What is occult blood

Occult blood means that you can’t see it with the naked eye. Occult gastrointestinal bleeding may occur anywhere in the gastrointestinal tract, from the oral cavity to the anorectum ¹. A review of multiple studies has shown that lesions in the upper gastrointestinal tract and small bowel are often the cause of iron deficiency anemia ².

Blood in the stool means there is likely some kind of bleeding in the digestive tract. It may be caused by a variety of conditions, including:

• Polyps
• Hemorrhoids
• Diverticulosis
• Ulcers
• Colitis, a type of inflammatory bowel disease

Blood in the stool may also be a sign of colorectal cancer, a type of cancer that starts in the colon or rectum. Colorectal cancer is the second leading cause of cancer-related deaths in the United States and the third most common cancer in men and in women. A fecal occult blood test is a screening test that may help find colorectal cancer early, when treatment is most effective.

In a review of five prospective studies of upper endoscopy and colonoscopy in patients with occult gastrointestinal bleeding, 20 to 30 percent of patients had a colorectal source, whereas 29 to 56 percent had an upper gastrointestinal tract source ³. No source was found in 29 to 52 percent of patients ³. Synchronous lesions, or simultaneous sources of occult bleeding in the upper gastrointestinal tract and the colon, were found in 1 to 17 percent of patients ⁴. Colonic lesions include colon cancer, colon polyps, vascular ectasias, and colitis. Causes of occult bleeding in the upper gastrointestinal tract include esophagitis, Cameron ulcers (a linear erosion in a hiatal hernia), gastric and duodenal ulcers, vascular ectasias, gastric cancer, and gastric antral vascular ectasia (Table 1). A small bowel source is likely in a high percentage of patients with recurrent bleeding and negative findings on esophagogastroduodenoscopy and colonoscopy.

In patients younger than 40 years, small bowel tumors are the most common cause of occult gastrointestinal bleeding; other causes include celiac disease ⁵ and Crohn disease ⁶. In patients older than 40 years, vascular ectasias and nonsteroidal anti-inflammatory drug–induced ulcers are the most common causes. However, celiac disease remains a possible cause in symptomatic ⁷ and asymptomatic ⁸ adults older than 50 years. Less common causes of occult gastrointestinal bleeding include infections (e.g., hookworm) and long-distance running. Researchers hypothesize that long-distance running induces gastrointestinal blood loss because of transient intestinal ischemia from decreased splanchnic perfusion during exercise ⁹.

Table 1. Prospective Studies of Patients with Occult Gastrointestinal Bleeding

Findings on esophagogastroduodenoscopy and colonoscopy		Frequency (%)
Colorectal source		20 to 30
	Angiodysplasia	1 to 9
	Colitis	1 to 2
	Colon cancer	5 to 11
	Polyps (adenomas)	5 to 14
Upper source		29 to 56
	Angiodysplasia	1 to 8
	Celiac disease	0 to 6
	Duodenal ulcer	1 to 11
	Esophagitis	6 to 18
	Gastric cancer	1 to 4
	Gastric ulcer	4 to 6
	Gastritis	3 to 16
Synchronous lesions (lesions found in both upper gastrointestinal tract and colon)		1 to 17
No source found		29 to 52

[Source ³ ]

Potential Causes of Occult Gastrointestinal Bleeding ³:

Mass lesions

• Carcinoma (any site) (most common).
• Large adenoma (> 1.5 cm)

Inflammation

• Ulcer (any site)
• Erosive esophagitis (most common).
• Cameron ulcers (linear erosions within a hiatal hernia)
• Celiac disease
• Colitis (nonspecific)
• Crohn disease
• Erosive gastritis
• Idiopathic cecal ulcer
• Ulcerative colitis

Vascular

• Vascular ectasias (any site) (most common).
• Gastric antral vascular ectasia
• Hemangioma
• Portal hypertensive gastropathy or colopathy

Infection

• Amebiasis
• Ascariasis
• Hookworm
• Strongyloidiasis
• Tuberculous enterocolitis
• Whipworm

Other

• Munchausen syndrome
• Long-distance running

Stool occult blood test

Fecal Occult Blood Test is designed to evaluate stool samples for hidden (“occult”) blood, meaning blood that cannot be seen with the naked eye. Although there are several possible causes of blood in the stool, one important cause is the presence of polyps or cancers in the digestive tract.

