The discovery of blood in one’s stool (hematochezia or melena) naturally causes anxiety. This visible or occult (hidden) bleeding often raises concerns about serious diseases like colorectal cancer. While blood in the stool can be a symptom of malignancy, most cases are caused by non-life-threatening conditions. Understanding the frequency of serious versus benign causes provides necessary context.
Statistical Context of Blood in Stool
Most instances of visible rectal bleeding are linked to benign, non-cancerous conditions. It is estimated that more than 96% of patients consulting a physician for this symptom will not have colorectal cancer. While the symptom must always be evaluated, the likelihood of a malignant cause is low.
For patients aged 45 years or older presenting with new-onset rectal bleeding, the positive predictive value for colorectal cancer is approximately 5.7%. This means roughly one in 10 patients in this group has either cancer or an advanced precancerous polyp (adenoma). The risk is highly stratified by age, with the incidence of colorectal cancer increasing rapidly after age 50.
For younger adults under the age of 50, the overall incidence of colorectal cancer is significantly lower. However, rectal bleeding is still a powerful predictor of disease in this population. It increases the odds of a colorectal cancer diagnosis by over eight times. Therefore, the symptom should be taken seriously as a red flag that warrants investigation.
Common Non-Malignant Causes of Bleeding
The most frequent causes of blood in the stool originate in the lower gastrointestinal tract and are typically benign anorectal conditions.
Hemorrhoids
Hemorrhoids are the most common culprit, representing swollen veins in the anus or lower rectum that bleed easily when strained. Bleeding is usually painless and presents as bright red blood on the toilet paper or dripping into the toilet bowl.
Anal Fissures
Anal fissures are small tears in the lining of the anal canal, often resulting from passing hard stools. The bleeding is accompanied by sharp pain during or immediately following a bowel movement. The bright red blood is usually small in volume and may be visible as a streak on the stool or toilet paper.
Other Gastrointestinal Conditions
Bleeding can also originate higher up in the colon due to conditions like diverticular disease. Diverticula are small, bulging pouches in the large intestine wall; if nearby blood vessels rupture, they can cause painless, sometimes substantial bleeding.
Inflammatory Bowel Disease (IBD), including Crohn’s disease and ulcerative colitis, causes inflammation and ulceration. This results in blood mixed with stool, often accompanied by diarrhea and abdominal pain.
Benign polyps are growths on the colon lining that are a source of bleeding and can be precancerous. These growths bleed intermittently, and their presence necessitates removal to prevent future malignancy. Other causes include vascular ectasias, dilated blood vessels in the colon that may bleed, particularly in older adults.
Identifying Warning Signs and Characteristics
The color and nature of the blood offer initial clues about the location of the bleeding source.
Blood Color and Location
Bright red blood typically indicates a source near the anus or rectum, such as hemorrhoids or a fissure. This lower GI bleeding is most often associated with benign conditions. Dark red or maroon blood suggests the bleeding is coming from higher up in the colon or lower small intestine.
Bleeding from the upper GI tract, such as from a stomach ulcer, results in stools that appear black and tarry, a condition known as melena. This black color is due to the blood being digested as it passes through the system.
Red Flags for Serious Disease
While any visible blood warrants attention, certain associated symptoms strongly suggest a more serious underlying issue, often called “red flags.” These include persistent changes in bowel habits, such as looser stools or a change in stool caliber.
Unexplained weight loss, chronic fatigue, severe abdominal pain, or symptoms of anemia alongside bleeding are causes for immediate medical concern. Blood that is consistently mixed in with the stool, rather than just on the surface, is a significant predictor of a cancer diagnosis.
Necessary Medical Evaluation
Any instance of new or persistent blood in the stool requires a medical evaluation to determine the source and rule out malignancy. The initial assessment involves a detailed patient history, focusing on the color and frequency of the bleeding and any associated symptoms. A physical examination, including a digital rectal exam, allows the doctor to check for common anorectal causes like hemorrhoids or fissures.
If the cause is not immediately obvious or if warning signs are present, the physician will move to diagnostic testing. Non-invasive screening tests, such as the Fecal Immunochemical Test (FIT) or Fecal Occult Blood Test (FOBT), detect minute amounts of occult blood. These tests help assess bleeding and may necessitate further investigation.
Endoscopic Procedures
The gold standard for directly visualizing the colon and rectum is an endoscopic procedure, such as a colonoscopy. This involves inserting a flexible tube with a camera through the rectum to examine the entire large intestine.
This procedure allows the physician to identify the exact source of bleeding, take tissue samples (biopsies), and remove polyps before they develop into cancer. Colonoscopy is typically recommended for patients over 45 who have rectal bleeding, especially if they have additional risk factors or concerning symptoms.