Colorectal screening with FIT, by the numbers

Worth the Test · Written by an emergency physician · Last reviewed September 2026

A fecal immunochemical test costs a few dollars, arrives in the mail, and asks for nothing but a sample and a stamp. It is also, by the arithmetic below, wrong about eleven times out of twelve when it raises an alarm. Both facts are true at once, and the interesting thing about FIT is that the second fact barely matters — because of what the test is for. FIT is the cleanest example in common use of a triage test: a cheap, repeatable first pass whose job is not to diagnose cancer but to decide who needs the expensive, definitive look.

Three numbers, and where they come from

• Sensitivity ≈ 79% — of people who truly have colorectal cancer, about 79 in 100 have a positive FIT on a single round. A meta-analysis of nineteen studies by Lee and colleagues puts the pooled figure at 79%, with individual studies ranging widely around it.
• Specificity ≈ 94% — about 94 of every 100 people without cancer are correctly cleared; roughly 6 in 100 are flagged anyway, most often from bleeding that has nothing to do with cancer.
• Prevalence ≈ 0.7% — in the large screening study by Imperiale and colleagues, 65 of 9,989 average-risk adults undergoing screening colonoscopy — about 0.7% — turned out to have colorectal cancer. That is the base rate a first screening round faces.

The same head-to-head study is a useful cross-check on the meta-analysis: in it, FIT ran at 73.8% sensitivity and 94.9% specificity for cancer — a little lower on both counts than the pooled figures, and close enough that the story below doesn't change whichever pair you use.

The 2×2, worked by hand

Send FIT kits to 1,000 average-risk adults and have every one returned. First split the population by the truth: 1,000 × 0.007 = 7 people with colorectal cancer, and 993 without.

Now apply the test. Of the 7 with cancer, 7 × 0.79 ≈ 6 are flagged (true positives) and about 1 is missed this round (a false negative). Of the 993 without cancer, 993 × 0.06 ≈ 60 are flagged anyway (false positives) and about 933 are correctly cleared (true negatives).

Follow the positive column. About 66 people get a letter saying their test was abnormal, and only about 6 of them have cancer. The positive predictive value:

PPV = (0.79 × 0.007) ÷ [(0.79 × 0.007) + (0.06 × 0.993)] = 0.00553 ÷ 0.06511 ≈ 0.085 — about 1 in 12.

Computed from exact rates that is 8.5%; dividing the rounded people, 6 ÷ 66, gives 9.1% — the same answer to the precision that matters. Eleven of every twelve positive FITs are false alarms, for the reason every rare-disease screen shares: the cancer-free crowd the false positives come from is more than a hundred times larger than the group with cancer. That mechanism has its own guide.

Why a 1-in-12 positive is fine here

A PPV of 8.5% would be alarming if a positive FIT led straight to surgery. It doesn't. It leads to a colonoscopy — a test that can actually look at the colon, remove what it finds, and settle the question. The whole design leans on that second step: FIT trades away predictive value to stay cheap, easy, and repeatable, precisely because a definitive follow-up exists to absorb its false alarms.

Follow-up has its own risks. The USPSTF’s 2021 recommendation estimates 17.5 serious bleeding events and 5.4 perforations per 10,000 colonoscopies following positive screening results. Both run higher than the rates for screening colonoscopy (14.6 and 3.1 per 10,000); the Task Force’s likely explanation is that follow-up colonoscopies involve more biopsies and more polyps removed.

Per 1,000 people screened, about 65 test positive (65.11 before rounding). If every one of them has a follow-up colonoscopy, those rates imply about 0.11 serious bleeding events and 0.035 perforations per 1,000 people screened — expected events across a population, not a forecast for any individual. The two estimates come from different sets of studies and can overlap, so they shouldn't simply be added into a count of people harmed; and they are procedure complication rates, not a treatment NNH.

A negative is genuinely reassuring — this round

Now the negative column. 934 people are cleared, and 933 of them are truly cancer-free — a negative predictive value above 99.8%. Before the test, a person's chance of harboring colorectal cancer was 0.7%; after a negative FIT it is about 0.16%. In likelihood-ratio terms the test earns its keep in both directions: LR+ = 0.79 ÷ 0.06 ≈ 13, a solidly useful positive, and LR− = 0.21 ÷ 0.94 ≈ 0.22, a real (if not decisive) push toward health. How those ratios move a probability is its own guide.

The catch is the phrase this round. One in five cancers slips past a single FIT, which is why the programs that use it repeat it every year. Repetition patches the sensitivity problem — a cancer missed this year gets another chance to bleed next year — but it compounds the false-alarm problem: at 94% specificity, ten annual rounds give a person nearly a one-in-two chance of at least one false positive along the way (1 − 0.94¹⁰ ≈ 46%, an independence illustration, not a general upper bound). That accumulation, and what the real-world data show it does, is worked through in repeated screening and false positives.

The test you return beats the test you don't

One more number decides more than any of the others, and it never appears in an accuracy table: the fraction of people who actually do the test. A colonoscopy-first strategy has higher single-shot sensitivity, but it asks for a day off work, a bowel prep, and sedation; mailed FIT asks for almost nothing. In the Imperiale study the newer multitarget stool-DNA test beat FIT on sensitivity (92.3% versus 73.8%) at the cost of specificity (86.6% versus 94.9%) — and every such comparison is moot for the person who returns neither kit. The calculator's uptake slider exists for exactly this reason: set it below 100% and watch missed cancers pile up in the "never tested" column no matter how good the test is. A mediocre test completed reliably outperforms an excellent test skipped.

What the U.S. guideline recommends

None of the arithmetic above says whether screening is worth doing; that comes from outcome evidence. In its 2021 recommendation, the U.S. Preventive Services Task Force concluded with high certainty that screening adults aged 50 to 75 has substantial net benefit and recommends it for all of them (grade A); it also recommends screening at ages 45 to 49, where it judged the net benefit moderate (grade B), and advises clinicians to offer screening selectively at ages 76 to 85 (grade C). Annual FIT is one of its recommended strategies, alongside stool DNA-FIT, CT colonography, flexible sigmoidoscopy and colonoscopy, each on its own schedule. The Task Force does not rank them, because no direct evidence compares the tests, and it notes that a stool test's benefit depends on repeating it on schedule and following every positive result with colonoscopy.

What generalizes

FIT is the friendly extreme of the screening map: a test whose low positive predictive value is a design choice rather than a flaw, because a definitive, therapeutic follow-up exists and the test is cheap enough to repeat. Compare it with PSA, where the follow-up is itself contested; with mammography, where the follow-up work-up is the main population-level harm; and with NIPT, where the confirmatory test carries real risk. Same arithmetic every time — what changes is what a positive costs.

Correction, September 2026. An earlier version of this page estimated the harms of colonoscopy after a positive FIT with screening-colonoscopy complication rates (14.6 serious bleeding events and 3.1 perforations per 10,000). It now uses the USPSTF’s rates for colonoscopies that follow a positive stool test. The corrections log lists every change.