Colon polyps are common as people age, but most polyps do not cause any health problems and are benign. However, a benign polyp can turn into a cancerous polyp and the cancer may even spread to other parts of the body (metastasize). If detected early, colon cancer can be successfully treated. It is therefore important to determine whether pre-cancerous or cancerous polyps are present.

Polyps are finger-like growths that protrude into the cavity (lumen) of the colon or the rectum. They can be fragile and bleed intermittently, such as when food waste brushes against them. This blood is mixed in with the stool and when the amounts are small, the blood can only be detected by tests for occult blood. This small amount of blood may be the first and sometimes the only sign of polyps or early colon cancer, making the stool-based tests valuable screening tools.

There are two principal methods for detecting occult blood in the stool. They are designed to detect hemoglobin, a molecule that is present in red blood cells. Hemoglobin has two essential parts: a chemical part called heme and a protein part called globin. Each method tests for a different part of the hemoglobin molecule.

1. The guaiac-based tests (gFOBT, Fecal Occult Blood Test) measure the heme (non-protein) part of hemoglobin from blood in the stool. Since the heme part of hemoglobin is common to blood from all sources, this method identifies the presence of blood from any source. This means that Fecal Occult Blood Tests will measure not only your blood but also blood from any dietary source, such as red meat, thereby causing false-positive test results. Compared to the Fecal Immunochemical Test (FIT), these methods have other limitations. Some foods and medications can interfere with this method, so you may be instructed to avoid them for a few days prior to testing. Since the heme part of hemoglobin is resistant to degradation in the intestine, these tests guaiac-based tests (gFOBT) also detect blood from other areas of the digestive tract, such as from bleeding stomach ulcers and bleeding gums. They are therefore less specific than the Fecal Immunochemical Test for the detection of bleeding from the colon.
2. The Immunochemical method (FIT) tests for the globin (protein) part of hemoglobin and Fecal Immunochemical Tests are designed to specifically detect human globin. Fecal Immunochemical Test generally will not detect globin from non-human blood such as found in beef and other meats, thereby making them superior to other tests. Hemoglobin from bleeding in the upper digestive tract is broken down before it reaches the lower digestive tract and so upper gastrointestinal bleeds are not usually detected by the Fecal Immunochemical Test. However, most health organizations, including the U.S. Multi-Society Task Force on Colorectal Cancer and the American Cancer Society recommend this test for screening for colon cancer.

Both Fecal Immunochemical Test and Fecal Occult Blood Test can be performed in people with common hemoglobin disorders (hemoglobinopathies), such as sickle cell disease.

How is the sample collected for testing?

Typically, you will be supplied with a kit to use at home. The collection method may vary based on the specific manufacturer of the kit. Follow the instructions that are provided with the kit. Multiple stool samples may have to be collected on different days and after the last sample is taken, all of the samples are brought to or mailed to your healthcare practitioner or laboratory.

• For Fecal Immunochemical Test (FIT), a common approach is to use a brush or other device to collect a sample from the surface of a stool, which is then inserted into a sample tube containing a solution and sent for testing. Generally, a single sample is required.
• For guaiac-based Fecal Occult Blood Test (gFOBT), the healthcare practitioner or laboratory will provide a test card that can have 1, 2 or 3 sections. The triple card is most commonly used. Stool should be collected into clean containers and should not be contaminated with urine or water. Using an applicator stick, a sample is collected from the surface of the stool and placed onto the specially treated pad on the test card and allowed to dry. When using cards with multiple test areas, the samples for each test area are collected on different days. For example, a 3-section card requires 3 stools, each collected on a different day. Collecting and testing multiple stool samples increases the chance of detecting cancer if it is present.

