Definition and differential diagnosis of functional diarrhoea
Functional diarrhoea is defined as liquid or semiliquid stools (not associated with pain or bothersome bloating) in 25% or more episodes of defecationRome Foundation Inc., 2016. It occurs in a subset of patients with chronic diarrhoea (commonly defined as diarrhoea persisting for longer than 4 weeks). Functional diarrhoea is distinguished from diarrhoea-predominant irritable bowel syndrome (IBS) by the absence of pain or discomfort, though there is considerable overlap in symptoms and therapies.
It is important to check for clinical features that warrant further investigation (see Clinical features that warrant further investigation of gastrointestinal symptoms), and to exclude other causes of chronic diarrhoea (see Differential diagnoses of chronic diarrhoea).
- functional gastrointestinal disorders—functional diarrhoea, irritable bowel syndrome
- inflammatory diseases—Crohn disease, ulcerative colitis, microscopic and collagenous colitis
- malabsorption syndromes—coeliac disease, villous damage caused by tropical sprue or chronic infection, pancreatic exocrine insufficiency, small intestinal bacterial overgrowth (SIBO), carbohydrate malabsorption, after surgery (eg bowel resection)
- chronic gastrointestinal tract infections—Clostridioides difficile (formerly known as Clostridium difficile), Giardia species, Entamoeba histolytica (amoebiasis), Cryptosporidium species
- drug-induced diarrhoea—metformin, antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), magnesium-containing products, antiarrhythmic and antihypertensive drugs, alcohol
- chemotherapy-induced diarrhoea—see the eviQ website
- bile salt diarrhoea—after cholecystectomy, bile salt malabsorption
- endocrine causes—hyperthyroidism, primary adrenal insufficiency (also known as Addison disease), diabetic autonomic neuropathies, hypoparathyroidism, neuroendocrine tumours
- laxative abuse
- factitious diarrhoea
Investigate whether there is a relationship between the patient’s symptoms and their diet, drugs (including over-the-counter drugs) or stress. Diarrhoea can be an unwanted effect of almost any drug or complementary therapy; common drug causes of diarrhoea are listed in Differential diagnoses of chronic diarrhoea. Anxiety can be associated with diarrhoea and may need treatment.
If there is doubt about the diagnosis, further investigation may be necessary. Investigations for chronic diarrhoea may include:
- stool microscopy (to identify infectious agents); see Faecal testing in acute infectious diarrhoea
- faecal calprotectin and C-reactive protein (to identify inflammation)
- faecal elastase (to identify pancreatic exocrine dysfunction)
- coeliac serology
- thyroid function tests
- HIV testing (chronic unexplained diarrhoea is an indicator condition for HIV testing1)
- endoscopy (with duodenal biopsy)
- colonoscopy (with biopsies to exclude microscopic colitis).
Further tests to look for endocrine causes and neuroendocrine tumours may be undertaken if there is clinical suspicion of these syndromes.