Does this test have other names?
gFOBT, stool occult blood
What is this test?
A fecal occult blood test checks a bowel movement [stool] sample for blood that can't be seen with the naked eye. Blood in the stool is a sign of bleeding in the digestive tract. This could indicate cancer, polyps, hemorrhoids, diverticulosis, or inflammatory bowel disease.
Why do I need this test?
You may need this test because the American Cancer Society recommends that all men and women at average risk for colon cancer start screening tests at age 45. One screening test option is a fecal occult blood test every year. This simple test can help find colon or rectal cancer. Your healthcare provider may give you the test.
What other tests might I have along with this test?
If the fecal occult blood test shows blood in the stool, your healthcare provider will likely recommend having a colonoscopy to find out the source and nature of the bleeding.
What do my test results mean?
Test results may vary depending on your age, gender, health history, the method used for the test, and other things. Your test results may not mean you have a problem. Ask your healthcare provider what your test results mean for you.
A fecal occult blood test uses chemicals to find out if there is blood in a sample of feces. If your test is negative, your result is normal.
If your test is positive, you had blood from your digestive tract in your stool sample. Additional testing, such as a colonoscopy, can help find out the location, cause, and extent of the bleeding.
How is this test done?
Your healthcare provider gives you a kit to help you collect and prepare bowel movement samples for the fecal occult blood test. You often need to collect samples from more than 1 bowel movement–typically 3 in a row. You then mail the samples or take them back to the provider or a lab. Follow the directions your provider gave you and that come with the kit.
Collecting and preparing the samples typically follows these steps:
Collect 1 of your stools in a dry container. Don't allow urine to mix with it.
Use a wooden applicator to put a small smear of stool [from the outside of the stool] on the card or slide you have been given.
Seal the sample and write your name and date on it.
Flush the unused stool down the toilet.
Repeat this process for the next 2 stools, or as instructed.
Does this test pose any risks?
This test poses no known risks.
What might affect my test results?
A positive result on a fecal occult blood test doesn't mean you have cancer. Other health conditions, such as ulcers or hemorrhoids, more commonly can cause a positive test result. Healthcare providers will do more tests find the cause.
Eating certain food can also affect the test results, even though the fecal occult blood test only detects human blood. Your healthcare provider may ask you to avoid certain foods a few days before the test to lower the chances of getting a false-positive result.
How do I get ready for this test?
Don't take NSAIDs [nonsteroidal anti-inflammatory drugs], such as ibuprofen or naproxen, or aspirin for 7 days before the test. Acetaminophen is safe to use. If you take aspirin daily to prevent heart disease, talk with your healthcare provider before you stop taking this medicine.
Don't take vitamin C supplements or drink or eat juice or fruits high in vitamin C for 7 days before the test. Vitamin C can cause a false-negative test result.
Don't eat red meat, such as beef, lamb, pork, and liver, for 3 days before testing. It's unlikely, but these foods could change your test result. In some cases, healthcare providers don't put any limits on food because this may make people less likely to do the test. Having the test is the most important thing.
Let your healthcare provider know about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.
Introduction
The fecal occult blood test [FOBT] is a diagnostic test to assess for occult blood in the stool. This test has commonly been used for colorectal cancer screening, especially in developed nations. Colon cancer is one of the most prevalent cancers in both men and women worldwide, therefore, early detection is imperative.[1] When used correctly for screening, this testing modality has established associations with decreased morbidity and mortality. Newer screening methods, including the fecal immunochemical test [FIT], have been developed. FIT uses antibodies to discern blood in the stool. These newer modalities have replaced the FOBT for colon cancer screening due to increased specificity, sensitivity, and decreased costs.[2]
Specimen Collection
Before stool collection and testing, it is imperative to ensure that the FOBT card and developer are not beyond their expiration dates. This test is performed in either the inpatient or outpatient setting. In the inpatient setting, the stool is often obtained manually during a digital rectal examination and placed onto heme occult testing cards. In the outpatient setting, the patient typically obtains a stool sample at home and then submits it to a laboratory. Laboratory selection is in conjunction with a healthcare provider, and they provide collection materials. When performing at home, the stool should be collected in a dry, clean container. An applicator stick is then used to apply a small amount of stool to the inside of the testing card, typically in a box labeled "A." The applicator stick should then be used to obtain a second sample from a different part of the stool, which is also placed inside of the testing card, typically in a box labeled "B." The testing card should then be stored at room temperature, away from heat and light, until transport to the appropriate laboratory.
The chemistry behind testing involves a catalyzed reaction. The heme occult testing card has alpha guaiaconic acid [guaiac] impregnated paper. A hydrogen peroxide reagent is then added to the paper. If heme is present in the stool sample, the alpha guaiaconic acid is oxidized by the hydrogen peroxide to a blue-colored quinone.[3] The blue color would signify a positive test result.
Indications
The most common indications for fecal occult blood tests are anemia, concern for gastrointestinal bleeding, and colon cancer screening. It can also be used to help discriminate irritable bowel syndrome [IBS] from inflammatory bowel disease [IBD], which is likely to yield a positive test result.