Is any test preparation needed to ensure the quality of the sample?

For the Fecal Immunochemical Test, there are no particular restrictions prior to testing. The test uses antibodies to detect only human blood from the lower digestive tract (colon).

For guaiac Fecal Occult Blood Tests (gFOBT), there are various restrictions:

• These tests detect the heme part of any blood that may be in the digestive tract. Therefore, steps should be taken to avoid introducing blood from any source other than your own into the digestive tract for a few days prior to the test. This includes avoiding various meats and especially red meat.
• Blood that arises from bleeding gums (caused by dental procedures or gum disease) may be detected by these tests. Avoid having any dental procedures up to three days before beginning to collect stool samples.
• Stomach bleeds such as caused by ulcers should be controlled prior to testing.
• Bleeding in the stomach may be triggered by the use of non-steroidal anti-inflammatory drugs (NSAIDs) such as aspirin, naproxen, and ibuprofen. Avoid taking these drugs for seven days prior to testing, if clinically possible.
• The test relies on a chemical reaction to produce the color change that gives a positive test. Foods such as broccoli, turnips, cauliflower and apples, and drugs such as colchicine may make the test appear positive even in the absence of blood (a false-positive result). If instructed to do so, avoid these foods and drugs three days prior to and during the testing period.
• Vitamin C, on the other hand, interferes with the chemical reaction and prevents the color formation that should occur when blood is present (a false-negative result). Vitamin C supplements and fruit juices that contain vitamin C should be avoided three days prior to and during testing.

Follow the instructions provided by your healthcare practitioner or included in test kit. Check with your healthcare provider before stopping any drugs to be certain that it is safe to do so.

How is the fecal occult blood test used?

The main use of tests for occult blood in the stool (Fecal Immunochemical Test and Fecal Occult Blood Test) is to screen for colon polyps or early colon cancer in people with average risk. Blood in the stool may be the only sign of early colon cancer. If detected early, treatment can begin immediately, improving the chance of a cure. Persons with average risk include those without a family history of colon cancer and those without certain genetic diseases.

The Fecal Immunochemical Test (FIT) is preferred over the guaiac Fecal Occult Blood Test because it is better at detecting cancer and it doesn’t require dietary restrictions before testing, according to the U.S. Multi-Society Task Force on Colorectal Cancer.

For people with an increased or high risk of colon cancer, a colonoscopy is usually recommended for screening because it is the most accurate and thorough screen available.

A secondary use of occult blood testing is to help determine if a person’s anemia is due to blood loss in the gastrointestinal tract (GI tract), such as from a bleeding ulcer. If someone has signs and symptoms of anemia, such as fatigue, a low hemoglobin and hematocrit level, and/or unusually dark stools, a healthcare practitioner may order the Fecal Occult Blood Test. As noted above, the Fecal Occult Blood Test can detect blood from any part of the gastrointestinal tract, while the Fecal Immunochemical Test is more reliable in cases of bleeding from the lower part of the gastrointestinal tract.

When is the fecal occult blood test ordered?

The American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer and U.S. Preventive Services Task Force recommend yearly testing when you choose occult blood testing (Fecal Immunochemical Test or Fecal Occult Blood Test) as the method of screening for colon cancer. The ACS advises that people of average risk begin screening at age 45. The U.S. Multi-Society Task Force on Colorectal Cancer and U.S. Preventive Services Task Force advise that screening for colon cancer begin at age 50.

A healthcare practitioner may sometimes order an Fecal Occult Blood Test when someone has unexplained anemia that might be caused by bleeding in the digestive tract.

What does the fecal occult blood test result mean?

Fecal occult blood test negative

A negative fecal occult blood test means no blood was detected in the stool at the time of the test.

