Occult blood is blood that’s hidden in a body fluid or tissue, invisible to the naked eye. The term “occult” simply means hidden. You can’t see it when you look at your stool or urine, but lab tests can detect it. Most often, the term comes up in the context of stool testing, where a fecal occult blood test checks for tiny amounts of blood that signal bleeding somewhere in the digestive tract.
Why Blood Can Be Hidden
When bleeding in the digestive tract is heavy, you’ll notice it: bright red blood in the toilet or black, tarry stools. But many conditions cause slow, low-volume bleeding that produces no visible change in stool color or consistency. A small polyp in the colon, for example, might ooze just enough blood to be chemically detectable without ever being noticeable. This is what makes occult blood testing valuable. It catches problems you wouldn’t otherwise know about.
Common Causes of Occult Blood in Stool
A positive test doesn’t automatically mean something serious. Several conditions can cause small amounts of hidden bleeding in the gastrointestinal tract:
- Colon polyps and colorectal cancer: Growths in the colon or rectum often bleed intermittently. Tumors develop their own blood supply, and as they grow, that tissue bleeds. This is the primary reason screening programs use occult blood tests.
- Stomach or duodenal ulcers: Open sores in the stomach lining or the first section of the small intestine can bleed slowly over weeks or months.
- Inflammatory bowel disease: Conditions like Crohn’s disease and ulcerative colitis cause chronic inflammation that damages the intestinal lining.
- Hemorrhoids and anal fissures: These are among the most common (and least dangerous) causes of a positive result.
- Esophageal or gastric varices: Enlarged veins in the esophagus or stomach, often related to liver disease, can leak blood.
Over time, even small amounts of chronic bleeding can deplete your iron stores. The principal concern with unexplained iron deficiency anemia in adults is that it may result from blood loss caused by an underlying gastrointestinal growth. When doctors find iron deficiency anemia without an obvious explanation, they often order an occult blood test as a first step.
How Stool Tests Detect Occult Blood
Two main types of stool tests are used, and they work in fundamentally different ways.
Guaiac-Based Test (gFOBT)
This older method detects the heme component of hemoglobin, the oxygen-carrying molecule in red blood cells. When a chemical developer is applied to a stool sample on a treated card, heme triggers a reaction that turns the card blue. The test typically requires three separate stool samples collected over several days, because bleeding from polyps and other sources can be intermittent.
The downside is that heme isn’t unique to human blood. Red meat, certain raw vegetables (broccoli, turnips, radishes, horseradish), and cantaloupe can all trigger a false positive. On the flip side, high doses of vitamin C (more than 250 milligrams per day) can cause a false negative by interfering with the chemical reaction. Aspirin, ibuprofen, and naproxen can also affect results. You’ll typically need to avoid these foods and medications for a few days before collecting samples.
Fecal Immunochemical Test (FIT)
FIT uses antibodies that specifically target the globin portion of human hemoglobin. Because it’s looking for a human-specific protein rather than a chemical found in many foods, it doesn’t require any dietary restrictions. It also needs fewer stool samples, often just one. FIT has largely replaced the guaiac test in most screening programs because of its convenience and accuracy.
How Accurate These Tests Are
FIT is particularly good at catching more advanced colorectal cancers. Research published in the journal Gastroenterology found that FIT detects stage II colorectal cancers with 92% sensitivity and stage IV cancers with 89% sensitivity. For earlier-stage cancers (stage I), sensitivity drops to about 68%, meaning roughly one in three early-stage cancers can be missed on a single test. This is one reason annual testing is recommended: what one year’s test misses, next year’s test may catch while the cancer is still treatable.
Neither test is designed to be a one-time diagnostic tool. Their strength lies in repeated use over years, which increases the cumulative chance of catching a problem early.
Screening Recommendations
The U.S. Preventive Services Task Force recommends colorectal cancer screening for all adults aged 45 to 75. Stool-based testing is one of several accepted strategies. The recommended schedule is a high-sensitivity gFOBT or FIT every year, or a newer combination test (stool DNA-FIT) every one to three years. A colonoscopy on a longer interval is the other main option.
Stool-based tests are often preferred as an initial screening method because they’re noninvasive, done at home, and inexpensive. You receive a kit with collection devices (usually a brush or applicator), a test card or tube, and a prepaid envelope to mail the sample to a lab. Samples should be kept at room temperature and returned as quickly as possible.
What Happens After a Positive Result
A positive occult blood test is not a diagnosis. It means blood was detected and the source needs to be identified. The standard next step is a colonoscopy, which allows a doctor to visually examine the entire colon and rectum. If polyps are found during the procedure, they can usually be removed on the spot. If the colonoscopy can’t be completed for any reason, a combination of sigmoidoscopy and imaging of the upper colon is an alternative.
Many people with a positive occult blood test turn out to have a benign cause, like hemorrhoids or a small area of inflammation. But the test exists because catching the serious causes early, particularly colorectal cancer and precancerous polyps, dramatically improves outcomes.
Occult Blood in Urine
The term “occult blood” also applies to urine, where it’s called microscopic hematuria: red blood cells present in the urine that you can’t see but that show up on a urinalysis. This is a common incidental finding, sometimes discovered during a routine checkup.
Causes range from minor to serious. Bladder or kidney infections, urinary tract stones, vigorous exercise, sexual activity, and an enlarged prostate can all produce microscopic blood in urine. More concerning causes include bladder or kidney cancer, kidney disease affecting the filtering units (glomeruli), blood-clotting disorders, and sickle cell disease. In menstruating women, contamination from menstrual blood can produce a false positive, so a repeat test after the period ends is standard.
When microscopic hematuria is confirmed, follow-up testing depends on the suspected cause. It may include blood tests to check kidney function, imaging (ultrasound, CT scan, or MRI) of the urinary tract, or cystoscopy, a procedure where a thin camera is guided through the urethra to examine the bladder lining directly.