Metallic Smelling Stool: Causes and Clinical Approach
A metallic odor in stool most commonly indicates the presence of blood in the gastrointestinal tract, which requires prompt evaluation to exclude serious pathology including malignancy, inflammatory bowel disease, or other sources of gastrointestinal bleeding.
Primary Cause: Gastrointestinal Bleeding
- Blood in stool produces a metallic smell due to iron content, and in men and postmenopausal women, any evidence of gastrointestinal bleeding (including occult blood) should be considered indicative of pathology until proven otherwise 1
- Iron-deficiency anemia with occult gastrointestinal bleeding is the most common form of GI bleeding and can be caused by virtually any lesion in the gastrointestinal tract, making evaluation nearly always indicated due to malignancy risk 1
- The metallic sensation/smell is related to metal compounds (iron from hemoglobin breakdown) which activate multiple chemosensory pathways 2
Critical Alarm Features Requiring Urgent Investigation
- Weight loss combined with any stool abnormality mandates colonoscopy within 2-4 weeks, as this essentially excludes functional disorders and suggests organic disease 3, 4
- Visible blood in stools or laboratory evidence of iron deficiency anemia requires immediate colonoscopy to exclude colonic neoplasia 4, 5
- Nocturnal symptoms or continuous (rather than intermittent) diarrhea strongly suggest organic disease over functional disorders 3, 4
- Symptom duration less than 3 months paradoxically increases likelihood of organic pathology 3, 4
Essential First-Line Investigations
- Complete blood count to assess for anemia, which has high specificity for organic gastrointestinal disease and would be expected with bleeding causing metallic odor 3, 4
- C-reactive protein and erythrocyte sedimentation rate, as abnormalities have high specificity for organic pathology including inflammatory bowel disease 3, 4
- Fecal occult blood testing to confirm presence of blood in stool 1
- Comprehensive metabolic panel including liver function tests to assess for systemic disease 6
Differential Diagnosis Beyond Bleeding
While blood is the primary cause of metallic odor, other considerations include:
- Dietary factors and medications should be reviewed systematically, as drugs cause approximately 4% of chronic diarrhea cases and can alter stool characteristics 3, 6
- Bile acid malabsorption can produce characteristic pale, malodorous stools (though typically not metallic), particularly after cholecystectomy or terminal ileum resection 3, 6
- Bacterial overgrowth produces volatile organic compounds including sulfur compounds (methanethiol, dimethyl disulfide, dimethyl trisulfide) that contribute to fecal odor, though these produce sulfurous rather than metallic smells 7
Specific Historical Elements to Elicit
- Previous gastrointestinal surgery including ileal resection, right colectomy, cholecystectomy, or gastric bypass, which predispose to malabsorption and altered stool characteristics 3, 4, 6
- Complete medication review including iron supplements, bismuth-containing products, or other metallic compounds that could alter stool odor 3
- Dietary history focusing on iron-rich foods or supplements 2
- Presence of abdominal pain, changes in bowel habits, or constitutional symptoms suggesting inflammatory bowel disease 5
Endoscopic Evaluation Strategy
- Colonoscopy with ileoscopy and multiple biopsies is the preferred examination when alarm features are present, with diagnostic yield of 15-20% in chronic diarrhea and up to 40% when inflammatory bowel disease is suspected 3
- Ileoscopy adds significant value, identifying pathology in 18% of patients with diarrhea and normal colonoscopy 3
- Biopsies should be obtained even from normal-appearing mucosa to exclude microscopic colitis, which can cause secretory diarrhea 3, 4