Occult blood is blood that’s present in a body fluid, usually stool or urine, but in amounts too small to see with the naked eye. The word “occult” simply means hidden. In stool, this typically means less than 50 milligrams of hemoglobin per gram, an amount that won’t change the color or appearance of a bowel movement but can be picked up by a lab test. If you’ve received a test result mentioning occult blood, it means the test detected trace amounts of blood that you wouldn’t have noticed on your own.
Why Hidden Blood Matters
Many conditions that cause bleeding inside the body start slowly. A small polyp in the colon, an irritated stomach lining, or a tiny ulcer may leak just enough blood to be chemically detectable long before you’d ever see red in the toilet or feel symptoms. That’s exactly what makes testing for occult blood useful: it can catch problems early, particularly colorectal cancer and precancerous growths called adenomas, when treatment is most effective.
Over time, even small amounts of chronic bleeding can lead to iron deficiency anemia. You might feel unusually tired, look pale, or get short of breath during activities that didn’t used to wind you. These symptoms sometimes prompt a doctor to order an occult blood test in the first place.
Occult Blood in Stool
Stool is the most common place occult blood is tested for, usually as part of routine colorectal cancer screening. The U.S. Preventive Services Task Force recommends screening for adults ages 45 to 75. Two main types of stool tests exist, and they work differently.
The guaiac-based fecal occult blood test (gFOBT) detects blood by reacting to heme, the iron-containing part of hemoglobin. Because heme stays relatively stable as it travels through the entire digestive tract, this test can pick up bleeding from anywhere: the esophagus, stomach, small intestine, or colon. The downside is that heme isn’t unique to human blood. Eating a large amount of red meat in the days before the test can trigger a false positive. Most gFOBT instructions ask you to avoid red meat for two to three days beforehand. Older guidelines also restricted certain raw fruits and vegetables (like turnips and horseradish), but current evidence shows those restrictions are unnecessary for the most commonly used versions of the test.
The fecal immunochemical test (FIT) takes a more targeted approach. It uses antibodies to detect human globin, the protein portion of hemoglobin. Because digestive enzymes break down globin as it passes through the upper gastrointestinal tract, FIT is more specific to bleeding in the lower gut, particularly the colon and rectum. It also won’t react to animal blood from food, so no dietary restrictions are needed. Both tests are done once a year.
What a Positive Result Actually Means
A positive occult blood test does not mean you have cancer. It means blood was detected, and the next step is finding out where it’s coming from. In one screening study of nearly 1,400 average-risk adults, 6.3% tested positive on FIT. Among those who went on to have a colonoscopy, about 17.5% had colorectal cancer, roughly 51% had advanced adenomas (precancerous polyps), and about 26% had simple, non-advanced adenomas. So the majority of positive results pointed to polyps that could be removed before they ever became cancerous.
The list of possible causes is long and ranges from completely benign to serious. Common sources include colon polyps, peptic ulcers, gastritis (inflammation of the stomach lining), esophagitis, inflammatory bowel disease, and hemorrhoids. Less common causes include small bowel tumors, swallowed blood from a nosebleed, and even coughing up small amounts of blood that gets swallowed. Non-cancerous causes are far more frequent than cancerous ones.
How Accurate Is the Test?
FIT catches roughly 73% of stage I colorectal cancers and around 80% to 82% of more advanced stages. Its specificity, the ability to correctly return a negative result when cancer isn’t present, runs between 87% and 95% depending on the study. Where the test is weakest is in detecting precancerous adenomas: sensitivity for advanced adenomas is only about 16% to 34%. This is why annual testing matters. A polyp that doesn’t bleed enough to trigger one year’s test may be caught the next.
Occult Blood in Urine
The term also applies to urine. Microscopic hematuria means red blood cells show up under a microscope (three or more per high-power field) even though your urine looks completely normal in color. A urine dipstick can flag this, though a positive dipstick without actual red blood cells under the microscope may point to other substances like myoglobin from muscle breakdown rather than true blood.
About half the time, doctors can identify a specific cause for microscopic hematuria. Common culprits include urinary tract infections, kidney stones, an enlarged prostate, bladder or kidney cancer, and kidney inflammation (glomerulonephritis). Strenuous exercise can also temporarily cause blood in the urine. In the other half of cases, no definite cause is found, and the finding is monitored over time.
What Happens After a Positive Stool Test
If your fecal occult blood test comes back positive, your doctor will typically recommend a colonoscopy. This is the standard follow-up because it lets the doctor directly examine the lining of the colon, identify the source of bleeding, and in many cases remove polyps during the same procedure. The goal is straightforward: find what’s bleeding and determine whether it needs treatment. A positive screening test is a starting point for investigation, not a diagnosis on its own.