What is the difference between melena and hematochezia?

Melena vs Hematochezia: Key Differences

Melena is the passage of black, tarry stools indicating digested blood from an upper gastrointestinal source, while hematochezia is the passage of bright red blood or blood clots per rectum, typically indicating a lower gastrointestinal source—though approximately 10-15% of patients with severe hematochezia actually have an upper GI bleed with rapid transit time. 1, 2

Defining Characteristics

Melena

• Appearance: Black, tarry stools with a sticky consistency and characteristic odor 3
• Blood origin: Digested blood that has been exposed to gastric acid 2
• Typical source: Upper GI tract (proximal to the ligament of Treitz)—esophagus, stomach, or duodenum 1, 3
• Common causes: Peptic ulcers, gastroduodenal erosions, esophagitis, varices, Mallory-Weiss tears 3
• Blood volume: Generally requires at least 50-100 mL of blood in the upper GI tract 4

Hematochezia

• Appearance: Bright red blood or blood clots passed rectally, also called "bright red blood per rectum" 2
• Blood origin: Blood that has not been exposed to gastric acid long enough for digestion 2
• Typical source: Lower GI tract (distal to the ligament of Treitz)—colon, rectum, or anus 2
• Common causes: Diverticulosis (20-41%), angiodysplasia (3-40%), ischemic colitis (10-21%), colorectal cancer/polyps (6-27%) 3

Critical Clinical Pitfall

The most important diagnostic trap is assuming hematochezia always indicates lower GI bleeding. 1, 2

• 10-15% of patients with acute severe hematochezia have an upper GI source identified on upper endoscopy 1, 2, 5
• This occurs when bleeding is massive and transit time through the GI tract is rapid enough that blood remains bright red 1, 3
• Duodenal ulcer is the most common cause (44%) of upper GI bleeding presenting as hematochezia 5
• Patients with hematochezia from upper GI sources have worse outcomes: higher transfusion requirements (5.4 vs 4.0 units), increased need for surgery (11.7% vs 5.7%), and higher mortality (13.6% vs 7.5%) compared to those presenting with melena 5

Diagnostic Approach Based on Presentation

For Melena

• First-line: Esophagogastroduodenoscopy (EGD) within 24 hours, as the source is almost always upper GI 3
• If EGD negative: Consider colonoscopy, as approximately 10-15% may have a lower GI source (slow bleeding from right colon can occasionally produce dark stools) 3
• If both negative: Evaluate small bowel with capsule endoscopy or enteroscopy 3, 6

For Hematochezia

• Hemodynamically unstable or severe bleeding: Perform upper endoscopy first to rule out upper GI source (15% prevalence in this setting) 1, 5
• Hemodynamically stable: Colonoscopy is typically first-line 1
• Key clinical clue: Presence of shock, high transfusion requirements, or hemodynamic instability despite resuscitation increases likelihood of upper GI source 5

Hemodynamic Significance

Both presentations require immediate assessment, but severity is determined by vital signs, not stool appearance. 3, 7

• High-risk features: Heart rate >100 bpm, systolic BP <100 mmHg, hemoglobin <100 g/L (or <7 g/dL) 3
• Patients with hematochezia from upper GI sources tend to be older (mean age 55 vs 50 years) and have more severe bleeding than those with melena 5
• Mortality in elderly patients (>65 years) with melena reaches up to 30% in those over 90 years 3

Special Considerations

• Black stools without tarry consistency: Likely from dietary iron, bismuth, or other non-bleeding causes—do not require urgent GI evaluation unless other concerning features present 3
• Maroon stools: Intermediate appearance suggesting either brisk upper GI bleeding or proximal lower GI bleeding 5
• Slow right colon bleeding: Can occasionally produce darker stools but typically not true melena 3