A fecal occult blood test (FOBT) uses a chemical process to measure blood that is hidden (occult) in a patient’s stool. This blood is often present in such small amounts that it can only be detected through chemical testing. The fecal blood test is safe and painless.

Blood in the stool may indicate a number of conditions, but the FOBT is used primarily to detect the presence of growths called polyps in the colon or rectum, or cancer. These conditions cause bleeding, as do other disorders such as inflammatory bowel diseases (including colitis), ulcers and hemorrhoids.

The American Cancer Society (ACS), World Health Organization (WHO) and U.S. Preventive Services Task Force all recommend that people over the age of 50 have an annual FOBT as a part of a routine colorectal exam. In some cases, this test will be performed during the exam in the physician’s office. More often, patients are given a kit to obtain their own sample, which is then returned to the physician or mailed to a laboratory. If blood is discovered during laboratory analysis, additional tests will be needed to locate the source of the bleeding. Such tests will focus on the colon, rectum and upper intestinal tract (including the esophagus, stomach and small bowel).

Despite the recommendation, a 2004 study found that 43 percent of Americans aged 50 and older failed to have a fecal occult blood test or lower endoscopy in the previous year, according to the U.S. Centers for Disease Control and Prevention (CDC). Many patients chose not to have the FOBT because of the nature of the test. However, when the FOBT is performed every one to two years in patients ages 50 to 80, it reduces the number of deaths from colorectal cancer by up to 30 percent, according to the National Cancer Institute.  

A fecal occult blood test (FOBT) is an analysis of a person’s stool that can reveal blood that is hidden (occult) in feces. It is typically performed by having the patient obtain stool samples at home and sending them into a laboratory for analysis. The FOBT is used to detect polyps or cancer in the colon or rectum. A polyp is a noncancerous (nonmalignant) growth that protrudes from the inner wall of the colon or rectum. It can develop into cancer if left untreated.

Blood vessels in the surface of colorectal polyps, adenomas and cancers are often fragile. The passage of feces through the colon and rectum frequently damages these vessels, releasing blood into the feces. However, it is rare for the bleeding to be so severe that the blood becomes visible in stool.

FOBT detects this hidden (occult) blood through a chemical reaction, but the test cannot determine where the blood originated. If blood is detected, further tests will be necessary to identify the source of the bleeding. In addition to polyps and cancer, the detected bleeding can occur from anywhere in the body - from the mouth to the anus - and can stem from conditions such as:

• Ulcers
  
• Hemorrhoids. Masses or clumps of tissues within the anal canal.
  
• Diverticulosis. Tiny pouches that form at weak spots in the colon wall.
  
• Inflammatory bowel disease (colitis)
  
• Crohn’s disease and ulcerative colitis. Chronic inflammatory diseases of the intestines.

When this test is performed every one to two years in patients ages 50 to 80, it reduces the number of deaths from colorectal cancer by up to 30 percent, according to the National Cancer Institute (NCI). The American Cancer Society (ACS) and other organizations recommend that people over the age of 50 have an annual FOBT as a part of a routine colorectal examination. However, compliance rates generally do not match that recommendation. The U.S. Centers for Disease Control and Prevention reports that in 2004, 57 percent of Americans aged 50 or older reported having had a fecal occult blood test or lower endoscopy within the previous year.


Although the screening rates for colorectal cancer are beginning to increase, the CDC estimates that 41.8 million people aged 50 and older have not been screened according to recommended guidelines.

Like all cancer tests, fecal occult blood tests have advantages and disadvantages. These include:

A fecal occult blood test (FOBT) is usually performed by the patient at home. A physician will give the patient a kit that explains the necessary steps. Kits can also be purchased at a pharmacy without a prescription. Cancers and other abnormalities bleed intermittently, so most patients are asked to take stool samples on three consecutive days.

There are many types of sample kits available. It is important that individuals carefully read the instructions enclosed with the kit to ensure accurate results. Most tests require the patient to obtain the sample and store it in a supplied container, or to use an applicator stick to apply the sample on a chemically treated card. These samples are then returned either in person or by mail in a special envelope to a laboratory or physician’s office. To ensure accuracy, these samples should be handled and stored as instructed until they are returned or mailed to the lab.

