What are the causes and treatments of dark stool?

Dark Stool: Causes and Management

Dark or black stools (melena) typically indicate upper gastrointestinal bleeding, but can also result from dietary substances or medications, making careful history-taking essential to distinguish true bleeding from benign causes.

Initial Assessment and History

Critical Historical Features to Obtain

• Dietary intake within the past 24-48 hours: Black licorice, bismuth subsalicylate (Pepto-Bismol), iron supplements, pork blood soup, and other dark foods can mimic melena 1
• Medication review: NSAIDs, anticoagulants, and iron supplements are key culprits 2
• Associated symptoms: Abdominal pain, weight loss, vomiting (especially hematemesis), dizziness, or syncope suggest true bleeding 3
• Stool characteristics: True melena is typically black, tarry, and sticky with a distinctive foul odor, versus formed brown stool with dark coloring from dietary causes 1, 2

Physical Examination Priorities

• Vital signs assessment: Check for hemodynamic instability (hypotension, tachycardia) indicating significant blood loss 4
• Digital rectal examination (DRE): Essential to confirm stool color and consistency, assess for masses, and rule out distal sources 4
• External anal examination: Identify hemorrhoids, fissures, or other anorectal pathology 5
• Abdominal examination: Assess for tenderness, masses, or signs of obstruction 3

Laboratory Evaluation

Initial Blood Work

• Complete blood count: Hemoglobin and hematocrit to assess anemia severity 4, 6
• Coagulation studies: PT/INR and PTT if bleeding is suspected 4
• Blood typing and cross-matching: If severe bleeding is present 4

Important caveat: In acute bleeding, hemoglobin may initially be normal due to hemoconcentration; repeat testing after resuscitation may reveal true anemia 6

Distinguishing Upper vs. Lower GI Sources

Upper GI Bleeding (Most Common with Melena)

• Melena characteristics: Black, tarry, sticky stools with foul odor indicate blood digested by gastric acid 6, 2
• Common causes: Peptic ulcers, gastritis, esophageal varices, Mallory-Weiss tears 6
• Nasogastric tube placement: Consider in unstable patients to assess for active upper GI bleeding 4

Lower GI Sources

• Hematochezia vs. melena: Bright red blood or maroon stools suggest lower GI source, though brisk upper GI bleeding can present this way 5, 2
• Common causes: Diverticular disease, angiodysplasia, inflammatory bowel disease, colorectal neoplasia 2

Diagnostic Approach Algorithm

For Hemodynamically Stable Patients

1. Obtain focused history emphasizing dietary intake, medications, and associated symptoms 1, 3
2. Perform DRE and external anal examination 4
3. Check hemoglobin/hematocrit and coagulation studies 4
4. If true melena suspected: Upper endoscopy (esophagogastroduodenoscopy) is the initial diagnostic procedure of choice 4
5. If lower GI source suspected: Anoscopy and flexible sigmoidoscopy as minimum evaluation 7

For Hemodynamically Unstable Patients

1. Immediate resuscitation: IV fluid/blood product resuscitation targeting hemoglobin >7 g/dL (>9 g/dL if massive bleeding or cardiovascular disease) 4
2. Nasogastric tube placement: Protect airway and decompress stomach 4
3. Urgent endoscopy: Upper endoscopy for suspected upper GI bleeding 4
4. CT angiography: If ongoing bleeding and hemodynamically stable after resuscitation 4
5. Surgical consultation: For life-threatening bleeding non-responsive to resuscitation 4

Special Considerations

When Hemorrhoids Are Present

Critical pitfall: Never attribute dark stools or positive fecal occult blood tests to hemorrhoids without complete evaluation 7

• Hemorrhoidal bleeding characteristics: Bright red blood dripping or squirting into toilet, blood on toilet paper 5, 7
• Mandatory colonoscopy: If positive fecal occult blood test, even with known hemorrhoids 7
• Complete colonic evaluation required: When bleeding is atypical for hemorrhoids, no source evident on anorectal exam, or significant risk factors for neoplasia present 7

Small Bowel Sources

• Consider when upper and lower endoscopy negative: Approximately 5% of GI bleeding cases have no lesions on routine endoscopy 6
• Common small bowel causes: Angiodysplasia, tumors, NSAID-induced ulcers, Meckel's diverticulum 6, 2
• Advanced imaging: Capsule endoscopy, enteroscopy, or CT enterography if initial workup negative 6

Management Approach

Conservative Management

• Discontinue offending agents: Stop NSAIDs, anticoagulants if possible 4
• Proton pump inhibitors: For suspected upper GI bleeding 4
• Monitor hemoglobin: Serial checks to assess ongoing bleeding 4

When to Escalate Care

• No improvement in 48 hours 4
• Symptoms worsen or overall condition deteriorates 4
• Development of warning signs: Severe vomiting, persistent fever, abdominal distension, frank blood in stools, or hemodynamic instability 4
• Persistent transfusion requirement: Consider surgical consultation and intraoperative enteroscopy 6

Age-Specific Considerations

• Patients <50 years with persistent bleeding: More aggressive workup to rule out small bowel neoplasm 6
• Elderly patients: Higher risk for diverticular disease and angiodysplasia; advanced age is a risk factor for acute lower GI bleeding 2