A fecal immunochemical test (FIT) measures the concentration of hemoglobin in your stool, reported in micrograms of hemoglobin per gram of feces (µg Hb/g). Most results fall well below 10 µg Hb/g, and the threshold for calling a result “positive” varies by screening program but commonly sits at 20 µg Hb/g or lower. That number, though, is far from universal, and understanding what your specific result means requires knowing which cutoff your program uses, what factors can push the number up or down, and how higher readings relate to actual risk.
How FIT Results Are Measured
Quantitative FIT devices collect a small stool sample on a probe, which is then analyzed in a lab using antibodies that bind specifically to human hemoglobin. The machine reports how much hemoglobin it detected, and the standard unit recommended by international guidelines is micrograms of hemoglobin per gram of feces (µg Hb/g). Some older lab reports use nanograms of hemoglobin per milliliter of buffer solution (ng Hb/mL), which is the raw reading from the device before adjusting for the sample weight. Converting between the two depends on the specific device, but for the widely used OC-Sensor system, you multiply ng/mL by 0.2 to get µg/g. So a reading of 100 ng/mL equals 20 µg/g.1PubMed Central. Colorectal cancer detection in an asymptomatic population: fecal immunochemical test for hemoglobin vs. fecal M2-type pyruvate kinase If your report uses ng/mL, ask your provider what that translates to in µg/g so you can compare it to published guidelines.
Qualitative FIT kits, by contrast, give you only a yes-or-no answer. They work like a pregnancy test: a line appears if hemoglobin exceeds a preset threshold built into the device. These kits can have different built-in thresholds, and because the cutoff is fixed by the manufacturer, there is no way to adjust sensitivity after the fact.2PubMed Central. Head-to-head comparison of the test performance of self-administered qualitative vs. laboratory-based quantitative fecal immunochemical tests in detecting colorectal neoplasm Expert task forces generally recommend quantitative FIT over qualitative versions because the numeric output allows programs to fine-tune their cutoff and gives clinicians more information about your individual risk level.3Gastroenterology. Fecal immunochemical testing (FIT) to screen for colorectal neoplasia
Where the “Positive” Cutoff Falls
There is no single worldwide standard for what counts as a positive FIT. Screening programs pick their own threshold based on how many people they can send for follow-up colonoscopy and how sensitive they want the test to be. In practice, cutoffs across different countries range from as low as about 8.5 µg Hb/g in parts of Belgium up to 120 µg Hb/g in England, Wales, and Northern Ireland.4PubMed Central. Variation in diagnostic performance of fecal immunochemical test (FIT) by cutoffs used in screening programs globally That is an enormous spread, and it means two people with the exact same stool hemoglobin level could get opposite results depending on where they live.
A U.S. multi-society task force on colorectal cancer has recommended favoring a lower threshold of 20 µg Hb/g or below to define a positive test, which catches more cancers at the cost of sending more people for colonoscopy.3Gastroenterology. Fecal immunochemical testing (FIT) to screen for colorectal neoplasia A meta-analysis of studies in people at increased risk found that cutoff values between 15 and 25 µg/g offered the best balance between sensitivity and specificity for detecting colorectal cancer, catching about 93% of cancers while correctly clearing about 94% of people who did not have cancer.5JAMA Internal Medicine. Diagnostic Accuracy of Fecal Immunochemical Test in Patients at Increased Risk for Colorectal Cancer
The trade-off is straightforward. Lower the cutoff and you catch more cancers, but you also flag more people who turn out to be fine. Raise the cutoff and you reduce unnecessary colonoscopies, but you miss more cancers. Programs with limited colonoscopy capacity sometimes accept a higher cutoff and screen more frequently to compensate. Research suggests that pairing a more sensitive (lower) cutoff with a slightly longer interval between screening rounds can be an effective compromise when colonoscopy appointments are scarce.6PubMed Central. Colorectal Cancer Screening within Colonoscopy Capacity Constraints: Can FIT-Based Programs Save More Lives by Trading off More Sensitive Test Cutoffs against Longer Screening Intervals?
