Melena Definition
Melena is defined as the passage of black, tarry stools indicating digested blood in the gastrointestinal tract, but specific quantitative thresholds for blood volume or duration are not established in clinical guidelines. 1, 2
Clinical Definition and Characteristics
The British Society of Gastroenterology and other major guidelines define melena purely by its physical characteristics rather than by specific blood volume or time parameters 1:
- Black, tarry appearance with sticky consistency and characteristic odor 2
- Indicates blood has been exposed to gastric acid and undergone digestion 3
- Typically requires at least 50-100 mL of blood loss to produce visible melena (general medical knowledge, though not explicitly stated in guidelines)
Key Distinguishing Features
Black stools without the sticky, tar-like consistency and characteristic odor do not represent true melena and generally do not require urgent GI evaluation unless other concerning features are present 2:
- Simple black stools from dietary causes (iron supplements, bismuth, certain foods) lack the tarry quality 2
- True melena suggests bleeding proximal to the ligament of Treitz in most cases 4
- Melena doubles the odds of finding a bleeding source in the proximal small intestine compared to other presentations 4
Clinical Severity Indicators
Guidelines focus on severity assessment rather than specific volume thresholds 1, 5:
- Patients presenting with both hematemesis and melena have more severe bleeding than those with melena alone 1
- Approximately 30% of bleeding ulcer patients present with hematemesis, 20% with melena alone, and 50% with both 6
- Massive upper GI bleeding can present as hematochezia (bright red blood per rectum) rather than melena if transit time is rapid 2
Duration Considerations
No specific duration threshold defines melena in clinical guidelines 1, 2:
- Rebleeding is defined by fresh hematemesis/melena plus hemodynamic instability (pulse >100 bpm, systolic BP <100 mmHg) or hemoglobin drop >20 g/L over 24 hours 1
- The focus is on hemodynamic status and ongoing bleeding rather than duration of melena passage 1, 6
Common Pitfalls
- Do not assume all black stools represent melena—obtain detailed medication and dietary history 2
- Do not delay endoscopy beyond 24 hours, as diagnostic and therapeutic success rates are highest within the first 36 hours 3
- Elderly patients (>65 years) with melena have significantly higher mortality rates and require more aggressive management 2, 7