Fecal Occult Blood Test
- Also Known As:
- Guaiac FOBT
- Stool Occult Blood Test
- Hemoccult Test
- Guaiac Smear Test
Test Quick Guide
The fecal occult blood test (FOBT) checks for blood hidden in the stool. The FOBT frequently includes an at-home sample collection kit and requires collecting two to three separate stool samples.
The FOBT is used as a tool for early detection of colorectal cancer. The FOBT can also detect bleeding in the gastrointestinal tract caused by other conditions. If blood is found in the stool, follow-up tests are typically needed to determine its cause.
About the Test
Purpose of the test
The purpose of the FOBT is to check for blood in the stool that cannot be seen with the naked eye. This can also be called occult blood.
The FOBT is an important test that can help look for cancer, precancerous conditions, or other medical problems. It may be used for health screening or diagnosis.
Screening refers to checking for disease when no signs or symptoms are present. Occult blood in the stool can be a sign of colorectal cancer, which is cancer of the colon or rectum. The FOBT may improve early detection of precancerous or cancerous growths. Correct use of the FOBT in cancer screening has been associated with lower disease and death rates.
Diagnosis is testing to determine the cause of disease after a patient already has symptoms. FOBT may be used as part of the diagnosis of conditions like anemia, gastrointestinal bleeding, and inflammatory bowel disease (IBD).
What does the test measure?
The FOBT measures whether there is any blood in a stool sample using a chemical reaction.
The way that the test detects blood is by looking for signs of a chemical reaction. A chemical substance called guaiac is infused into a small paper card that is part of the FOBT at-home kit. After samples are collected and smeared onto the card, they are sent to a lab for testing.
During testing, a second chemical called peroxidase is added to the samples. If blood is present in the stool, a chemical reaction occurs when it mixes with the peroxidase and guaiac. In this way, the test can identify whether there is occult blood in the stool.
Fecal immunochemical testing (FIT) is another method of looking for traces of blood in the stool. This alternative method is described in detail on our Fecal Immunochemical Testing page.
When should I get a fecal occult blood test?
Annual fecal occult blood testing is one type of test that can be used for colorectal cancer screening. The United States Preventive Services Task Force (USPSTF) recommends screening for colorectal cancer beginning at age 45. This testing may begin earlier for people who are at higher risk of developing colorectal cancer, including people with a family history of the disease or who have inflammatory bowel conditions.
Depending on a person’s symptoms, a fecal occult blood test may be used for diagnosis of various health conditions. For example, detecting gastrointestinal bleeding may help determine a cause of anemia. FOBT can also be involved in identifying several types of health issues affecting the gastrointestinal system.
Whether for screening or diagnosis, a doctor is in the best position to determine if FOBT or any other test is appropriate in your specific case.
Finding a Fecal Occult Blood Test
How to get tested
The most common method for FOBT testing is to use an at-home kit. Your doctor may prescribe and mail a FOBT kit directly to you. Alternatively, you can purchase a kit online or over-the-counter without a prescription.
In some cases, a stool sample for FOBT may be taken during a rectal examination in a hospital or doctor’s office. However, this is not the preferred or standard way to obtain a stool sample.
Can I take the test at home?
FOBT samples are typically collected at home over three consecutive days. A test kit includes the materials for collecting a stool sample, including a test card. After samples are collected, the test card is mailed to a lab for processing.
How much does the test cost?
The cost of an FOBT kit will depend on whether you have medical insurance that covers this test. You may be responsible for some out-of-pocket costs such as copays or deductibles. Some types of insurance cover the full cost of FOBT testing. You can check with your doctor or insurance company prior to obtaining your FOBT kit.
Taking a Fecal Occult Blood Test
The FOBT requires a stool sample that is collected on special test paper. In most cases, the sample is self-collected at home, but in some situations it may be obtained by a doctor during a rectal exam.
Before the test
Certain foods and drugs may affect the results of your FOBT. It’s important to follow instructions carefully and consult your doctor if you have questions or concerns. Your doctor may recommend you avoid the following before the test:
- Nonsteroidal, anti-inflammatory drugs (NSAIDs): Drugs such as ibuprofen, naproxen, or aspirin may need to be avoided for seven days prior to your test. However, if you have certain medical conditions, it’s important to talk with your doctor before stopping NSAIDs or other medications.