Higher Numbers Mean Higher Risk
A FIT result is not simply positive or negative in terms of what it tells your doctor. The actual hemoglobin concentration carries information about how likely it is that something serious is going on. In a large Taiwanese screening dataset, the median stool hemoglobin was about 57 µg/g for non-advanced adenomas (small, less worrisome polyps), roughly 82 µg/g for advanced adenomas, and around 163 µg/g for colorectal cancer.7Journal of Global Oncology. Rank-Based Fecal Hemoglobin Concentration for Assessing Colorectal Neoplasms of National-Wide Colorectal Cancer Screening in Taiwan In other words, higher readings correlate with more advanced disease. A study modeling the relationship between FIT concentration and the likelihood of finding a colorectal tumor confirmed a strong log-linear pattern: as the hemoglobin reading climbs, the chance that a colonoscopy will find a clinically significant lesion rises steeply.8PubMed Central. Application of quantitative estimates of fecal hemoglobin concentration for risk prediction of colorectal neoplasia
This has practical implications for how quickly you should follow up. Someone whose FIT comes back at 200 µg/g warrants more urgent attention than someone at 25 µg/g, even though both are technically “positive.” One UK study of symptomatic patients found that all 25 cancers in their cohort occurred in the FIT-positive group, with none in the FIT-negative group, giving the test a sensitivity and negative predictive value of 100% in that population. The positive predictive value, however, was only about 11%, meaning roughly nine out of ten people with a positive FIT did not have cancer.9PubMed Central. Does the Value of the Faecal Immunochemical Test (FIT) Result Matter, and Can This Be Used as a Triage Criterion for Symptomatic FIT-Positive Patients? So a positive result does not mean you have cancer. It means something is causing blood to show up in your stool and it needs investigating.
What Can Push Your Number Up Without Cancer
FIT detects human hemoglobin, period. It cannot tell whether that blood came from a tumor, an inflamed patch of intestine, or a hemorrhoid that bled during a bowel movement. Several non-cancerous conditions can produce a positive result:
- Hemorrhoids: One study found that in about 4% of people with false-positive FIT results, hemorrhoids were the only abnormal finding on colonoscopy.10Gastrointestinal Endoscopy. Effect of hemorrhoids on the diagnostic accuracy of fecal immunochemical testing for colorectal cancer screening
- Diverticular disease: Small outpouchings in the colon wall can bleed intermittently and raise stool hemoglobin.11PubMed Central. Factors associated with false fecal immunochemical test results in colorectal cancer screening
- Colorectal inflammation: Inflamed intestinal lining appears red and swollen on endoscopy and can leak small amounts of blood. Having three or more non-advanced adenomas also raised the odds of a false-positive result in one study.12Scientific Reports. Risk factors associated with false-positive fecal immunochemical test results in colorectal cancer screening
Lifestyle and body factors also play a role. Older age, higher body mass index, smoking, and alcohol consumption have all been associated with higher odds of a false-positive result.11PubMed Central. Factors associated with false fecal immunochemical test results in colorectal cancer screening These factors may increase low-grade intestinal bleeding independently of any tumor.
False Negatives and Why a Low Number Is Not a Guarantee
A result below the cutoff is reassuring but not ironclad. Some colorectal lesions simply do not bleed enough to be detected, or they bleed intermittently and happen to be quiet on the day you collect your sample. Flat lesions that do not protrude into the intestinal space are less likely to bleed at all.13British Journal of Biomedical Science. Faecal Immunochemical Test (FIT) Sensitivity; A Five Year Audit This is a key reason why screening programs recommend repeating FIT every year or two rather than relying on a single negative result to rule out cancer permanently.