In some cases, the FOBT may be performed in a physician’s office. In these cases, the patient may provide a sample as part of a colorectal examination. More often, the test is completed through a home kit.

A fecal immunochemical test (FIT) is a newer type of stool blood test kit that detects a specific portion of a human blood protein. The test is performed in the same manner as an FOBT, but the fact that it is more specific reduces the number of false positives. Some patients may find this easier to use than traditional FOBT, as only two samples are necessary and vitamins and foods do not affect the test. However, as with traditional testing, FIT cannot detect tumors that are not bleeding.

Patients should consult their physician about any preparatory steps that may be necessary to ensure the success of their fecal occult blood test (FOBT). Various foods, medications and supplements can cause false positive or negative results during testing.

For example, aspirin use can cause inflammation of the stomach (gastritis) and subsequent bleeding, which could then lead to a positive FOBT even though the source of the blood is not due to a tumor. Conversely, taking a vitamin C supplement could suppress the mechanism used in the test and create a false negative result. In this case, cancer would be present, but not detected by the FOBT.

Patients should follow their physician’s advice, and should never stop taking any medication without their physician’s approval. As a general rule, patients should avoid the following for three days (seven days in the case of NSAIDs) prior to testing:

• Nonsteroidal anti-inflammatory drugs (NSAIDs), including ibuprofen and aspirin (no more than one aspirin a day)
  
  
• Anticoagulants (which reduce the blood's ability to clot)
  
  
• Vitamin C from supplements or citrus fruits in excess of 250 milligrams daily
  
  
• Red meat (beef or lamb)
  
  
• Raw fruits and vegetables, especially melons, radishes, turnips and horseradish

In addition, the test should not be completed if the patient has certain medical conditions, such as bleeding hemorrhoids. With women, the test should not be done during menstruation.

After a sample reaches the laboratory, a chemical (usually guaiac) is applied. If the chemical comes into contact with blood, it will appear as a different color.  Once the sample is analyzed, the physician will contact the patient with the results.

If the test results are negative, no blood was found in the sample. A positive result indicates that blood was found. Patients should remember that several factors – including foods and medicines consumed before the test – may skew the accuracy of test results.

Positive test results will lead to further testing to determine the cause of the bleeding. Tests that may be performed include:

• Colonoscopy. Examination of the entire colon with a long tube and camera that is inserted into a patient’s rectum.
  
  
• CT colonography (virtual colonoscopy). Uses computed tomography to produce detailed pictures of the colon. The test is noninvasive but is not as sensitive as a colonoscopy and does not allow physicians to collect biopsy samples. Nevertheless, it may be the preferred test for some patients with gastrointestinal conditions.
  
  
• Flexible sigmoidoscopy. Similar to a colonoscopy, this procedure uses a shorter tube to investigate the lower parts of the colon (sigmoid colon and descending colon). In some cases, this test will be performed in conjunction with the original FOBT.
  
  
• Double contrast barium enema (DCBE). A chalky substance is used to open and fill the colon. Air is then pumped in to expand the colon, and x-rays are taken.
  

In some cases, other tests may be performed that allow the physician to look inside esophagus, stomach and duodenum, which are potential sources of bleeding. These include:

• Upper endoscopy
• Upper gastrointestinal x-ray
• Small bowel x-ray

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their condition. Patients may wish to ask their doctors the following questions about fecal occult blood test (FOBT):

1. When should I have my first FOBT?
   
   
2. Should I stop taking any of my regular medications?
   
   
3. Do I need any preparation before I obtain my sample?
   
   
4. Will I need to conduct the test at home or will it be done in your office?
   
   
5. If it is done at home, where will I obtain the kit?
   
   
6. How many samples will I need to take?
   
   
7. When will I receive the test results and from whom?
   
   
8. What will follow if my test is positive?
   
   
9. What are the chances of a false-positive or false-negative test result?
   
   
10. How will you determine the cause of my bleeding?
    
    
11. How often will I need to conduct FOBTs?
    
    
12. If I have a colonoscopy, do I still need to conduct FOBTs?