What causes a metallic smell in stool?

Metallic Smell in Stool: Causes and Clinical Approach

A metallic smell in stool is not a recognized clinical finding in gastroenterology guidelines or medical literature, as the characteristic odor of stool is primarily caused by methyl sulfide compounds (methanethiol, dimethyl disulfide, and dimethyl trisulfide), not metallic-smelling compounds. 1

Understanding Normal and Abnormal Stool Odor

The smell of stool represents the end products of microbial activity and metabolism in the gastrointestinal tract. 2 The actual compounds responsible for fecal odor are:

• Methyl sulfides (methanethiol, dimethyl disulfide, dimethyl trisulfide) are the major components of fecal odor 1
• Small amounts of hydrogen sulfide gas contribute to the characteristic smell 1
• Skatole and indole produce a naphthalene-like "mothball" odor rather than the typical fecal smell 1

What Patients May Be Describing

When patients report a "metallic" smell, they may actually be experiencing:

1. Gastrointestinal Bleeding

• Blood in stool can create an iron-like or metallic perception, though this is typically described as a "bloody" rather than metallic odor 3
• Colonic, inflammatory, or secretory diarrhea typically presents with liquid loose stools with blood or mucous discharge 3
• Inspection of the stool is helpful in distinguishing inflammatory from malabsorptive pathology 3

2. Malabsorption with Steatorrhea

• Malabsorption is accompanied by steatorrhea and the passage of bulky, malodorous pale stools 3
• Giardiasis presents with explosive, watery, foul-smelling stools or semisolid stools with evidence of steatorrhea 4
• The "foul-smelling" quality may be misinterpreted as metallic by some patients 4

3. Medication or Supplement Effects

• Iron supplements can cause dark, tarry stools with an altered odor that patients may describe as metallic
• Certain laxatives alter stool color and potentially odor: phenolphthalein and anthraquinones turn stool red, while bisacodyl turns it purple-blue 3, 5
• Methylene blue used in endoscopic procedures may cause a green hue to stool for up to 24 hours 5

Recommended Clinical Approach

Initial Assessment

Obtain a detailed history focusing on:

• Presence of blood or mucus in stool (suggests colonic/inflammatory pathology) 3
• Stool consistency and appearance (bulky pale stools suggest malabsorption; liquid stools with blood suggest inflammatory disease) 3
• Medication and supplement use, particularly iron, bismuth products, or laxatives 3, 5
• Duration of symptoms (less than 3 months suggests organic disease) 3
• Associated symptoms: weight loss, nocturnal diarrhea, abdominal pain 3

Physical Examination and Testing

Perform screening blood tests including:

• Full blood count, erythrocyte sedimentation rate, C-reactive protein 3
• Urea and electrolytes, liver function tests 3
• Iron studies, vitamin B12, folate 3
• Thyroid function tests 3

Stool examination should include:

• Visual inspection during rigid sigmoidoscopy without bowel preparation 3
• Stool culture for bacterial pathogens (Salmonella, Shigella, Campylobacter, E. coli O157) 3
• Ova and parasite examination if infectious etiology suspected 3
• Testing for Giardia lamblia and Cryptosporidium if malabsorption suspected 3, 4

Important Clinical Pitfalls

• Do not dismiss the patient's concern, but recognize that "metallic smell" is not a standard clinical descriptor and requires translation into recognized pathological findings 1
• If the patient describes truly foul-smelling diarrhea with steatorrhea, consider giardiasis, particularly in patients with recent travel, immunocompromise, or achlorhydria 4
• If symptoms persist despite negative initial workup, consider factitious diarrhea, which represents 20% of cases at tertiary referral centers and is the most common cause of diarrhea of previously undetermined origin 3, 6
• Patients with connections to health professions or psychiatric history are at higher risk for factitious diarrhea and may require supervised inpatient stool collection 3, 6