Pre-analytical factors can also drag your number down artificially. Hemoglobin degrades in the stool sample over time, especially in warm conditions. Lab experiments showed that samples stored at 30–35°C for ten days lost an average of 41% of their hemoglobin concentration. Within four days, however, the drop was modest and not statistically significant, supporting the stability of mail-return programs as long as samples reach the lab relatively quickly.14Clinical Chemistry. Hot Mail: Temperature Exposure during Mail Return of an Immunochemical Fecal Occult Blood Test If you live in a hot climate, mailing your sample promptly matters.
Do Medications Affect the Result
A common question is whether blood thinners or aspirin will cause a false positive. The evidence is surprisingly reassuring on this point. Multiple studies have looked at people taking oral anticoagulants, aspirin, or clopidogrel and found that these medications do not significantly increase the false-positive rate. One study found no significant difference in the median stool hemoglobin concentration between users and non-users of anticoagulants combined with antiplatelets.15PubMed Central. Effect of oral anticoagulants on the outcome of faecal immunochemical test Another confirmed on multivariate analysis that antiplatelet and anticoagulant use did not independently increase the risk of a positive FIT.16PubMed Central. The effect of antiplatelet therapy and oral anticoagulants on the accuracy of faecal immunochemical testing This is why most screening programs do not ask you to stop blood thinners before collecting your sample.17Gastroenterology. Effects of Oral Anticoagulants and Aspirin on Performance of Fecal Immunochemical Tests in Colorectal Cancer Screening
Unlike the older guaiac-based stool tests, FIT uses antibodies specific to human hemoglobin. That means red meat and certain vegetables will not cause false positives the way they could with the previous generation of tests. You do not need to follow any special diet before collecting your sample.
Sex and Age Shift the Baseline
Your “normal” stool hemoglobin level is not identical to everyone else’s. In a large community-based cohort of people who did not develop colorectal cancer, the 95th percentile of baseline stool hemoglobin was about 13.4 µg/g for people aged 50–59, 16.2 µg/g for those aged 60–69, and 21.0 µg/g for those aged 70–75. Men also ran higher than women: 17.8 µg/g versus 13.4 µg/g at the 95th percentile.18PubMed Central. Influence of Varying Quantitative Fecal Immunochemical Test Positivity Thresholds on Colorectal Cancer Detection: A Community-Based Cohort Study This means men are more likely to test positive at any given cutoff, and older people are more likely to test positive than younger people, even without cancer.
A study looking at nine different FIT brands across a range of thresholds consistently found higher sensitivity and lower specificity in men compared to women. It also found that a negative FIT result is less reliable for ruling out advanced neoplasia in men than in women, and in older people compared to younger ones.19PubMed Central. Consistent Major Differences in Sex- and Age-Specific Diagnostic Performance among Nine Faecal Immunochemical Tests Used for Colorectal Cancer Screening Some programs have experimented with sex-specific cutoffs to account for this. A Swedish screening program, for instance, used a cutoff of 40 µg/g for women and 80 µg/g for men.20PubMed Central. Interval cancer after two rounds of a Swedish population-based screening program using gender-specific cut-off levels in fecal immunochemical test Whether sex-specific thresholds improve outcomes in the long run is still being studied, but the biological rationale is sound: men simply tend to have more hemoglobin in their stool at baseline.
Why FIT Mostly Catches Lower GI Problems
FIT uses antibodies that bind to intact human hemoglobin. Hemoglobin breaks down as it travels through the digestive tract, so blood originating high up — in the esophagus or stomach — is largely degraded by the time it reaches the colon and the stool sample. This makes FIT fairly specific for lower gastrointestinal bleeding.21PubMed Central. Positive Fecal Immunochemical Test Results Are Associated with Increased Risks of Esophageal, Stomach, and Small Intestine Cancers If you have an upper GI condition like a stomach ulcer, FIT is unlikely to detect it. That is not a flaw — it is by design. The test was built to screen for colorectal cancer, not upper-tract diseases. But it means a negative FIT says nothing about the health of your stomach or esophagus.