Potential Diagnosis
Occult fecal blood can be present secondary to several etiologies, some of which will be mentioned here. Neoplastic causes include adenocarcinoma, gastrointestinal metastasis, lymphoma, and leiomyosarcoma. Inflammatory causes include Crohn disease, ulcerative colitis, gastritis, peptic ulcer disease, and diverticular bleeding.[4] Vascular causes include angiodysplasia, venous ectasia, variceal bleeding, hemangioma, gastric antral vascular ectasia, and Dieulafoy's lesion.[5] Infectious causes include Salmonella, enteroinvasive and enterohemorrhagic Escherichia coli, Shigella, Neisseria, Yersinia, tuberculosis, Campylobacter, and Strongyloides.[6]
Normal and Critical Findings
If the patient's fecal occult blood test does not turn blue, it is negative. If the card turns blue, this is positive and requires further gastroenterological workup.
Interfering Factors
One problem with FOBT is the need for medication and dietary restrictions before testing. These restrictions are in order to decrease the risk of false negative and false-positive results. Many studies assessing the risk of these false results exist. One particular retrospective study evaluated the medications that could create false-positive results and encouraged patients to avoid these medications, if possible, for seven days before testing. The listed medications include acetylsalicylic acid, unfractionated or low-molecular-weight heparin, warfarin, clopidogrel, nonsteroidal anti-inflammatory drugs, and selective serotonin reuptake inhibitors. The study found that 10.9% of the patients with positive fecal occult blood testing and no dietary or medication restriction beforehand had normal follow up endoscopic evaluations.[3]
Three days prior to fecal occult blood testing, avoidance of certain foods should be to help prevent false test results. False-positive results have been associated with red or rare meat as well as raw fruits and vegetables, including but not limited to horseradish, raw turnips, cantaloupe, broccoli, cauliflower, parsnips, and red radishes. False-negative results are also known to occur in patients taking ascorbic acid [vitamin C] in excess of 250mg/day.
Inappropriate collection in patients with hematuria or menses may also result in false positive test results.
Clinical Significance
Colon cancer is the third most commonly diagnosed cancer worldwide.[2] It occurs in all populations regardless of race, ethnicity, gender, or socioeconomic status. The absence of appropriate screening leads to the delay of both diagnosis and treatment. Fecal occult blood testing is one of many methods used for colon cancer screening, and its use is valid in asymptomatic patients. It helps improve the detection of early-stage cancers by guiding patient selection for follow-up tests such as colonoscopies. FOBT is not necessary for high-risk or symptomatic patients, and these patients should promptly obtain a referral to a gastroenterologist for further workup and management.
When misused or administered incorrectly, fecal occult blood testing has resulted in unnecessary testing, increased healthcare costs, and prolonged hospital stays. Therefore, it should only be performed when indicated. Many organizations focus on educating healthcare providers about these indications. The United States Preventative Services Task Force recommends colorectal cancer screening using fecal occult blood testing, sigmoidoscopy, or colonoscopy for individuals starting at age 50 to age 75. The American College of Gastroenterology recommends that screening begins at the age of 45 in African Americans. In patients with first-degree relatives with advanced adenomas or colorectal carcinoma diagnosed prior to the age of 60, screening should begin at the age of 40 or 10 years earlier than the youngest diagnosed relative. In patients with first-degree relatives diagnosed with advanced adenomas or colorectal cancer after the age of 60, screening can begin at the usual age of 50.[7] Recommendations also exist that the fecal immunochemical test [FIT] replace the older guaiac-based fecal occult testing due to increased sensitivity and specificity. FIT targets human globin often found with lower gastrointestinal bleeding, and it has been shown to improve detection rates for colorectal cancer when compared to FOBT. It also does not necessitate any dietary modifications; therefore patient adherence has improved.[3]
Further information regarding colorectal screening and follow-up testing is beyond the scope of this article.
Review Questions
References
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2.Schreuders EH, Ruco A, Rabeneck L, Schoen RE, Sung JJ, Young GP, Kuipers EJ. Colorectal cancer screening: a global overview of existing programmes. Gut. 2015 Oct;64[10]:1637-49. [PubMed: 26041752]
3.Narula N, Ulic D, Al-Dabbagh R, Ibrahim A, Mansour M, Balion C, Marshall JK. Fecal occult blood testing as a diagnostic test in symptomatic patients is not useful: a retrospective chart review. Can J Gastroenterol Hepatol. 2014 Sep;28[8]:421-6. [PMC free article: PMC4210232] [PubMed: 25014182]
4.Fu Y, Wang L, Xie C, Zou K, Tu L, Yan W, Hou X. Comparison of non-invasive biomarkers faecal BAFF, calprotectin and FOBT in discriminating IBS from IBD and evaluation of intestinal inflammation. Sci Rep. 2017 Jun 01;7[1]:2669. [PMC free article: PMC5453945] [PubMed: 28572616]
5.Dalle I, Geboes K. Vascular lesions of the gastrointestinal tract. Acta Gastroenterol Belg. 2002 Oct-Dec;65[4]:213-9. [PubMed: 12619428]
6.Papaconstantinou HT, Thomas JS. Bacterial colitis. Clin Colon Rectal Surg. 2007 Feb;20[1]:18-27. [PMC free article: PMC2780149] [PubMed: 20011357]
7.Rex DK, Boland CR, Dominitz JA, Giardiello FM, Johnson DA, Kaltenbach T, Levin TR, Lieberman D, Robertson DJ. Colorectal Cancer Screening: Recommendations for Physicians and Patients from the U.S. Multi-Society Task Force on Colorectal Cancer. Am J Gastroenterol. 2017 Jul;112[7]:1016-1030. [PubMed: 28555630]