How accurate are at-home stool tests for bowel cancer?

This page explains how accurate at-home stool tests for colon cancer are, and what that means for how often to test and what to do after a positive result.

A test mailed to your house, done in your own bathroom, is an appealing alternative to bowel prep (cleaning out the colon) and a hospital appointment. The fair question is whether you are giving something up by choosing it.

The US Preventive Services Task Force publishes the actual numbers, so you can see the trade-off rather than guess at it.

Two measures matter. Sensitivity is how often a test correctly picks up people who do have the condition. Specificity is how often it correctly clears people who do not.

For detecting colorectal cancer, USPSTF reports these figures for FIT (fecal immunochemical test), high-sensitivity gFOBT (guaiac fecal occult blood test) and the stool DNA-FIT test:

• FIT: sensitivity 0.74, specificity 0.94
• High-sensitivity gFOBT: sensitivity 0.50 to 0.75, specificity 0.96 to 0.98
• Stool DNA-FIT: sensitivity 0.93, specificity 0.84

A sensitivity of 0.74 means roughly three in four cancers present at the time of testing would be picked up. That is useful, but not perfect.

Notice the trade-off in the stool DNA test. It catches more cancers, but its specificity is lower. That means more people without cancer will get a positive result and go on to colonoscopy.

The figures for finding advanced adenomas — growths that have not yet become cancer but are on that path — are much weaker:

• FIT: 0.23
• High-sensitivity gFOBT: 0.06 to 0.17
• Stool DNA-FIT: 0.43

This is the real difference between stool testing and colonoscopy. Colonoscopy can find and remove growths before they ever become cancer. Stool tests are far less able to find these growths.

Repeating the test on schedule is what makes stool-based screening work. One round is not the program.

Because each single round misses things, USPSTF pairs each test with an interval:

• FIT: every year
• High-sensitivity gFOBT: every year
• Stool DNA-FIT: every one to three years

A single round of an imperfect test is not the whole program, which is why the repeat schedule matters.

USPSTF is clear here: positive results on stool-based screening tests require follow-up with colonoscopy for the screening benefits to be achieved.

That point matters: a positive stool test that is not followed up is close to no screening at all. A positive result is not a diagnosis. The colonoscopy is how you find out the cause.

If you get a positive result, ask your care team to arrange the colonoscopy. If getting that appointment is difficult, tell your doctor's office rather than waiting quietly.

USPSTF recommends colorectal cancer screening with a grade A for adults aged 50 to 75, and a grade B for adults aged 45 to 49. If you have a family history of bowel cancer, inflammatory bowel disease, or a previous adenoma, your situation sits outside average-risk screening and should be planned with your clinician.

The best screening test, in the end, is one that actually gets done on time. For many people that is the one that arrives in the mail.

A test kit that sits in a drawer contributes nothing, and one that is done carelessly can give a misleading answer. The instructions on collecting the sample and mailing the kit promptly matter because the lab chemistry depends on them.

Two practical points come up often. Read the kit's instructions about medicines and foods before you collect, because instructions vary between kits. And send the sample back quickly rather than letting it sit at home for a week.

If the process is too hard, say so. Doctors' offices would far rather send a replacement kit than record a patient as not screened.

It is also worth naming the embarrassment, since it stops a lot of people. Handling a stool sample is unpleasant and a bit embarrassing, and everyone who does it feels that way. USPSTF gives screening its top grades for these ages, which makes it worth a few awkward minutes.