The short answer
Stool tests check a sample of your bowel movement for hidden blood or DNA changes that can be signs of colorectal cancer or polyps. You collect the sample at home and mail it to a lab. They are simple and need no prep, but a positive result must be followed up with a colonoscopy.
Stool tests look for hidden blood or DNA changes in a bowel movement that you can't see.
The main types are gFOBT, FIT, and the stool DNA test (FIT-DNA).
You collect the sample yourself at home and send it to a lab — no bowel prep or sedation.
How often you test depends on the type: FIT and gFOBT usually every year, stool DNA every 3 years.
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The full explanation.
The simple version
Stool tests screen for colorectal cancer by checking a sample of your bowel movement for signs of trouble you cannot see. Both polyps and colorectal cancers can bleed tiny amounts, and some abnormal cells shed DNA into the stool. These tests pick up those hidden clues.
The big appeal: you do them at home. There is no bowel prep, no sedation, and no time off work. You collect a small sample using a kit and mail it to a lab.
Stool tests are the simplest way to start colorectal screening — but a positive result always needs a colonoscopy to follow up.
The three main types
The U.S. Food and Drug Administration has approved several stool tests. The three you are most likely to hear about are:
- gFOBT (guaiac fecal occult blood test). Uses a chemical to detect heme, a part of the blood protein hemoglobin. Because it can also react to some foods, you may need to avoid things like red meat beforehand.
- FIT (fecal immunochemical test). Uses antibodies to detect blood protein specifically. It usually needs no diet changes, which makes it easier to do correctly.
- FIT-DNA / stool DNA test (Cologuard). Detects hidden blood plus certain DNA markers shed from the colon and rectum lining.
FIT and the stool DNA test usually need no diet changes; gFOBT often does — always read your kit's instructions.
How you do the test
Each kit comes with instructions and everything you need to collect a small stool sample. You do this in the privacy of your own home, seal the sample, and send it to a laboratory. The lab checks it and sends the result to your care team.
There is no cleaning out of the colon and no sedation. The most important thing is to follow the kit steps carefully — collecting the sample the wrong way, or waiting too long to mail it, can affect the result.
How often to repeat
Because a single stool test only looks at one moment in time, these tests are repeated more often than a colonoscopy:
- FIT — generally every year or two
- gFOBT — generally every year or two
- Stool DNA test (FIT-DNA) — generally at least every 3 years
Staying on schedule is what makes stool testing work. A one-time test that isn't repeated offers much less protection.
Stool tests only protect you if you keep doing them on time.
What a positive result means
A positive stool test means blood or DNA changes were found. This is not the same as a cancer diagnosis. Hidden blood can come from harmless causes such as hemorrhoids, and DNA changes can have many explanations.
But a positive result does mean one thing for certain: you need a colonoscopy to look directly at the colon and find out what is going on. This step is essential. Skipping the follow-up colonoscopy after a positive stool test undoes the whole point of screening.
A positive stool test is not a diagnosis — it's a signal that a colonoscopy is needed.
Costs and access
Colorectal cancer screening is a preventive service that many insurance plans and Medicare are required to help cover, and stool tests are widely available. One thing to keep in mind: if your stool test is positive and you need a follow-up colonoscopy, some insurers may treat that colonoscopy as a diagnostic exam and bill it differently.
It is worth asking your plan ahead of time how both the stool test and any follow-up are covered, so there are no surprises.
Check with your insurer about coverage for both the stool test and a possible follow-up colonoscopy.
Words to know
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Common questions
▸How does a stool test find cancer?
Both polyps and colorectal cancers can bleed small amounts that you cannot see. Stool tests check for that hidden (occult) blood, and one type also checks for DNA changes shed by abnormal cells. Finding these signs early can point to a problem that needs a closer look.
▸What's the difference between gFOBT, FIT, and the stool DNA test?
gFOBT uses a chemical to detect a part of blood called heme, and you must avoid certain foods before it. FIT uses antibodies to detect blood protein and usually needs no diet changes. The stool DNA test (FIT-DNA, sold as Cologuard) checks for both blood and certain DNA markers from the colon lining.
▸How often do I need to test?
It depends on the test. If FIT or gFOBT is your only screening, experts generally recommend doing it every year or two. The stool DNA test is generally done at least every 3 years. Your care team will tell you the right schedule for the test you choose.
▸Do I have to change my diet or do a bowel prep?
There is no bowel prep and no sedation. For gFOBT you may need to avoid certain foods, like red meat, beforehand. FIT and the stool DNA test usually do not require diet changes. Follow the kit's instructions exactly so your result is accurate.
▸What happens if my stool test is positive?
A positive result means blood or DNA changes were found — it does not mean you have cancer. But it does mean you need a colonoscopy to look directly at the colon and find the cause. Getting that follow-up colonoscopy is essential; skipping it undoes the benefit of screening.
▸Are stool tests as good as a colonoscopy?
Stool tests are a valid screening choice and are easier to do, but they work differently. They must be repeated more often, and unlike colonoscopy they cannot remove polyps — any positive result leads to a colonoscopy. The best test is often the one you will actually complete on schedule.
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