What are the possible causes of bloody stool in a patient?

Causes of Bloody Stool

The most common causes of bloody stool are diverticulosis, angiodysplasia, hemorrhoids, and ischemic colitis, with the specific etiology heavily dependent on patient age and clinical presentation. 1

Primary Etiologies by Frequency

Most Common Lower GI Bleeding Sources

• Diverticulosis accounts for 20-41% of acute lower GI bleeding cases and is the single most frequent cause in older adults 1
• Angiodysplasia represents 3-40% of cases, with higher prevalence in elderly patients due to age-related vascular changes 1
• Hemorrhoids and anorectal lesions cause 5-14% of significant bleeding episodes, though they are the most common cause of minor rectal bleeding 1
• Ischemic colitis accounts for 12-21% of cases and should be suspected in patients with vascular comorbidities 1

Inflammatory and Infectious Causes

• Inflammatory bowel disease (IBD) including Crohn's disease and ulcerative colitis causes bloody diarrhea with purulent, mucoid stool 2, 3
  • Crohn's disease shows clinically obvious rectal bleeding in approximately one-third of patients, with massive hemorrhage occurring in 2.5% 3
  • Ulcerative colitis typically presents with pancolitis and diffuse mucosal ulceration causing bleeding 4, 2
• Bacterial colitis from Shiga toxin-producing E. coli (STEC), Salmonella, Shigella, Campylobacter, and Yersinia causes bloody, purulent diarrhea with fever and severe abdominal pain 4, 5

Neoplastic Causes

• Colorectal cancer and polyps account for 6-27% of acute lower GI bleeding, with higher rates in critically ill patients 1
• Undetected tumors may present with frank bloody stools after chemotherapy, including adenomatous polyps and metastatic lesions 6

Upper GI Sources Presenting as Bloody Stool

• Peptic ulcer disease and other upper GI sources cause 10-15% of cases presenting with acute severe hematochezia when bleeding is brisk enough to accelerate transit time 1
• Massive upper GI bleeding can present as bright red blood per rectum rather than melena if transit is rapid 7

Critical Diagnostic Distinctions

Character of Blood Guides Source Localization

• Bright red blood (hematochezia) typically indicates lower GI source distal to the ligament of Treitz, but can represent massive upper GI bleeding with rapid transit 1
• Melena (black, tarry, sticky stools) indicates digested blood from upper GI sources including peptic ulcers, gastroduodenal erosions, esophagitis, varices, and Mallory-Weiss tears 7
• Darker blood mixed in stool suggests more proximal bleeding source compared to bright red blood 7

Age-Related Patterns

• The incidence of lower GI bleeding increases dramatically with age, showing a >200-fold increase from age 20 to 80 years 1
• This age-related increase is explained by the rising prevalence of diverticulosis and angiodysplasia in elderly patients 1
• Diverticular disease is uncommon under age 40, but affects nearly one-third of the population by age 50 8

Special Clinical Scenarios

Right-Sided vs. Left-Sided Colonic Bleeding

• While 90% of diverticula occur in the left colon, bleeding originates from the right colon in at least 50% of diverticular hemorrhage cases 8
• Right-sided abdominal pain with maroon or bright red blood is highly suggestive of non-occlusive mesenteric ischemia (NOMI) 9

Drug-Induced and Iatrogenic Causes

• Vasoconstrictive medications including metoclopramide can precipitate NOMI in critically ill patients with vascular comorbidities 9
• Chemotherapy-related bleeding has various causes including pseudomembranous colitis (even without prior antibiotics), ischemic colitis, and undetected tumors 6
• Anticoagulation, NSAIDs, diabetes mellitus, hypertension, and ischemic heart disease are associated with increased risk of diverticular hemorrhage 8

Rare but Important Causes

• Meckel's diverticulum presents with painless rectal bleeding in adolescents and should be considered when other causes are excluded 4
• Postpolypectomy bleeding, aortocolonic fistula, trauma from fecal impaction, and anastomotic bleeding account for 3-28% of cases in various series 1

Critical Clinical Pitfalls

Do Not Assume Benign Causes Without Evaluation

• Small bowel sources account for 0.7-9.0% of severe hematochezia and are easily missed 1
• Slow bleeding from the right colon can occasionally produce dark stools, though typically not true melena 7

Red Flags Requiring Urgent Intervention

• Loss of rectal sensation is an ominous sign suggesting transmural ischemia with nerve damage requiring immediate surgical consultation 9
• Shock index >1 (heart rate ÷ systolic BP) indicates hemodynamic instability and predicts poor outcomes 4, 9
• Persistent hemodynamic instability despite resuscitation warrants immediate surgical consultation and may indicate bowel infarction 9

Mortality Considerations

• Lower GI bleeding has a reported mortality rate of 2-4%, with most bleeding stopping spontaneously in 80-85% of cases 1
• Elderly patients (>65 years) have significantly higher mortality rates requiring more aggressive management 7
• Diverticular hemorrhage ceases spontaneously in approximately 90% of cases, though recurrence is common 8