Positive FIT Test: What Happens Next?

A fecal immunochemical test looks for one thing. Human hemoglobin, the protein that carries oxygen in red blood cells.

It does that with antibodies, which is the detail that separates it from the older test. The guaiac test, gFOBT, uses a chemical reaction to find heme, a component of hemoglobin. That reaction cannot tell human heme from the heme in a steak, so people have to change what they eat beforehand. FIT does not require that.

So a positive FIT means blood was in the sample. It does not say how much, where it came from, or why.

NCI puts the range plainly. Both polyps and colorectal cancers can bleed. Hidden blood in stool may also point to conditions that are not cancer, such as hemorrhoids.

Polyps deserve their own sentence, because the word frightens people more than it should. NCI states that colorectal polyps are common in people older than 50, and that most do not become cancer. One type, the adenoma, is more likely to. That is precisely why finding and removing them is worth doing.

Most colorectal cancers begin as a growth in the lining of the colon or rectum. Some are raised polyps on a stalk. Some sit flat against the surface. Some are barely indented. That variety is part of why a direct look matters. See stool tests for colorectal cancer for how the different kits compare.

NCI is unambiguous. People who have a positive finding with a stool test are advised to have a colonoscopy.

The reason is what colonoscopy can do that nothing else on the list can. It examines the rectum and the entire colon with a flexible lighted tube that carries both a lens and a tool for removing tissue. If something abnormal is found anywhere along the way, it can be taken out during the same procedure.

That is not true of the alternatives. NCI notes that if polyps or other abnormal growths are found during a virtual colonoscopy, a standard colonoscopy usually has to follow to remove them. Sigmoidoscopy reaches only part of the colon, and a follow-up colonoscopy may still be recommended.

Repeating the stool test is not a substitute either. It would answer the same narrow question a second time, and the answer would still send the same way.

Preparation is the part people dread, and NCI does not soften it. Colonoscopy requires a thorough cleansing of the entire colon beforehand. Most patients receive some form of sedation during the test.

If an abnormality is found, it is removed by polypectomy, or a biopsy is taken. Either way the cells are examined to see whether cancer is present. That microscope result, not the colonoscopy itself, is the actual answer.

There is a real risk to name. NCI lists a small risk of tearing or perforation of the lining of the colon. It is small, and it is the reason the procedure is not used casually as a first-line screen for everyone.

For context on where colonoscopy sits among the options, see colonoscopy and colorectal cancer screening.

Knowing the alternatives makes the next conversation easier, because the choice of test shapes the follow-up.

NCI lists three stool tests. FIT. The older guaiac test. And multitarget stool DNA testing, sold as Cologuard, which detects hemoglobin alongside DNA markers shed from the lining of the colon. A newer multitarget stool RNA test, ColoSense, was approved in 2024. NCI notes it is not yet commercially available and that experts have not yet recommended a screening interval for it.

There is also a blood-based option. NCI states that Shield is approved for screening adults aged 45 and older at average risk, analyzing plasma DNA for certain changes.

Every one of those shares the same limitation as FIT. A positive result routes to a colonoscopy, because none of them can look at the colon or remove anything.

This is worth saying because the silence is real. NCI's screening fact sheet says a follow-up colonoscopy will be needed if the test is positive. It does not give a number of weeks.

What it does give is the surrounding schedule. FIT used on its own is generally recommended every year or two. Stool DNA testing is suggested at least every three years. Screening colonoscopy runs every ten years for people at average risk. Screening starts at 45 for average risk and generally continues to 75, with the decision between 76 and 85 based on life expectancy, other conditions and previous results.

Anyone handed a specific follow-up interval should ask which guideline it came from, since it is not coming from this page.

NCI includes a warning that has nothing to do with medicine and costs people real money.

Colorectal cancer screening is a preventive service that many insurance plans must cover. But a colonoscopy done to follow up a positive screening test is considered a diagnostic exam. It may not be covered, or may not be covered as fully as a screening colonoscopy would be.

It goes further. NCI notes that some insurers treat even a screening colonoscopy that finds a polyp needing removal as diagnostic, and charge accordingly. Its advice is to check coverage with the insurer before the test, and to find out what the out-of-pocket cost would be if something is found.

That is a phone call worth making while waiting for the appointment rather than after it.

• Call 911 or go to an emergency department if, after the colonoscopy, bleeding from your bottom is heavy or keeps getting worse, or severe stomach pain sets in and does not let up. Perforation and significant bleeding are the two rare complications the procedure carries.
• Call 911 or go to an emergency department if, while you are still waiting for the appointment, you pass a large amount of blood and also faint, feel your heart racing when you stand up, or go pale and clammy. That combination is bleeding fast enough to matter.
• Contact the hospital where you had the colonoscopy straight away if you feel hot, cold, or shivery, or run a high temperature in the days after it. That is the urgency the NHS attaches to this; in the UK you can also call 111.

• https://www.cancer.gov/types/colorectal/screening-fact-sheet
• https://www.nhs.uk/tests-and-treatments/colonoscopy/what-happens-on-the-day/
• https://www.cancer.gov/about-cancer/screening/screening-tests