Chronic Diarrhea + Severe Iron Deficiency Anemia: Differential Diagnosis

1. Celiac Disease (Gluten-Sensitive Enteropathy) - TOP DIAGNOSIS

• Immune-mediated enteropathy triggered by gluten (wheat, rye, barley) causing villous atrophy in the proximal small intestine
• Presentation ranges from asymptomatic iron-deficiency anemia alone (due to duodenal disease) to severe diarrhea, weight loss, and multi-nutrient malabsorption
• Diarrhea mechanism: villous atrophy causes steatorrhea + secondary lactase deficiency + a secretory component from crypt hyperplasia
• Isolated iron-deficiency anemia refractory to oral iron is a classic red-flag trigger for celiac testing
• Global prevalence ~1.4%; common in adults aged 30-60 years
• Associated features: bloating, fatigue, osteoporosis, dermatitis herpetiformis, elevated liver enzymes, aphthous stomatitis
• Associated diseases: Type 1 diabetes, autoimmune thyroid disease

2. Crohn's Disease (Small Bowel Involvement)

• Chronic transmural granulomatous inflammation that can affect any segment of the GI tract
• Small bowel involvement (particularly terminal ileum and proximal bowel) causes malabsorption
• Anemia is common and may be due to iron deficiency, anemia of chronic disease, or vitamin B12 deficiency
• Diarrhea is prominent; may be accompanied by abdominal pain, weight loss, perianal disease, and extraintestinal manifestations
• Elevated inflammatory markers (CRP, ESR), fecal calprotectin elevated

3. Small Intestinal Bacterial Overgrowth (SIBO)

• Abnormal colonization of the small bowel with colonic-type bacteria
• Causes malabsorption of iron, vitamin B12, fat-soluble vitamins
• An important cause of otherwise unexplained diarrhea, especially in the elderly
• Presents with diarrhea, bloating, and nutritional deficiencies
• Risk factors: motility disorders, anatomical abnormalities, prior GI surgery, proton pump inhibitor use

4. Hookworm Infection (Ancylostoma duodenale / Necator americanus)

• One of the most common causes of moderate-to-severe iron deficiency anemia globally
• Adult worms attach to the small intestinal mucosa and cause direct blood loss
• Chronic infection leads to hypochromic microcytic anemia + diarrhea
• Consider in patients from or with travel history to endemic tropical/subtropical regions

5. Tropical Sprue / Tropical Malabsorption

• Malabsorptive syndrome seen in tropical regions
• Causes chronic diarrhea, steatorrhea, and multiple nutritional deficiencies including iron and B12
• Can present similarly to celiac disease but is not gluten-mediated

6. Inflammatory Bowel Disease - Ulcerative Colitis

• Chronic relapsing colitis causing bloody diarrhea with iron loss through GI bleeding
• Iron deficiency develops from chronic blood loss + possibly reduced absorption
• Usually presents with hematochezia, urgency, tenesmus

7. Microscopic Colitis (Collagenous / Lymphocytic Colitis)

• Causes chronic, watery, non-bloody diarrhea with a normal-looking colonoscopy
• Biopsy is required for diagnosis
• Can cause iron deficiency due to chronic diarrhea and malabsorption
• More common in middle-aged women; associated with NSAID use

8. Chronic Giardiasis

• Giardia lamblia infection of the proximal small intestine
• Causes malabsorption, fatty diarrhea, bloating, and iron deficiency
• Can cause prolonged/chronic infection particularly in immunocompromised patients or in children

9. Whipple's Disease (Tropheryma whipplei)

• Rare but important: small bowel infiltration causing severe malabsorption
• Triad: diarrhea, weight loss, arthralgias
• Can cause multi-system disease including neurological involvement

Summary Table

Diagnostic Approach

1. Bloods: CBC, serum ferritin, iron studies, TTG-IgA + total IgA, ESR/CRP, folate, B12, albumin
2. Stool: Fecal calprotectin, culture, ova and parasites (including Giardia antigen), C. difficile toxin
3. Endoscopy: Upper GI endoscopy with duodenal biopsies (celiac, Whipple's) + colonoscopy with random biopsies (IBD, microscopic colitis)
4. Imaging: CT enterography if Crohn's suspected
5. Breath test: Hydrogen breath test for SIBO
6. Serology: Consider stool antigen for Giardia

Key clinical pearl: The combination of chronic diarrhea + severe iron deficiency anemia should prompt immediate testing for celiac disease (TTG-IgA), as this is the most commonly missed and highly treatable cause. Iron deficiency refractory to oral iron supplementation is a particularly strong indicator.