Fecal occult blood test positive

• For the Fecal Immunochemical Test (FIT), a positive result indicates abnormal bleeding in the lower digestive tract. While this bleeding could be caused by colon cancer, other possible causes include ulcers, polyps or hemorrhoids.
• For the guaiac-based Fecal Occult Blood Test (gFOBT), a positive test result indicates that abnormal bleeding may be occurring somewhere in the digestive tract. This blood loss could be due colon cancer or to cancerous tumors. It could also be due to benign polyps, to ulcers, diverticulosis, inflammatory bowel disease, hemorrhoids or to blood swallowed due to bleeding gums or nosebleeds.

A positive result from either method requires follow-up testing. This usually involves direct imaging of the colon and rectum. A colonoscopy is typically recommended because it allows examination of the colon and the removal of any precancerous polyps and/or cancerous areas that are found.

Bleeding, especially from polyps and tumors, is intermittent, so blood is not uniformly distributed in all stool samples. Taking multiple samples on different days increases the chance of detecting bleeding that is intermittent. This is especially true for Fecal Occult Blood Test. The Fecal Immunochemical Test (FIT) is more sensitive, so a single sample is usually considered acceptable.

An occult blood test (Fecal Immunochemical Test or Fecal Occult Blood Test) can give a false-negative result if the cancer or polyps do not bleed during the time the sample is taken. In the absence of polyps or colon cancer, a false-positive result may be obtained if you have bleeding from other sources, such as hemorrhoids, ulcers and inflammatory bowel disease.

What kind of procedures might follow a positive fecal occult blood?

Multiple diagnostic procedures are available to investigate the gastrointestinal tract in patients with occult bleeding. The choice and sequence of procedures will depend on clinical suspicion and any associated symptoms. Upper gastrointestinal bleeding (identified as the source of bleeding proximal to the ampulla of Vater) can be detected by esophagogastroduodenoscopy. Proximal small bowel bleeding can be detected with push enteroscopy, which reaches the proximal jejunum. Bleeding of the mid and distal small bowel can be detected with capsule endoscopy, deep enteroscopy, and computed tomographic (CT) enterography. Lower gastrointestinal bleeding (colonic bleeding) can be detected with colonoscopy ¹⁰. Intraoperative enteroscopy remains an option for those rare patients who have recurrent bleeding from a source not yet identified with the previously mentioned methods. Small bowel barium studies have a very low yield and have been largely replaced by capsule endoscopy.

Esophagogastroduodenoscopy and colonoscopy will find the bleeding source in 48 to 71 percent of patients ⁴. In patients with recurrent bleeding, repeat esophagogastroduodenoscopy and colonoscopy may find missed lesions in 35 percent of those who had negative initial findings ⁴. If a cause is not found after esophagogastroduodenoscopy and colonoscopy have been performed, capsule endoscopy has a diagnostic yield of 63 to 74 percent ¹⁰.

Other than stool occult blood tests, are there other ways of screening for colon cancer?

Yes. There are imaging tests that may be used for the detection of precancerous polyps and colon cancer. One of these procedures may be chosen instead of screening annually with a fecal occult blood test:

• Colonoscopy is recommended by most health organizations as the preferred method of screening for colon cancer. It involves a thorough examination of the rectum and entire colon using a flexible tube. If polyps or potentially cancerous areas are found, they may be removed during the procedure and examined by a pathologist to see if cancer is present. It is recommended every 10 years for average-risk individuals. Colonoscopy however requires significant preparation, is invasive, and is much costlier than tests for occult blood in the stool. In individuals with abnormal results on screening tests such as Fecal Occult Blood Test, colonoscopy is then used as a diagnostic test for colon cancer.
• Sigmoidoscopy is an examination of the rectum and lower colon with a lighted instrument. It also allows for the removal of any polyps. If this is the chosen method of screening, once every 5 to 10 years is recommended.
• CT colonoscopy (virtual colonoscopy) is a less invasive procedure that uses computed tomography (a CT scan) to visualize the entire colon. The recommended screening interval is 5 years.
• Capsule colonoscopy is a procedure which uses a vitamin-sized capsule that is swallowed and contains a wireless camera that transmits pictures as it travels through the digestive tract. Not enough evidence is available on this test yet and it is not widely available. If chosen for screening, it should be done every 5 years.

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