What can cause a positive occult blood test?

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]

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.

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. 

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]

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.

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.

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

1.

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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]

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]

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]

Dalle I, Geboes K. Vascular lesions of the gastrointestinal tract. Acta Gastroenterol Belg. 2002 Oct-Dec;65(4):213-9. [PubMed: 12619428]

Papaconstantinou HT, Thomas JS. Bacterial colitis. Clin Colon Rectal Surg. 2007 Feb;20(1):18-27. [PMC free article: PMC2780149] [PubMed: 20011357]

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]