasia 2
Colonoscopy with biopsies is mandatory in patients with chronic diarrhea and weight loss, with a diagnostic yield ranging from 7-31% 3

Do not diagnose irritable bowel syndrome in patients with weight loss, as this is an exclusion criterion for functional disorders 1, 4
Do not perform flexible sigmoidoscopy alone in patients with alarm features, as full colonoscopy is required to evaluate the entire colon 2, 3
Do not skip colonic biopsies, even with normal-appearing mucosa, as microscopic colitis can be detected only with biopsies 2, 3

Blood work and stool studies should be completed within 1-2 weeks, with colonoscopy scheduled urgently (within 2-4 weeks) given alarm features 1, 2
If initial workup is unrevealing, proceed to upper endoscopy and specialized testing within 4-6 weeks 2