What are the diagnostic steps and treatment options for green or black stool?

Diagnostic Approach to Green or Black Stool

Green stool is typically benign and results from rapid intestinal transit, dietary factors (iron-fortified formulas, green vegetables), or bile pigments, while black stool (melena) indicates upper gastrointestinal bleeding until proven otherwise and requires urgent evaluation. 1, 2

Initial Clinical Assessment

History and Physical Examination

• Determine true stool color objectively rather than relying solely on patient descriptions, as patients use 23+ different descriptors that often conflict with actual stool appearance 2
• Assess for associated symptoms: abdominal pain, weight loss, hemodynamic instability, or signs of anemia 3
• Perform digital rectal examination to evaluate for hemorrhoids, anal fissures, or palpable masses 3
• Examine all hernia orifices and surgical scars for potential obstruction 3

Key Distinguishing Features

Black Stool (Melena):

• Card color 4 (black) has a 95-98% positive predictive value for upper GI bleeding when identified by patients or direct stool examination 2
• Dark blood mixed within stool suggests proximal bleeding source 3, 1
• Associated findings: tarry consistency, foul odor, potential hemodynamic compromise 4

Green Stool:

• Green color correlates with elevated bile acid content (7.7 mg/g vs 3.76 mg/g in brown stools) 5
• Common in rapid transit states, iron supplementation (particularly whey-predominant formulas at 12 mg/L), or dietary factors 6
• Green stools without other concerning features should be considered a normal variation 6

Laboratory Evaluation

Initial Testing

• Complete blood count to assess for anemia (strong recommendation for all patients with stool color changes) 3
• Stool hemoccult testing if bleeding suspected 3
• C-reactive protein or erythrocyte sedimentation rate if inflammatory process suspected 3

Additional Testing Based on Clinical Context

• Metabolic panel (glucose, calcium, thyroid-stimulating hormone) only if other clinical features warrant 3
• Stool for ova and parasites if diarrhea-predominant symptoms or endemic exposure 3
• Faecal calprotectin if age <45 years with diarrhea to exclude inflammatory bowel disease 3

Endoscopic Evaluation

For Black Stool/Suspected Upper GI Bleeding

• All patients with melena require sigmoidoscopy at minimum 3
• Full colonoscopy or air-contrast barium enema indicated for: 3, 1
  • Dark blood or blood mixed in feces
  • Guaiac-positive stools
  • Anemia
  • Age >50 years without recent colorectal cancer screening
  • Family history of colorectal cancer or personal history of polyps

For Bright Red Blood (If Present)

• Anoscopy to visualize internal hemorrhoids 3, 1
• Flexible sigmoidoscopy mandatory even if hemorrhoids identified 3, 1
• Colonoscopy preferred over sigmoidoscopy as initial approach for bright red bleeding, as 20/217 patients (9%) with bright red hematochezia had lesions beyond 60 cm, including 8 cancers 7

Management Algorithm

Green Stool Without Alarm Features

1. Reassure patient that green stool is typically benign 6
2. Review dietary intake (iron supplements, green vegetables) and medications 6
3. If associated with diarrhea, consider bile acid malabsorption and trial of cholestyramine or bismuth subsalicylate 5
4. No further workup needed if CBC normal and no other concerning symptoms 3

Black Stool (Melena)

1. Assess hemodynamic stability immediately - obtain IV access, initiate fluid resuscitation if unstable 3
2. Insert nasogastric tube to assess for upper GI source 3
3. Transfuse packed red blood cells to maintain hemoglobin >7 g/dL (>9 g/dL if massive bleeding or cardiovascular disease) 3
4. Proceed urgently to upper endoscopy if melena confirmed 3, 1
5. Consider colonoscopy if upper endoscopy negative 3, 1

Common Pitfalls to Avoid

• Do not rely on patient verbal descriptions alone - use objective color assessment or direct stool visualization, as 22% of patients have discrepancies between verbal description and actual color 2
• Do not attribute fecal occult blood to hemorrhoids without adequate colonic evaluation 3
• Do not perform flexible sigmoidoscopy alone for hematochezia - colonoscopy is more cost-effective and safer, saving $12-116 per patient while reducing perforation risk 7
• Do not assume green stool indicates pathology in the absence of other symptoms - it is often a normal variant 6
• Do not delay endoscopy in patients with true melena - the positive predictive value for upper GI bleeding approaches 98% 2