- Excess vitamin C: You typically need to avoid consuming more than 250 milligrams (mg) of vitamin C per day for seven days leading up to your test. This includes total vitamin C intake from supplements and other sources.
- Red or rare meat: It is normally necessary to avoid rare or red meat for three days prior to the test because traces of blood in these meats can cause false positive results.
- Some other foods: The doctor may advise you to not eat certain foods for several days. These may include certain raw fruits and vegetables.
Ask your doctor for specific pretest instructions and follow them closely in order to receive the most dependable test result.
During the test
For most at-home stool collections for FOBT, you follow a series of steps to obtain the test sample:
- Prepare the card by writing your name, the date, and other information needed for mailing the card back.
- Flush the toilet and allow the bowl to refill. Then place tissue paper provided in the kit over the toilet bowl.
- During a bowel movement, allow stool to drop onto the paper. Then use one of the sticks provided in the kit to poke into the stool and take a small sample. Smear the stool onto the area clearly marked on the card.
- Flush the remainder of the stool and discard the stick.
- Repeat these steps two more times over the next two days. When completed you should have three separate stool samples collected over three consecutive days.
- Place the test card in the envelope provided and seal the envelope. Mail the test card to your doctor’s office or to the lab as soon as possible to help ensure your sample does not degrade over time.
Follow the instructions carefully as you prepare for the test, collect your samples, and mail your card to the lab. Contact your doctor if you have any questions about properly taking the FOBT test.
After the test
There are no known risks involved in using an at-home FOBT kit. After your samples have been collected, there are no further requirements or restrictions. You can resume your usual diet, medications, and activities.
Fecal Occult Blood Test Results
Receiving test results
The turnaround time for lab test results is typically within a few business days depending on the lab where you mailed your FOBT test card. If you obtained your FOBT through your doctor’s office, you can ask about the standard turnaround time for test results. You may also be able to access your medical records more quickly if you have access to an online medical chart.
Interpreting test results
Your FOBT lab results will be listed as negative, positive, or inconclusive. This is determined based on whether the test paper changes color when exposed to specific chemicals.
A negative result means no blood was detected in the stool samples. A positive result means blood was detected. Inconclusive means there was a problem with the sample.
While FOBT is used for early detection of colorectal cancer, a positive result does not mean that you have cancer. FOBT can detect gastrointestinal bleeding caused by many other medical conditions. Some examples of other medical issues that can cause a positive FOBT result include:
- Inflammatory bowel disease including colitis
Because many conditions can lead to a positive FOBT, further testing is frequently necessary to diagnose a specific underlying cause.
In order to best understand the significance of your FOBT test result, you should talk with your doctor. The doctor can tell you if the test was positive, negative, or inconclusive and address whether any follow-up testing is appropriate.
Are test results accurate?
The FOBT can detect blood in the stool and has been found to aid in the early detection of colorectal cancer. However, it is not a perfect test. Sometimes samples will have false positive or false negative results.
False positive results occur when results are positive but the person does not really have the disease or condition. For FOBT, an example of a false positive is when traces of blood from red meat are found in the stool.
False negative results occur when results are negative but the person actually has the disease or condition. For example, false negative FOBT results are known to occur in patients taking over 250 milligrams per day of vitamin C. Additionally, because bleeding can occur intermittently, testing over three days may not identify all people who have blood in their stool.
It is essential to follow all of the instructions before and during testing in order to enhance the accuracy of the test. You can talk to your doctor about any questions you have about the accuracy of your specific test result.
Do I need follow-up tests?
If blood is found in your stool, additional testing may be recommended in order to determine the cause and location of the bleeding.
The most common follow-up test to the FOBT is a colonoscopy, which is an exam to view the rectum and entire length of the colon. This test allows the doctor to view the inside of the colon and remove any suspicious areas or growths with small surgical tools.
Other follow-up tests may also be appropriate depending on your specific situation, including any symptoms you have or results from other medical tests.
Comparing and contrasting a FOBT and a fecal immunochemical test (FIT)
Both the FOBT and the FIT are stool-based tests that can be used for colorectal cancer screening. Based on current guidelines, either one may be used annually to improve early detection of colorectal cancer.
Both tests can be performed at home and require obtaining a stool sample. However, there are some key differences between these tests.
While the FOBT requires some dietary and medication restrictions, there are typically no restrictions with the FIT test. Another key difference is the FOBT requires three separate stool samples collected over three consecutive days. The FIT requires just one stool sample.
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