What Is the FIT Test Results Range?
Understand how FIT test results are measured, what influences levels, and how thresholds can vary between laboratories for more accurate interpretation.
The FIT (Fecal Immunochemical Test) is a widely used screening tool for detecting hidden blood in stool, an early indicator of colorectal cancer or other gastrointestinal conditions. It is non-invasive, easy to use at home, and commonly recommended for individuals over 50 or those with risk factors for colorectal disease.
Key Components Of The Test
FIT detects trace amounts of hemoglobin in stool, signaling potential bleeding in the lower gastrointestinal tract. Unlike guaiac-based fecal occult blood tests (gFOBT), which react to heme from both blood and certain foods, FIT specifically identifies human hemoglobin using antibodies. This specificity reduces false positives from diet and increases reliability in detecting colorectal abnormalities.
The test kit includes a collection device, a buffer solution, and a sample vial with an applicator. The process involves collecting a small stool sample with the applicator and placing it into the vial with the buffer, which stabilizes the hemoglobin for analysis. The sample is then sent to a laboratory for immunoassay testing to quantify hemoglobin levels.
FIT is highly sensitive to bleeding from the colon and rectum but does not detect blood from the upper digestive tract, as hemoglobin from these sources is broken down by digestive enzymes before reaching the stool. This focus makes FIT particularly effective for colorectal cancer screening, detecting bleeding from malignancies or precancerous lesions in the large intestine.
How Results Are Measured
FIT results are determined by measuring the concentration of hemoglobin in stool using an immunoassay. This technique employs antibodies that bind to human hemoglobin, generating a measurable signal proportional to blood levels. Automated analyzers assess the optical density of the reaction, producing results in micrograms of hemoglobin per gram of stool (µg/g).
The threshold for a positive result varies by laboratory and test manufacturer but typically falls between 10 and 50 µg/g. Lower cutoffs increase sensitivity, detecting more cases of early colorectal cancer and advanced adenomas, while higher thresholds improve specificity, reducing false positives and unnecessary follow-ups. Some health systems choose lower thresholds to maximize early detection.
Results are reported as either negative or positive, or quantitatively with an exact hemoglobin concentration. A negative result means hemoglobin levels are below the detection threshold, suggesting no significant bleeding. A positive result indicates elevated levels, warranting further evaluation, usually via colonoscopy. Some clinical settings use quantitative reporting to assess risk more precisely and guide follow-up recommendations.
Range Thresholds Across Different Laboratories
FIT result thresholds are not universally standardized, leading to variations in cutoff values. Most laboratories define a positive result as hemoglobin levels exceeding 10 to 50 µg/g, but some institutions adjust criteria based on screening goals and population risk profiles. European programs often use lower thresholds, such as 10 µg/g, to enhance early detection, while some North American labs set higher cutoffs, around 20–50 µg/g, to reduce false positives and unnecessary colonoscopies.
Threshold selection impacts clinical outcomes. Lower cutoffs improve sensitivity, detecting more early-stage colorectal cancers and advanced adenomas, but increase follow-up procedures. Higher thresholds enhance specificity, reducing false positives but potentially missing early lesions. A study in The BMJ found that raising the cutoff from 10 to 20 µg/g reduced colonoscopies by 33% but slightly decreased cancer detection rates. Some laboratories use a tiered approach, retesting borderline results before recommending invasive procedures.
Factors That Can Influence Levels
Several factors can affect FIT results. The location of gastrointestinal bleeding is key—lesions in the colon and rectum are more likely to yield detectable hemoglobin, while upper digestive tract bleeding may go undetected due to enzymatic breakdown. This specificity makes FIT unsuitable for identifying upper gastrointestinal conditions.
Stool consistency can also impact accuracy. Liquid stools may dilute hemoglobin, increasing false negatives, while firmer samples retain more concentrated traces of blood. Bowel movement frequency plays a role as well—slower transit times may degrade hemoglobin, while frequent bowel movements may preserve it, influencing test sensitivity.