What is the most common cause of black stool?

Most Common Cause of Black Stool (Melena)

The most common cause of black stool (melena) is peptic ulcer disease, specifically duodenal and gastric ulcers, which account for approximately 50% of upper gastrointestinal bleeding cases in Western populations. 1

Primary Etiologies of Melena

Black, tarry stools (melena) indicate bleeding from the upper gastrointestinal tract proximal to the ligament of Treitz, where blood undergoes degradation by gastric acid and intestinal bacteria. 1

Most Common Causes in Order of Frequency:

Peptic Ulcer Disease (Most Common Overall)

• Duodenal ulcers are the single most frequent cause, followed by gastric ulcers and gastric erosions. 1
• In pediatric populations, duodenal ulcer was identified as the most common diagnosis (n=22 out of 39 identified sources). 2
• These lesions account for approximately 50% of upper GI bleeding cases in the United States. 3

Esophageal Varices (Geographic Variation)

• In tropical countries like India, esophageal varices account for approximately 42-50% of upper GI bleeding cases, making them equally or more common than peptic ulcer disease in these populations. 3
• Varices are the most common cause of massive upper GI bleeding (66.7% of severe cases). 3
• These require underlying portal hypertension from cirrhosis or chronic liver disease. 4

Other Significant Causes:

• Gastric erosions are frequently identified alongside ulcers. 1
• Mallory-Weiss tears (7.5% of cases) typically occur after prolonged vomiting. 1, 3
• Esophagitis from various causes including reflux disease. 1, 2
• Neoplasms (gastric cancer, duodenal tumors) must be excluded, particularly in older patients. 1
• Angiodysplasia is the most common small bowel source when upper endoscopy is negative. 5

Critical Clinical Context

Risk Factors That Increase Likelihood:

• NSAID use significantly increases risk of peptic ulcer disease and erosive gastritis. 6, 7
• Helicobacter pylori infection is strongly associated with peptic ulcer disease. 4
• Chronic liver disease/cirrhosis predisposes to variceal bleeding. 4, 3
• Advanced age increases risk across all etiologies. 6

Important Diagnostic Considerations:

• The upper gastrointestinal tract (esophagus, stomach, duodenum) is the source in approximately 87% of melena cases (34 out of 39 identified sources in one pediatric study). 2
• Small intestine sources account for approximately 13% when upper endoscopy is negative. 2
• In patients with iron deficiency anemia and occult bleeding, upper GI lesions are found in 72% versus colonic lesions in 45%. 7

Diagnostic Approach

Esophagogastroduodenoscopy (EGD) is the primary diagnostic and therapeutic modality for melena, providing diagnosis in 97.5% of cases. 1, 3

Key Clinical Predictors for Upper GI Source:

• Vomiting in association with melena significantly increases likelihood of finding a source on EGD. 2
• Hemoglobin level ≤3 g/dL below normal limit correlates with identifiable upper GI lesions. 2
• Abnormal abdominal ultrasonography findings predict positive EGD results. 2

Critical Pitfall to Avoid:

• One-third of patients with a benign upper GI bleeding lesion have a concurrent colonic lesion, necessitating complete evaluation of both upper and lower GI tracts in patients with iron deficiency anemia. 7
• Fecal occult blood testing does not reliably distinguish upper from lower GI bleeding sources, though positive tests correlate highly with neoplastic lesions (74% positive predictive value). 7