How the Fecal Occult Blood Test Works
Overview
A fecal occult blood test (FOBT) checks for hidden blood in stool that you can’t see with your eyes. This test can help find early signs of problems like colorectal cancer, polyps, or digestive tract issues before symptoms appear. Many doctors recommend this test because it is easy to do at home and serves as an important step in staying healthy.
People might not realize they have bleeding inside their digestive system, so the FOBT helps catch problems early. By understanding what this test is and why it matters, anyone can take a smart step toward protecting their health and preventing serious illness.
What Is a Fecal Occult Blood Test?
A fecal occult blood test (FOBT) checks for tiny amounts of hidden blood in stool. This test helps find possible problems in different parts of the digestive system, such as the colon or rectum.
Purpose of FOBT
Doctors use a fecal occult blood test to look for blood in stool that cannot be seen without a microscope. Hidden blood can be an early sign of colon cancer, polyps, or ulcers. Screening with this test helps catch diseases before symptoms appear. Adults at risk for colon cancer often need to do the test every year.
If blood appears, it does not always mean cancer—a variety of conditions, including hemorrhoids or minor injuries, can also cause blood in stool. A positive result usually prompts doctors to order more testing, like a colonoscopy, to find out where the blood is coming from.
Types of FOBT
There are two main types of fecal occult blood tests. The first, called the guaiac-based FOBT (gFOBT), uses a special card coated with a chemical. Patients smear a small sample of stool on the card, and the chemical checks for hidden blood. The second type is the fecal immunochemical test (FIT). FIT uses antibodies to find human blood in the stool and usually responds only to blood from the lower intestines.
Both tests are simple, can be done at home, and do not require a doctor’s visit to collect the sample. Some tests may require avoiding certain foods or medicines before collecting the sample, especially the guaiac-based tests. Labs process the results, and if blood is found, doctors may advise follow-up tests.
How the Fecal Occult Blood Test Works
The fecal occult blood test (FOBT) helps find hidden blood in stool that cannot be seen with the naked eye. Doctors use this to check for problems in the digestive system, such as bleeding.
There are two main types of FOBT: guaiac-based FOBT (gFOBT) and fecal immunochemical test (FIT). Both tests look for tiny amounts of blood but use different methods.
- Guaiac-based FOBT (gFOBT): Uses a chemical called guaiac to detect blood. Patients collect stool samples and apply them to a test card, such as the Hemoccult card. When a developer solution is added, the test area changes color if blood is present.
- Fecal Immunochemical Test (FIT): Uses antibodies to find blood in the stool. Patients collect a small sample using a collection device. The sample is then tested in a lab.
Table: Comparison of FOBT Methods
| Test Type | What it Detects | How it Works | Sample Needed |
|---|---|---|---|
| gFOBT (Guaiac) | Hidden blood | Guaiac + developer | Stool on card |
| FIT | Hidden blood | Antibody reaction | Stool in tube |
Usually, people collect samples at home. Doctors recommend avoiding certain foods and medicines before the gFOBT because they might affect results. With FIT, these restrictions are less common.
When and Why FOBT Is Used
The Fecal Occult Blood Test (FOBT) is a common tool for checking hidden blood in stool. Doctors often use it to screen for colorectal cancer and detect early signs of problems in the digestive tract. FOBT is usually recommended:
- For adults over 45 or 50 years old.
- As a yearly screening for colon cancer.
- For people with an average risk but no symptoms.
Some reasons for using FOBT include:
- Screening for colorectal cancer.
- Finding polyps before they turn into cancer.
- Checking for bleeding from ulcers or other digestive issues.
- Helping understand unexplained stomach pain or changes in bowel habits.
A positive FOBT result does not mean someone has cancer. It shows there may be bleeding from somewhere in the digestive tract.
| Possible Reasons for Blood in Stool |
|---|
| Colorectal cancer |
| Polyps |
| Ulcers |
| Hemorrhoids |
| Inflammatory bowel disease |
If the test finds blood, doctors usually order more tests to locate the bleeding and find the cause. FOBT is noninvasive and can often be done at home. This makes it a useful first step in checking for problems such as early colon cancer.
FOBT Procedure and Sample Collection
The fecal occult blood test (FOBT) requires careful steps to make sure the results are accurate. The process involves proper preparation, correct use of applicator sticks, and safe handling of stool samples.
Preparing for the Test
Before beginning the FOBT, people may need to avoid certain foods and medications. Foods like red meat, some raw vegetables, and vitamin C supplements can affect results. It is important to follow any instructions from a healthcare provider on what to avoid before testing.
Clean hands and a clean toilet are helpful for safe sample collection. Toilet bowl cleaners, chemicals, or colored water should not be used during testing because they can change the sample or impact results.
Women should not collect stool during menstruation, and anyone should avoid collecting samples during active bleeding from hemorrhoids or urinary blood. Write the name, date, and other requested information on the collection card before starting. This helps the lab process the sample correctly.
Using Applicator Sticks
Applicator sticks help take a small amount of stool and put it on the collection card. The stool should not touch toilet water. Some people put a piece of clean paper or plastic wrap in the toilet to catch the stool before it touches water. Use the stick to collect a tiny sample from the stool’s surface. Smear the sample onto the space on the collection card, usually in thin layers.
