Blood in Stool: What Could Be Causing It

What Does Blood in Stool Actually Mean?

Blood in stool is not a diagnosis. It is a sign that points toward an underlying source somewhere along the GI tract. The color, volume, and timing of bleeding all carry diagnostic weight. Bright red blood coating the outside of stool typically indicates a distal source such as the rectum or anus. Dark red or maroon blood mixed throughout stool suggests a more proximal colonic source. Black, tarry melena almost always originates above the ligament of Treitz.

Hematochezia vs. Melena: Why the Distinction Matters

Hematochezia refers to the passage of red or maroon blood per rectum. Melena describes black, tarry, foul-smelling stool produced when hemoglobin is degraded by gut bacteria during transit. A 2016 review published in the American Journal of Gastroenterology confirmed that melena reliably predicts an upper GI source in more than 90% of cases, while hematochezia is lower GI in origin in roughly 80 to 85% of presentations [1].

Occult bleeding, by contrast, produces no visible color change. It is detected only by fecal immunochemical testing (FIT) or guaiac-based fecal occult blood testing (gFOBT). The U.S. Preventive Services Task Force recommends annual FIT for colorectal cancer screening starting at age 45 in average-risk adults [2].

How Much Blood Is Too Much?

Any visible blood warrants evaluation. Large-volume bleeding, hemodynamic instability (heart rate above 100 bpm, systolic blood pressure below 90 mmHg), or syncope requires emergency admission. A validated risk score, the Oakland Score, stratifies patients with acute lower GI bleeding into safe-discharge versus admission groups with an AUROC of 0.91 in its derivation cohort [3].

The Most Common Causes of Blood in Stool

Most outpatient rectal bleeding has a benign anorectal cause. Identifying features help narrow the differential before any procedure is ordered.

Hemorrhoids

Internal hemorrhoids bleed painlessly; external hemorrhoids bleed with pain. Hemorrhoidal disease affects an estimated 75% of people at some point in their lives [4]. Bleeding is typically bright red, appears on toilet paper or the surface of stool, and stops spontaneously. Risk factors include low-fiber diet, prolonged straining, pregnancy, and chronic constipation.

Treatment escalates from dietary fiber supplementation (20 to 35 g/day per ACG guidance) through rubber band ligation to surgical hemorrhoidectomy for refractory cases [4]. A 2020 Cochrane review found rubber band ligation superior to injection sclerotherapy for grades I, III hemorrhoids, with a recurrence rate of 10 to 15% at one year versus 30 to 40% for sclerotherapy [5].

Anal Fissures

An anal fissure is a longitudinal tear in the anoderm, almost always in the posterior midline. It produces bright red blood and sharp, tearing pain during and after defecation. Chronic fissures can develop a sentinel skin tag and are associated with elevated internal anal sphincter tone.

First-line treatment is topical 0.2% glyceryl trinitrate (GTN) or topical diltiazem 2%, which reduce resting sphincter pressure and allow healing in 50 to 70% of cases within 8 weeks [6]. Botulinum toxin injection and lateral internal sphincterotomy are reserved for refractory cases.

Diverticulosis and Diverticular Bleeding

Colonic diverticula are present in approximately 35% of adults over age 50 and in roughly 60% of those over 80 [7]. Most diverticula never bleed. When they do, bleeding is typically sudden, painless, large-volume, and bright red to maroon. Diverticular bleeding accounts for 30 to 40% of all acute lower GI bleeding hospitalizations [7].

Bleeding stops spontaneously in 75 to 80% of cases. Recurrent or ongoing bleeding requires colonoscopic hemostasis, angiographic embolization, or segmental colectomy. A 2021 study in Gastrointestinal Endoscopy found that endoscopic band ligation at the time of colonoscopy reduced re-bleeding rates compared to standard epinephrine injection alone (11% vs. 33%, P<0.01) [8].

Inflammatory and Infectious Causes

Inflammation and infection in the colon produce blood mixed with mucus. The pattern differs from anorectal bleeding.

Inflammatory Bowel Disease

Crohn's disease and ulcerative colitis (UC) together affect approximately 3 million Americans [9]. UC classically produces bloody diarrhea with urgency and tenesmus. Crohn's disease may produce bloody stool when the colon or rectum is involved, though it more often produces non-bloody diarrhea when confined to the small bowel.

The American College of Gastroenterology's 2019 UC guidelines state: "The primary goals of UC treatment are to achieve and maintain corticosteroid-free remission" [10]. Mesalamine (2.4 to 4.8 g/day orally) remains the backbone of treatment for mild-to-moderate UC, while biologic agents such as infliximab and vedolizumab are used for moderate-to-severe or steroid-dependent disease [10].

