Blood in the Stool & Colorectal Cancer: When to See a Doctor

Blood in the stool is one of the signs the National Cancer Institute lists for both colon cancer and rectal cancer: "blood (either bright red or very dark) in the stool." NCI also says plainly that "these and other signs and symptoms may be caused by colon cancer or by other conditions."

Both halves of that sentence matter. Blood in the stool often comes from something that is not cancer; NCI notes it "may also indicate the presence of conditions that are not cancer, such as hemorrhoids." But polyps and colorectal cancers bleed too, and nothing about the color, the amount, or the fact that it stopped tells you which one you are dealing with. That is what an evaluation is for.

An NCI-reported study of adults under 50 diagnosed with colorectal cancer looked at 17 possible signs and found four that stood out: abdominal pain, rectal bleeding, diarrhea, and iron deficiency anemia. "Rectal bleeding had the strongest association with a diagnosis of early-onset colorectal cancer, followed by iron deficiency anemia."

The delays in that study are the part worth sitting with. People with one sign were diagnosed a median of 9.7 months after it appeared. With two signs, 5.8 months. With three or four signs, still 4.8 months. Even among people with rectal bleeding, who were diagnosed sooner than the rest, "the median delay was still 7 months."

Dr. Yin Cao, one of the study's authors, said it directly: "If you have any of these signs or symptoms, don't wait to see a doctor." She added that people "should at least go to their primary care doctor, and, if needed, the primary doctor will refer them to a gastroenterologist."

With rectal bleeding, the volume, the colour and what comes with it decide how fast to act.

• Call 911 or go to an emergency department if… bleeding does not stop, the toilet water turns red, or you pass large clots.
• Call 911 or go to an emergency department if… bleeding comes with dizziness, fainting, breathlessness, a racing heart, or pale clammy skin.
• Call 911 or go to an emergency department if… you have severe abdominal pain with vomiting, a swollen belly, and no stool or gas passing.
• Call your care team the same day if… your stool is black and tarry or dark red, or you have bloody diarrhoea.
• Call your care team the same day if… bleeding comes with fever, unintended weight loss, or new tiredness and breathlessness.

Haemorrhoids are a fine conclusion after an examination — not a reason to skip one.

Alongside blood in the stool, NCI lists:

• A change in bowel habits, including diarrhea or constipation
• A feeling that the bowel does not empty completely
• Stools that are narrower or a different shape than usual
• General abdominal discomfort: frequent gas pains, bloating, fullness, or cramps
• Weight loss for no known reason
• A change in appetite
• Fatigue
• Vomiting

Iron deficiency anemia, found on a blood test, is also a warning sign even when you have not seen any blood.

NCI describes these tests:

Physical exam and history. Your doctor asks about your personal and family health history and examines you.

Digital rectal exam. "The doctor or nurse inserts a lubricated, gloved finger into the lower part of the rectum to feel for lumps."

Fecal occult blood test. A stool test, either guaiac-based or immunochemical (FIT), that checks stool "for blood that can only be seen with a microscope."

Sigmoidoscopy and colonoscopy. Thin, lighted tubes used to look for "polyps, abnormal areas, or cancer." In colonoscopy, "the rectum and entire colon are examined using a colonoscope, a flexible lighted tube with a lens for viewing and a tool for removing tissue." Any abnormal growths found in the colon and rectum can be removed during the procedure.

Biopsy. "Removal of cells or tissues so they can be viewed under a microscope to check for signs of cancer." For rectal cancer, tissue may also be tested by immunohistochemistry and for microsatellite instability.

CEA assay. A blood test that measures the level of carcinoembryonic antigen.

Most groups recommend that people at average risk start colorectal cancer screening at age 45, continuing to age 75, with individual decisions between 76 and 85. Screening is for people without symptoms. If you have visible blood in your stool, you are not in that group, and a normal stool test does not close the question. NCI notes that people with a positive stool test "are advised to have a colonoscopy"; if you are bleeding, ask directly whether you need one regardless of what a stool test shows.

Book an appointment rather than watching for another episode. Write down when the bleeding started, how often it has happened, the color, whether it is mixed into the stool or on the paper, and any change in bowel habits, weight, or energy. Bring your medicine list and your family history of colorectal cancer or polyps.

If the first explanation you are given is hemorrhoids, ask what the plan is if the bleeding continues, and when the colon itself will be looked at.

• National Cancer Institute. Colon Cancer Treatment (PDQ) — Patient Version. https://www.cancer.gov/types/colorectal/patient/colon-treatment-pdq
• National Cancer Institute. Rectal Cancer Treatment (PDQ) — Patient Version. https://www.cancer.gov/types/colorectal/patient/rectal-treatment-pdq
• National Cancer Institute. Screening Tests to Detect Colorectal Cancer and Polyps. https://www.cancer.gov/types/colorectal/screening-fact-sheet
• National Cancer Institute. Possible Signs of Colorectal Cancer in Younger Adults. https://www.cancer.gov/news-events/cancer-currents-blog/2023/colorectal-cancer-young-people-warning-signs