Repeat this process if the instructions ask for samples from more than one part of the stool or from different stools on separate days. Use each applicator stick only once and throw it away. Wash hands after handling the applicator and before touching anything else.
Handling Stool Samples
After placing stool samples on the collection card, close the card tightly to keep the samples safe. Store the card away from heat, sunlight, and chemicals to protect the test. Keep the collection card in a clean, dry place before taking it to the lab or sending it in. Most tests ask for samples on three different days to increase accuracy. Label each sample with the correct date.
Never collect samples from stools that have touched toilet water or have been mixed with urine. Follow specific directions from a healthcare provider, as procedures or storage times can vary. Sending samples to the lab quickly helps make sure results are reliable.
Factors Affecting FOBT Accuracy
Several factors can change how accurate a fecal occult blood test (FOBT) is. The test’s sensitivity—how well it finds tiny amounts of blood—may be higher or lower depending on these factors.
- Diet can have a strong effect. Some foods, like red meat or certain vegetables, can create false positives. Eating large amounts of vitamin C may cause false negatives, meaning blood is present but not detected by the test.
- Medications are also important. Drugs such as aspirin and other anti-inflammatory medicines might increase the chance of a false positive by causing bleeding in the digestive tract.
- Sample collection is crucial. Proper technique is key since a single, poorly collected sample may not show hidden blood. It is important to follow instructions for collecting samples exactly.
- Quality control in labs matters. Incorrect storage, delayed processing, or using expired test kits can reduce test reliability.
The following table lists some common factors and their effects:
| Factor | Possible Effect on FOBT |
|---|---|
| Diet (red meat) | False positive |
| High vitamin C intake | False negative |
| Aspirin/NSAIDs | False positive |
| Poor sample collection | Missed detection |
| Poor quality control | Unreliable results |
Doctors review all these factors when interpreting results. This helps avoid errors and ensures a more accurate screening for hidden blood in stool.
Comparing FOBT With Other Screening Methods
Fecal occult blood testing (FOBT) detects hidden blood in the stool, which can be a sign of colorectal cancer or other conditions. While FOBT is simple and noninvasive, newer and more sensitive methods, such as the fecal immunochemical test (FIT) and colonoscopy, offer key differences in accuracy, convenience, and use.
FOBT vs. FIT
The traditional guaiac-based FOBT (gFOBT) checks for the presence of blood in stool using a chemical reaction. In contrast, the fecal immunochemical test (FIT) directly looks for human hemoglobin using antibodies. FIT is often more sensitive and specific, as it mainly detects blood from the lower digestive tract.
FIT does not require special dietary restrictions, while gFOBT may give false positives if the person consumes red meat or certain vegetables before the test. FIT is also easier to use, which can improve test completion rates among patients.
Many guidelines now prefer FIT over gFOBT for routine screening because it is better at finding colorectal cancer and advanced adenomas. Both tests are usually repeated every year.
| Feature | gFOBT | FIT |
|---|---|---|
| Type of detection | Chemical | Antibody |
| Diet restrictions | Yes | No |
| Sensitivity for cancer | Moderate | Higher |
| Convenience | Medium | High |
| Frequency | Yearly | Yearly |
FOBT vs. Colonoscopy
Doctors directly examine the entire colon with a flexible camera during a colonoscopy. Unlike FOBT, which only detects potential bleeding, colonoscopy allows physicians to find and remove growths such as polyps during the procedure.
Doctors typically recommend colonoscopy every ten years if results are normal, but patients must prepare their bowels, receive sedation, and visit a clinic. Patients can do FOBT at home with simple sample collection, and the test is much less invasive. FOBT can miss some early cancers or polyps because it only looks for bleeding.
Colonoscopy is much more sensitive and can catch cancers or growths before symptoms start. However, colonoscopy carries some risks, including bleeding, infection, and a small chance of injury to the colon. Patients who cannot have colonoscopy or do not want an invasive test may use FOBT or FIT as an alternative.
Limitations and Considerations of FOBT
Fecal occult blood tests (FOBT) have some important limitations. They only show if there is blood in the stool, not what is causing the bleeding. FOBT does not have perfect sensitivity. It may miss some cancers or polyps, especially if the bleeding is not constant or is too small to detect.
Table: Common Limitations of FOBT
| Limitation | Details |
|---|---|
| Sensitivity | Can miss some cases of cancer or polyps |
| False Positives | Non-cancer causes like hemorrhoids or certain foods can cause a positive result |
| Quality Control | How samples are collected and stored can affect results |
Certain foods, supplements, and medications can affect the results. For example, iron supplements or some pain relievers might lead to a false positive result. There are two main types of FOBT: guaiac-based (gFOBT) and immunochemical (FIT). The FIT test does not usually require special diet or medication changes, while the gFOBT does.
Accurate results depend on proper quality control. Incorrect handling or testing errors can produce unreliable outcomes. If someone gets a positive FOBT, doctors will recommend follow-up tests, like a colonoscopy, to find out why there is blood in the stool.
