Diagnosis
Chronic diarrhea is arbitrarily defined as lasting longer than 4 weeks and is often due to noninfectious causes, except for infec tion with Giardia lamblia. This diagnosis should be considered in patients with exposure to young children or potentially contaminated water (lakes and streams).
Gastroenterology and Hepatology
Medications are often overlooked as a cause of chronic diarrhea. Look for PPIs, magnesiumcontaining antacids, metformin, colchicine, antibiotics, and sorbitol (added as a sweetener to gum and candy).
Select colonoscopy for most patients with chronic diarrhea. Patients undergoing colonoscopy should have the terminal ileum viewed to assess for Crohn disease and undergo random biopsies of the colonic mucosa to assess for microscopic colitis. If the colonoscopy is nondiagnostic, a 48 to 72hour stool collection with analysis of fat content measures the amount of diar rhea and steatorrhea. Fat excretion above 14 g/d is diagnostic of steatorrhea. Patients with steatorrhea should undergo evaluation for smallbowel malabsorption disorders (e.g., celiac disease), bacterial overgrowth, and pancreatic insufficiency.
Stool electrolytes (sodium and potassium) can be measured in liquid stool to calculate the fecal osmotic gap, which helps to diagnose osmotic diarrhea. The gap is calculated as 290 − 2 × [Na + K]; an osmotic gap >100 mOsm/kg H2O indicates an osmotic
diarrhea. A gap <50 mOsm/kg H2O indicates a secretory diarrhea. Measured stool osmolarity <250 mOsm/kg H2O suggests
factitious diarrhea associated with chronic laxative abuse or adding water to the stool.
Osmotic diarrhea is most commonly caused by lactase deficiency. Osmotic diarrhea is associated with eating, improves with fasting, and typically is not nocturnal. Secretory diarrhea is characterized by largevolume, watery, nocturnal bowel movements and is unchanged by fasting (see also Celiac Disease).
STUDY TABLE: Differential Diagnosis of Chronic Diarrhea
If you see this… Diagnose or do this…
Bloating, abdominal discomfort relieved by a bowel movement,
no weight loss or alarm features IBS; test for celiac disease Diarrhea mainly in women aged 45–60 years, unrelated to food
intake (nocturnal diarrhea), normal colonoscopy Microscopic colitis; stop NSAIDs, PPIs; biopsy
Diarrhea with dairy products Lactose intolerance; dietary exclusion or hydrogen breath test Use of artificial sweeteners or fructose Carbohydrate intolerance; attempt dietary exclusion or hydrogen
breath test
Nocturnal diarrhea and diabetes mellitus or SSc Small bowel bacterial overgrowth; hydrogen breath test or empiric antibiotic trial
Coexistent pulmonary diseases and/or recurrent Giardia infection CVI and selective IgA deficiency; obtain measurement of immune globulins
Somatization or other psychiatric syndromes, history of laxative
use Self-induced diarrhea; obtain tests for stool pH, sodium, potassium, and magnesium Severe secretory diarrhea and flushing Carcinoid syndrome; obtain test for 24-hour urinary excretion of
5-HIAA
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❖Test Yourself
A 36yearold woman has watery diarrhea that is not nocturnal. She has six to seven highvolume bowel movements daily, and her symptoms improve with fasting. Fecal leukocytes and stool culture are negative. Stool sodium is 70 mEq/L and stool potassium is 10 mEq/L.
ANSWER: The diagnosis is osmotic diarrhea.
A 55yearold woman who takes daily NSAIDs for OA has watery diarrhea without weight loss. Colonoscopy is normal.
ANSWER: The diagnosis is likely microscopic colitis.
Malabsorption
Diagnosis
Patients with chronic diarrhea, especially those who report an oily residue in their stool, should be evaluated for possible fat malabsorption. The four most common disorders causing malabsorption with steatorrhea are celiac disease, small bowel bacte rial overgrowth, shortbowel syndrome, and pancreatic insufficiency.
Gastroenterology and Hepatology
STUDY TABLE: Chronic Diarrhea and Malabsorption Syndromes
If you see this… Do this…
History of IBS and iron deficiency anemia Diagnose celiac disease
Obtain IgA anti-tTG or IgA anti-endomysial antibody assays and small bowel biopsy if positive
Order a gluten-free diet Chronic pancreatitis, hyperglycemia, history of pancreatic
resection, CF Diagnose pancreatic insufficiency
Obtain tests for excess fecal fat, x-rays for pancreatic calcifications, and consider pancreatic function tests
Treat with pancreatic enzyme replacement therapy Previous surgery, small bowel diverticulosis, dysmotility (SSc or
diabetes mellitus), combination of vitamin B12 deficiency and elevated folate level
Diagnose bacterial overgrowth
Order empiric trial of antibiotics or hydrogen breath test Resection of >200 cm of distal small bowel (or viable small
bowel <180 cm) Diagnose short-bowel syndromeReplace nutrient and electrolyte deficiencies History of resection of <100 cm of distal ileum, now with
voluminous diarrhea, weight loss, and evidence of malnutrition Diagnose short-bowel syndrome with bile acid enteropathy Order empiric trial of cholestyramine
Arthralgia; fever; neurologic, ocular, or cardiac disease Diagnose Whipple disease
Select small bowel biopsy and PCR for Tropheryma whippelii Order antibiotics for 12 months
Travel to India or Puerto Rico, malabsorption, weight loss,
malaise, folate or vitamin B12 deficiency, and steatorrhea Diagnose tropical sprue Order a small bowel biopsy
Treat with a sulfonamide or tetracycline and folic acid Prolonged traveler’s diarrhea, diarrhea after a camping trip, or
outbreak in a day-care center Diagnose giardiasis
Identify Giardia parasites or Giardia antigen in the stool Treat with metronidazole
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◆Don’T BE TrickED
• Do not use cholestyramine if ileal resection is >100 cm (will worsen bile salt deficiency and steatorrhea).
• Use cholestyramine if diarrhea begins after cholecystectomy.
❖
❖Test Yourself
A 54yearold woman has a 4month history of diarrhea and weight loss. Laboratory studies show hypocalcemia, microcytic anemia, and an increased PT.
ANSWER: The probable diagnosis is celiac disease. Order an IgA antitTG antibody assay and, if positive, follow with a small
bowel biopsy.