How can you tell if there is blood in your stool?

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If your stool is bright red or crimson, there is often have blood in your stool.

You need go to see a doctor. Generally, hemorrhoid can cause bright red blood in the stool, and intestinal cancer or gastrointestinal bleeding can cause  crimson blood in the stool.

Generally we can tell from the color. The most accurate way is to have a fecal occult blood test.

At home, people can identify blood by examining the feces or by using a home-use fecal occult blood test available at pharmacies. This type of test helps identify blood that isn't visible to the human eye.

When viewing the stool, you should:

  • Examine the stool carefully. If what appears to be bright red blood on a tissue after wiping, or if toilet water contains blood, the cause probably is an open wound or other form of bleeding in the lower digestive tract, according to the National Digestive Diseases Information Clearinghouse. So, it might be due to an anal fissure or hemorrhoid.
  • Look for dark-colored stool. Black stool, or stool with red blood mixed in, suggests upper digestive tract bleeding, the National Digestive Diseases Information Clearinghouse says. When it is the case, you need to tell your health care provider. On the other hand, it may be nothing to worry about: some foods, like blueberries or black licorice, can cause feces to take on a dark appearance that may be resemble the color of blood.
Some tips for doing a fecal occult blood test:
  • Purchase a fecal occult blood test from a pharmacy. Your health care provider might provide one.
  • Open the package and review the instructions. Depending on the type of kit, it will contain either a flushable pad or a piece of material that resembles thick tissue paper.
  • Cleanse the genital and anal regions with an alcohol swab, or follow the test kit instructions for cleaning the area.
  • Place the tissue or flushable card into the toilet. Empty the bowels onto the materials.
  • Examine the flushable card for a change of color. Typically, no change after five minutes indicates no blood is present. 
  • Withdraw the tissue paper with stool sample, and touch the stool with a tester card that comes in the kit. Like the flushable card, this tester card will change color within five minutes if blood is present. Repeat the test once per day for three days or as directed by the test kit, the Cleveland Clinic says.
  • Report the findings of visual and fecal occult testing to a medical professional. He should evaluate any indication of blood to ensure the cause is not serious.
  • Note all other symptoms that accompany the blood in the stool, and report them to a health care professional.

What's more, antidiarrheals, especially those containing bismuth salicylate, can cause darkening of the stool too.