Cancer Featured The Brief: Colorectal Cancer Blog Wellness

What does blood in your poop actually look like?

Q: I saw a Colorectal Cancer Awareness poster that said to look out for blood in my stools. What exactly am I looking for?

A: A person’s stool (poop) is a window into their health and a precious warning sign of a problem, so look at it before you flush it. Keeping an eye on your stools is an important step in being aware of colorectal cancer and identifying early signs of the disease.

The topic of what one’s stool says about their health is very extensive and I won’t get into it all here, but let’s talk about seeing blood.

Firstly, blood in your poop is not normal. All blood passed in the stool, around the stool or after the stool is abnormal and represents a problem of varying severity.

So to answer the question, you are just looking for blood. And if you see it, you need to report it to your family physician for investigation. It’s just that simple. Having said that, there are certainly more worrisome characteristics of the blood in one’s stool and certain features that you should be aware of in order to report them to your physician. To understand these characteristics, one must imagine where this blood is coming from.

Colorectal cancer is a disease of the cells of the inner lining of the colon or rectum. The cells begin to grow out of control, first forming a cluster of cells called a polyp and eventually becoming a cancer as the cells burrow deeper into the wall. Because these polyps and eventual cancers are growing on the inner lining of the bowel, they ooze or bleed into the bowel and that blood then becomes incorporated with the stool as it passes through.

With that background, one can begin to imagine just how blood in the stool might look if it is coming from a colorectal cancer. If the cancer is in the right side of the colon (the beginning of the colon and furthest from the anus) then the blood might be undetectable by the eye by the time it reaches the toilet. If there is lots of blood coming from a cancer in the right colon, it may no longer look like red blood by the time it reaches the toilet but rather like black tar mixed in with the stool.

If the cancer is further along in the colon or even in the rectum, the blood will still be red smeared along the stool as it hits the toilet or the blood will be mixed in with the stool but still bright red in appearance.

Finally, if the cancer that is bleeding is in the rectum, just before the exit, you might see only blood coming into the toilet either prior to or after the stool as it is bleeding directly out of the anus rather than mixing with stool.

So by understanding how blood in the stool could be a warning sign of a cancer, it also becomes clearer why we use a fecal immunochemical test (FIT) to look at the stool microscopically (and why it’s so important!) This test checks for occult (hidden) blood in the stool as a screening test for colorectal cancer — it can find blood that isn’t visible to the eye when you look in the toilet.

The test isn’t specific — many other things can lead to hidden blood in the stool and the test can’t tell which it is causing the blood. But it is very sensitive, which means that almost all colorectal cancer patients would be positive if the test was done. That means very few false negatives, which is ideal for a screening test.

It is obviously very important to understand that many different colorectal problems can lead to blood in the stools, not just cancer. But do remember that regardless of the cause, blood in your poop it is not normal and needs discussion and investigation by your primary care physician. The characteristics of the blood will offer clues but by no means will it make a diagnosis and almost certainly the next step will be a colonoscopy to definitively sort out what’s going on.

In very simple terms, please do this for me: Turn and look at your stool before flushing. It could save your life.


(This post was updated in August 2020 to reflect a change in Ontario’s colorectal screening test from the fecal occult blood test to the FIT)

About the author

Dr. Shady Ashamalla

Dr. Ashamalla is a colorectal cancer surgeon at Sunnybrook’s Odette Cancer Centre specializing in minimally invasive surgical treatments. He’s also a teacher and researcher in surgical education and simulation, and he's a consultant surgeon to the Toronto Maple Leafs. Read more posts on his blog.