What causes dark stool (melena)?

Differential Diagnosis of Dark Stool (Melena)

Primary Diagnostic Consideration

Melena is a clinical diagnosis indicating upper gastrointestinal bleeding and requires immediate hemodynamic assessment followed by urgent upper endoscopy—do not delay for stool testing. 1

Key Clinical Features to Establish

• Confirm true melena by digital rectal examination: characteristic black, tarry, sticky stool with distinctive odor indicating digested blood 1
• Distinguish from benign causes of black stool: dietary iron supplements, bismuth subsalicylate, or certain foods produce simple black stools without the tar-like consistency or odor of melena 1
• Assess hemodynamic status immediately: calculate shock index (heart rate/systolic blood pressure)—a value >1 indicates hemodynamic instability requiring ICU admission 1
• Check for orthostatic hypotension as this indicates significant blood loss 1

Most Common Causes of Melena

Upper Gastrointestinal Sources (Most Common)

The upper GI tract is the most common bleeding source in both adults and children presenting with melena 2, 3:

• Peptic ulcer disease (duodenal and gastric ulcers): the most frequent diagnosis, particularly duodenal ulcers 4, 2
• Gastric erosions and gastritis 4
• Esophagitis 4, 2
• Esophageal varices: especially in patients with cirrhosis or portal hypertension 2
• Mallory-Weiss tears: particularly with history of vomiting 4
• Vascular malformations (angiodysplasia) 4

Small Bowel Sources (Less Common)

• Small intestinal bleeding: accounts for approximately 13% of melena cases when upper endoscopy is nondiagnostic 2, 3
• Angiectasia: the most common cause of obscure bleeding, accounting for up to 80% of small bowel sources 5
• Small bowel tumors: particularly important to consider in patients younger than 50 years 5

Lower Gastrointestinal Sources (Uncommon but Important)

• Right-sided colonic lesions: colonoscopy after nondiagnostic upper endoscopy finds a suspected bleeding source in only 4.8% of melena cases 6
• Colorectal tumors: patients with melena have 2.87 times higher odds of colon tumor compared to screening populations 6
• Colitis and right-sided arteriovenous malformations 6

Risk Stratification Factors

High-risk features predicting serious pathology and worse outcomes 4, 1:

• Age >65 years
• Shock index >1 or orthostatic hypotension
• Hemoglobin ≥3 g/dL below normal limit
• Significant comorbidities (cardiovascular, renal, or liver disease)
• Fresh red blood on rectal examination despite melena
• History of previous GI bleeding

In pediatric patients, additional predictive factors for upper GI source include 2:

• Vomiting
• Abnormal abdominal ultrasonography findings
• Hemoglobin ≥3 g/dL below normal limit

Special Population Considerations

Patients with Underlying Conditions

• Cirrhosis/portal hypertension: consider esophageal varices, portal hypertensive gastropathy 2, 7
• Chronic NSAID or anticoagulant use: increased risk of peptic ulcer disease 5
• Alcohol abuse: direct toxic effects on intestinal epithelium, peptic ulcer disease 5
• Immunocompromised/AIDS patients: consider opportunistic infections causing hemorrhagic colitis (e.g., cytomegalovirus) 8

Medication-Induced Causes

Up to 4% of chronic diarrhea with dark stool may be medication-related 5:

• NSAIDs (peptic ulcers, small bowel ulceration)
• Anticoagulants
• Antihypertensives (ACE inhibitors)
• Chemotherapeutic agents

Critical Diagnostic Pitfalls to Avoid

• Never delay endoscopy to obtain fecal occult blood testing—melena is a clinical diagnosis warranting immediate evaluation 1
• Do not assume all melena originates from the upper GI tract: massive upper GI bleeding with rapid transit can present as bright red blood per rectum (hematochezia) 1
• Do not confuse simple black stools from iron or bismuth with true melena—the latter has distinctive tar-like consistency and odor 1
• In patients with nondiagnostic upper endoscopy, recognize that colonoscopy has low yield (4.8%) but should still be performed due to increased colorectal cancer risk 6
• Do not overlook small bowel sources: if upper endoscopy and colonoscopy are negative, consider capsule endoscopy for obscure bleeding 5, 2

Diagnostic Algorithm Priority

1. Confirm melena by digital rectal examination 1
2. Assess hemodynamic stability (shock index, orthostatic vitals) 1
3. For unstable patients: CT angiography immediately for fastest localization 1
4. For stable patients: upper endoscopy (EGD) within 24 hours as first-line diagnostic and therapeutic modality 4, 1, 2
5. If EGD nondiagnostic: consider colonoscopy (especially if age >50 or cancer risk factors) 6
6. If both negative with ongoing bleeding: capsule endoscopy or enteroscopy for small bowel evaluation 5, 2