Infectious Colitis

Bacterial pathogens including Campylobacter jejuni, Salmonella species, Shigella species, Entamoeba histolytica, and enterohemorrhagic Escherichia coli (EHEC, particularly O157:H7) cause bloody diarrhea. EHEC is of particular concern because antibiotic use may increase the risk of hemolytic uremic syndrome (HUS); the CDC recommends avoiding antibiotics in confirmed EHEC infections [11].

Stool culture, PCR-based GI panels, and ova-and-parasite testing form the diagnostic backbone. Most bacterial infectious colitis resolves within 5 to 10 days with supportive care.

Ischemic Colitis

Ischemic colitis results from transient reduction in blood flow to the colon, most often in the watershed areas (splenic flexure and rectosigmoid junction). It presents with sudden left-sided abdominal pain followed by bright red or maroon rectal bleeding. Risk factors include older age, cardiovascular disease, hypotension, and constipation medications with vasoconstrictive properties. A 2018 cohort study found that NSAID use was an independent risk factor for ischemic colitis (adjusted OR 1.74, 95% CI 1.21 to 2.50) [12].

Upper Gastrointestinal Sources That Cause Black Stool

Melena means the bleeding source is proximal. Upper GI causes require different initial workup and management.

Peptic Ulcer Disease

Peptic ulcer disease (PUD), caused primarily by Helicobacter pylori infection and NSAID use, is the most common cause of upper GI bleeding and accounts for roughly 50% of all upper GI bleeding hospitalizations [13]. Duodenal and gastric ulcers bleed into the small intestine; when blood transits the colon slowly, it presents as melena. High-volume upper GI bleeds can occasionally produce maroon or even bright red stool.

The Rockall score and the Glasgow-Blatchford score (GBS) stratify risk for adverse outcomes. A GBS of 0 identifies patients safe for outpatient management with a sensitivity of 98.6% for predicting the need for clinical intervention [14]. Intravenous proton pump inhibitor (PPI) therapy (e.g., esomeprazole 80 mg bolus then 8 mg/hr infusion) followed by upper endoscopy within 24 hours is standard of care for high-risk upper GI bleeding [13].

Esophageal and Gastric Varices

Variceal bleeding complicates portal hypertension, most commonly from cirrhosis. It produces rapid, large-volume hematemesis and melena. The six-week mortality from a first variceal bleed is approximately 15 to 20% in compensated cirrhosis and up to 50% in decompensated disease [15]. Vasoactive drugs (terlipressin, octreotide), endoscopic band ligation, and prophylactic antibiotics (ceftriaxone 1 g IV daily for 7 days) are the standard triple therapy per Baveno VII consensus guidelines [15].

Mallory-Weiss Tears

A Mallory-Weiss tear is a longitudinal mucosal laceration at the gastroesophageal junction, classically triggered by forceful vomiting. It accounts for 5 to 10% of upper GI bleeding episodes and stops spontaneously in roughly 90% of cases [13].

Colorectal Cancer and Polyps

Colorectal cancer (CRC) is the third most common cancer in the United States, with an estimated 152,810 new cases in 2024 [16]. Rectal bleeding is a presenting symptom in 40 to 50% of CRC cases, typically as blood mixed with or coating stool, sometimes accompanied by a change in bowel habits, unexplained weight loss, or iron-deficiency anemia.

Advanced adenomatous polyps bleed intermittently and rarely visibly. Fecal immunochemical testing (FIT) detects blood from these lesions at a sensitivity of 79% and specificity of 94% for CRC in average-risk populations [2].

Any patient over 45 with new rectal bleeding and no clear anorectal source on examination warrants colonoscopy to exclude CRC. The ACG and ACS both recommend starting CRC screening at age 45 for average-risk individuals [2][16].

The HealthRX Lower GI Bleeding Triage Framework stratifies patients into three tracks based on symptom pattern:

Track A (Likely benign, age <45, no red flags): Examine anorectum, treat presumptively for hemorrhoids or fissure, reassess at 4 to 6 weeks. FIT if no clear source found.

Track B (Age 45 or older, or any age with iron-deficiency anemia, weight loss, or change in bowel habits): Expedited colonoscopy within 2 to 4 weeks.

Track C (Hemodynamic instability, large-volume bleeding, melena with altered vital signs): Emergency department referral, inpatient admission, urgent endoscopy or intervention.

Rare but Important Causes

Several less common conditions must not be missed.

