Test Quick Guide

The fecal occult blood test (FOBT) checks for blood hidden in the stool. An 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. It 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 (blood invisible in stool), also 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 (cancer of the colon or rectum). The FOBT may improve the 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 the disease if you already have symptoms. FOBT may be used to diagnose 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. Guaiac, a chemical substance, 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 hydrogen peroxide is added to the samples. If blood is present in the stool, a chemical reaction occurs when it mixes with the hydrogen peroxide 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. FIT has largely replaced FOBT for large-scale, population-based colorectal cancer screening.

When should I get this test?

Annual FOBTs are one type of test that can be used for colorectal cancer screening. The U.S. Preventive Services Task Force (USPSTF) recommends screening for colorectal cancer beginning at age 45.

This testing may begin earlier if you are at higher risk of developing colorectal cancer, including if you have a family history of the disease or inflammatory bowel conditions.

Depending on your symptoms, an FOBT may be used to diagnose various health conditions. For example, if you see blood in your stool, your doctor may order a FOBT. Detecting gastrointestinal bleeding may help determine the cause of anemia (for example, unexplained iron deficiency anemia). FOBT can also identify 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 can I get a fecal occult blood test?

The most common method for FOBT testing is to use an at-home kit. Your doctor may prescribe and mail an 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. But this is not the preferred or standard way to obtain a stool sample.

Can I take the test at home?

You can take an FOBT 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 before obtaining your FOBT kit.

At-home FOBT kits also range in price, depending on the lab you use to analyze your samples.

Taking a Fecal Occult Blood Test

The FOBT requires a stool sample collected on a 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

FOBT depends on the peroxidase activity of hemoglobin, and is therefore not specific for human blood. 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 before your test. However, it’s important to talk with your doctor before stopping NSAIDs or other medications if you have certain medical conditions.
  • Excess vitamin C: You typically must avoid consuming more than 250 milligrams (mg) of vitamin C per day for seven days before 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 not to 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 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:

  1. Prepare the card by writing your name, the date, and other information needed for mailing the card back.
  2. Flush the toilet and allow the bowl to refill. Then place the tissue paper provided in the kit over the toilet bowl.
  3. 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 marked on the card.
  4. Flush the remainder of the stool and discard the stick.
  5. 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.
  6. Place the test card in the envelope provided and seal the envelope. Mail the test card to your doctor’s office or 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 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 three to five 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 negative, positive, or inconclusive. This is determined by whether the test paper changes color when exposed to specific chemicals.

A negative result means no blood was detected in the stool samples, and a positive result indicates blood was detected. Inconclusive means there was a problem with the sample.

While FOBT is used for the 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:

  • Polyps
  • Ulcers
  • Inflammatory bowel disease including colitis
  • Hemorrhoids
  • Diverticulosis

Because many conditions can lead to a positive FOBT, further testing is frequently necessary to diagnose a specific underlying cause.

Talk with your doctor to best understand the significance of your FOBT test result. The doctor can tell you if the test was positive, negative, or inconclusive and address whether any follow-up testing is appropriate. Some questions to ask your doctor about your test result include:

  • What do my results indicate about my health?
  • What next steps should I take based on my results?
  • Do you recommend any follow-up testing?


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