What Happens After a Positive Result
A positive FIT is a referral to colonoscopy, not a diagnosis. How quickly that colonoscopy happens makes a real difference. A large study of more than 70,000 patients found that getting a colonoscopy within six months of a positive FIT showed no significantly increased risk of cancer outcomes compared to the fastest follow-up window of 8 to 30 days. But waiting longer started to matter. At 10 to 12 months, the odds of any colorectal cancer were about 48% higher, and the odds of advanced-stage disease nearly doubled. Beyond 12 months, the picture worsened sharply: the odds of any colorectal cancer more than doubled, and the odds of stage IV disease were nearly four times higher than in people who got a prompt colonoscopy.22JAMA. Association Between Time to Colonoscopy After a Positive Fecal Test Result and Risk of Colorectal Cancer and Cancer Stage at Diagnosis A systematic review of multiple studies confirmed this pattern, finding that colonoscopy performed beyond nine months was associated with both higher cancer incidence and more advanced stage at diagnosis.23PubMed Central. Association Between Time to Colonoscopy After Positive Fecal Testing and Colorectal Cancer Outcomes: A Systematic Review
The practical takeaway: if your FIT comes back positive, aim to schedule your colonoscopy within a few months. There is no need to panic — you have a comfortable window — but there is also no reason to sit on the referral indefinitely. The data strongly suggest that delays beyond about nine months start costing lives.
Tracking Your Numbers Over Time
Because FIT is designed to be repeated, the trend in your results across multiple rounds carries valuable information beyond any single reading. A study tracking more than 4,400 people through three sequential rounds of FIT found that those whose hemoglobin values remained stable and low had the lowest rate of advanced colorectal neoplasia, at about 5%. People whose values showed a serial increase leading to a positive result in the third round had the highest detection rate, at roughly 33%.24PubMed Central. Changes in faecal haemoglobin values over sequential rounds of faecal immunochemical tests (FIT) in a surveillance population
Researchers have also proposed using your baseline hemoglobin concentration to personalize how often you should be rescreened. A Taiwanese study suggested that someone with a very low initial reading (1–9 µg/g) could safely extend their screening interval to four years, while someone in the 10–19 µg/g range might go three years. People near or above the positive threshold would stay on the standard annual or biennial schedule, or be referred directly for colonoscopy.25JAMA Oncology. Precision Colorectal Cancer Fecal Immunological Test Screening With Fecal-Hemoglobin-Concentration–Guided Interscreening Intervals This kind of risk-stratified approach is not yet standard practice in most countries, but it illustrates an important point: your FIT number is not just a pass/fail mark. It sits on a continuum, and where you fall on that continuum tells a more nuanced story about your colorectal health than a simple “negative” label does.
Why Programs Use Different Cutoffs
It is reasonable to wonder why countries cannot just agree on one number. The answer involves both biology and logistics. A program like England’s, historically using a very high cutoff around 120 µg/g, does so in part because of the sheer volume of people being screened and the limited colonoscopy capacity available. At that threshold, sensitivity for colorectal cancer dropped to about 56%, but specificity was nearly 99%, meaning very few people were sent for unnecessary procedures.4PubMed Central. Variation in diagnostic performance of fecal immunochemical test (FIT) by cutoffs used in screening programs globally Belgium’s Flanders program, with a cutoff near 8.5 µg/g, caught up to 98% of cancers but flagged a much larger share of the population — about 18% — for follow-up. Each approach reflects a different judgment about the acceptable trade-off between missing some cancers and overwhelming endoscopy services with people who turn out to be fine.
Population characteristics matter too. As noted above, background stool hemoglobin levels vary by age and sex, and the prevalence of colorectal cancer in the target population affects the positive predictive value of any given cutoff. A threshold that works well in a 50-to-60-year-old population may perform differently in a 70-to-75-year-old population simply because the underlying disease rate is higher in the older group. These are the kinds of considerations that keep the “right” cutoff a moving target rather than a fixed number carved in stone.