Angiodysplasia

Angiodysplasias are dilated, thin-walled vessels in the colonic mucosa. They account for 4 to 7% of lower GI bleeding episodes and are most prevalent in adults over 60 [7]. Bleeding is typically recurrent, low-grade, and painless. Argon plasma coagulation (APC) at colonoscopy is the standard treatment, with re-bleeding rates of approximately 20 to 25% at one year [7].

Meckel's Diverticulum

Meckel's diverticulum is the most common congenital anomaly of the GI tract, occurring in roughly 2% of the population. It contains ectopic gastric mucosa in approximately 50% of symptomatic cases, leading to acid-induced ulceration and painless, often substantial rectal bleeding. It is the most common cause of significant lower GI bleeding in children under age 5 [17]. Technetium-99m pertechnetate scintigraphy (Meckel's scan) detects ectopic gastric mucosa with a sensitivity of approximately 85% in children [17].

Radiation Proctitis

Patients who have received pelvic radiation for prostate, cervical, or rectal cancer may develop chronic radiation proctitis, typically presenting 6 to 18 months after treatment with rectal bleeding, urgency, and tenesmus. APC is effective in 80 to 90% of cases; sucralfate enemas also reduce bleeding in randomized trials [18].

How Blood in Stool Is Diagnosed

Diagnosis begins with a thorough history and physical examination before any testing is ordered.

History and Physical

The clinician should document blood volume, color, relationship to stool, duration, associated symptoms (pain, diarrhea, weight loss, fever), medication use (NSAIDs, anticoagulants, aspirin), and personal or family history of CRC, IBD, or polyps. Digital rectal examination and anoscopy can identify hemorrhoids, fissures, and low rectal masses without any additional equipment.

Laboratory Tests

A complete blood count (CBC) quantifies anemia. Iron studies identify iron deficiency as evidence of chronic occult bleeding. Coagulation studies (PT/INR, aPTT) are relevant if anticoagulant use or liver disease is suspected. Stool culture and PCR-based GI multiplex panels are ordered when infectious colitis is suspected.

Endoscopy

Colonoscopy is the definitive diagnostic and therapeutic test for lower GI bleeding, identifying a source in 70 to 90% of cases [3]. Upper endoscopy (EGD) is indicated when melena is present or when lower GI workup is unrevealing. Flexible sigmoidoscopy evaluates the rectum and sigmoid colon and may suffice in younger patients with a clear anorectal source.

Imaging

CT angiography (CTA) of the abdomen and pelvis detects active bleeding at rates as low as 0.3 mL/min and is preferred over nuclear medicine red cell scans in most acute settings due to speed and anatomical detail [3]. Mesenteric angiography allows both diagnosis and therapeutic embolization.

Treatment Principles by Cause

Treatment must match the underlying source.

• Hemorrhoids: Fiber supplementation, sitz baths, rubber band ligation for grades I, III, hemorrhoidectomy for grade IV.
• Anal fissures: Topical GTN 0.2% or diltiazem 2%, stool softeners, lateral internal sphincterotomy for chronic cases.
• Diverticular bleeding: Colonoscopic hemostasis, angiographic embolization, or surgery for refractory cases.
• Peptic ulcer: IV PPI, endoscopic hemostasis (clips, thermal therapy, injection), H. Pylori eradication (triple or quadruple therapy per ACG 2017 guidelines) [13].
• UC: Mesalamine for mild-to-moderate disease, corticosteroids for flares, biologics for moderate-to-severe or steroid-dependent disease.
• Variceal bleeding: Vasoactive drugs plus endoscopic band ligation plus prophylactic antibiotics, with TIPS for refractory cases.
• CRC: Surgical resection, with adjuvant chemotherapy depending on stage per NCCN guidelines [16].

Anticoagulated patients with GI bleeding present a specific challenge. A 2017 study in JAMA Internal Medicine found that resuming anticoagulation within 90 days of a GI bleed was associated with a 68% lower risk of thromboembolism and no significant increase in re-bleeding compared with non-resumption [19].

When to Seek Emergency Care

Go to the emergency department immediately for any of the following:

• Dizziness, fainting, or rapid heart rate with rectal bleeding
• Blood clots or large volumes of red blood per rectum
• Black, tarry stool with abdominal pain or hemodynamic instability
• Rectal bleeding in a patient on anticoagulants or with known liver disease
• Rectal bleeding accompanied by fever above 38.5°C and severe abdominal pain

A 2019 systematic review in Alimentary Pharmacology and Therapeutics found that patients presenting with acute lower GI bleeding and a heart rate above 100 bpm had a 3.7-fold higher risk of in-hospital mortality compared with hemodynamically stable patients [20].