Small Bowel and Malabsorption
Questions here test a structured approach to malabsorption: which nutrient is malabsorbed, which segment or organ is responsible, and which test confirms it. Expect celiac disease, bile acid diarrhea, small intestinal bacterial overgrowth and a small set of specific infections.
Celiac disease
- Screen with tissue transglutaminase IgA plus total IgA, since IgA deficiency affects roughly 2% and causes false negatives. Patients must be eating gluten for valid results.
- Take at least four duodenal biopsies from the second part plus one from the bulb; report by Marsh grade, with Marsh 3 indicating villous atrophy.
- HLA-DQ2/DQ8 has high negative predictive value and is useful mainly to exclude disease.
- Seronegative celiac is considered when histology is typical but antibodies are negative; look for IgA deficiency, immunosuppression or an alternative cause of villous atrophy such as tropical sprue, giardiasis, common variable immunodeficiency or olmesartan use.
A framework for malabsorption
- Fat malabsorption: steatorrhea, weight loss, fat-soluble vitamin deficiency. Fecal elastase under 200 micrograms per gram suggests pancreatic exocrine insufficiency and under 100 is severe.
- Bile acid diarrhea: chronic watery diarrhea after ileal resection, cholecystectomy or in microscopic colitis. SeHCAT retention under 10% at 7 days is diagnostic; treat with bile acid sequestrants.
- Small intestinal bacterial overgrowth: glucose or lactulose breath testing is imperfect; duodenal aspirate culture is the reference. Risk factors include motility disorders, blind loops, strictures and prolonged PPI use.
- Protein-losing enteropathy: hypoalbuminemia with elevated fecal alpha-1 antitrypsin clearance; causes include intestinal lymphangiectasia, Menetrier disease and Crohn disease.
Specific entities
- Whipple disease: weight loss, arthralgia and diarrhea; PAS-positive macrophages on duodenal biopsy, confirmed by Tropheryma whipplei PCR; treat with ceftriaxone followed by trimethoprim-sulfamethoxazole for about a year.
- Giardiasis: bloating, malabsorption; stool antigen or PCR; tinidazole or metronidazole.
- Vitamin B12 deficiency from terminal ileal disease or resection, pernicious anemia, or bacterial overgrowth.
Worked example
A 32-year-old woman has iron deficiency anemia and bloating. TTG IgA is eight times the upper limit with normal total IgA; duodenal biopsy shows Marsh 3a changes. She has celiac disease: start a gluten-free diet, check bone density and micronutrients, and repeat serology at 6-12 months to confirm response.
Traps
- Testing serology after the patient has already started a gluten-free diet.
- Taking a single duodenal biopsy.
- Attributing iron deficiency to menstrual loss without excluding celiac disease.
- Ignoring IgA deficiency when TTG is negative but clinical suspicion is high.