Tools
Fecal Occult Blood Test
Introduction
The fecal occult blood test (FOBT) is a diagnostic modality for detecting microscopic blood in stool, primarily employed in colorectal cancer (CRC) screening. CRC ranks among the most prevalent malignancies in men and women worldwide, and early detection significantly reduces associated morbidity and mortality.[1] The fecal immunochemical test (FIT), a newer method, utilizes antibodies to detect human hemoglobin and has largely supplanted FOBT due to superior specificity, enhanced sensitivity, and lower cost.[2]
Blood entering the upper gastrointestinal tract undergoes complete digestion of the globin portion of hemoglobin by proteolytic enzymes. Heme is converted by bacterial action into porphyrins. Hemoglobin reaching the lower intestine is largely undigested. In healthy individuals, gastrointestinal blood loss typically ranges from 0.5 to 1.5 mL per day, below the detection threshold of standard FOBTs.[3]
Detection methods are based on the identification of hemoglobin or its degradation products.[4] Fecal blood may also be identified via macroscopic examination for erythrocytes or hematin crystals or spectroscopic analysis of hemoglobin and its derivatives.[5]
Specimen Collection
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Specimen Collection
FOBT may be performed in both inpatient and outpatient settings. In the inpatient setting, stool may be obtained manually during digital rectal examination and applied to heme occult testing cards. In the outpatient setting, the patient typically collects a stool sample at home and submits it to a laboratory.[6]
Sampling should occur prior to contact with toilet water, as hemoglobin may leach from the specimen, and toilet sanitizers can interfere with results. Blood is not uniformly distributed within feces, and gastrointestinal bleeding may be intermittent. Therefore, samples should be collected from multiple areas and repeated on 3 separate days.[7]
Specimens should be protected from extreme heat and humidity. Dried samples may be stored at room temperature for up to 14 days.[8] Delays between collection and analysis may result in false-negative results due to degradation of the pseudoperoxidase activity of heme in moist feces. Collection directly onto filter paper in the test kit, followed by drying, prevents this degradation.[9]
Certain immunological methods employ collection devices containing liquid preservatives, allowing sample application by smearing feces onto the provided card.[10] Patient preparation is necessary for optimal results. Samples should not be collected in the presence of visible blood in stools or urine, including conditions such as menstruation, active hemorrhoids, and urinary tract infection.
Procedures
Verification of the FOBT card and developer for expiration before stool collection and testing is essential. Clinical laboratory tests commonly detect hemoglobin, heme, or heme-derived porphyrins.[11] Hemoglobin is identified via immunological methods, heme via guaiac-based methods exploiting the pseudoperoxidase activity of heme, and porphyrins via fluorimetric analysis.[12]
The predominant method for detecting fecal occult blood is based on heme detection. In these assays, the pseudoperoxidase activity of heme catalyzes the liberation of nascent oxygen from hydrogen peroxide.[13] The liberated oxygen subsequently oxidizes a chromogen. Historically employed chromogens include benzedrine and o-toluidine. Although sensitive, these compounds are carcinogenic and are no longer utilized in clinical laboratories. Alternative chromogens such as imipramine hydrochloride and desipramine hydrochloride have been used, with guaiac, a natural resin extracted from Guaiacum officinale, representing the most commonly employed chromogen in contemporary testing.[14] The sensitivity of the guaiac-based method is lower than that of o-toluidine. However, the incorporation of stabilizers has enhanced the sensitivity of this technique.[15]
These tests utilize a card containing high-quality filter paper impregnated with guaiac, which remains stable over extended periods as the guaiac is not in solution.[16] The developing solution consists of stabilized hydrogen peroxide in an aqueous alcoholic medium. Hemoglobin and its iron-containing degradation products catalyze the release of oxygen from hydrogen peroxide via pseudoperoxidase activity. The liberated oxygen subsequently oxidizes α-guaiaconic acid, a phenolic compound present in guaiac.[17] The reaction produces a quinone structure that rearranges to a blue dye through internal electron transfer.
Immunological methods exhibit higher specificity by employing antibodies targeting blood components, most commonly the globin chain of hemoglobin.[18] Hemoglobin forms a complex with a conjugate composed of a monoclonal or polyclonal antibody linked to a dye or enzyme, which generates a colored product from the substrate within the system.[19] Various immunochemical detection systems have been described, including enzyme immunoassays, hemagglutination, latex agglutination, and colloidal gold agglutination assays.[20]
Numerous commercial kits have been developed for the detection of blood using immunochemical methods, with some systems fully automated, resulting in increased reproducibility.[21] Immunochemical assays may be conducted in hospitals, at home, or in clinical laboratories.[22] These methods do not require dietary restrictions and exhibit greater sensitivity, with lower detection limits compared with guaiac-based methods. Automated systems further enhance reproducibility by eliminating the subjectivity associated with visual result interpretation. Immunochemical methods can detect as little as 0.3 mL of blood added to stool and demonstrate increased specificity for lower gastrointestinal tract bleeding, particularly from the colon, as globin from hemoglobin in the upper gastrointestinal tract is hydrolyzed by proteolytic enzymes.
One device employs a combination of immunological and guaiac-based approaches. Hemoglobin is immobilized by a monoclonal antibody and subsequently visualized using the guaiac-based reaction. Nonhemoglobin peroxidases may produce a background blue color, which is disregarded in result interpretation.[23]
The heme-porphyrin test is based on the conversion of heme in hemoglobin entering the gastrointestinal tract to porphyrins, likely mediated by gut bacteria. In this assay, porphyrins are extracted from feces and quantified using spectrofluorimetry.[24] This approach allows precise quantitation of hemoglobin entering the gastrointestinal tract and demonstrates superior sensitivity for detecting bleeding in the upper gastrointestinal tract.[25] Guaiac-based and immunological methods are less reliable for upper gastrointestinal tract bleeding, as hemoglobin may be degraded by proteolytic enzymes within the gut.[26]
A recently described method employs matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI-TOF-MS). In this technique, the fecal sample is mixed with water, ultrasonicated, and centrifuged. The resulting supernatant is combined with a matrix solution for MALDI-TOF-MS analysis.[27] Blood is identified by the detection of water-soluble α- and β-globin chains. This method is 10- to 100-fold more sensitive than conventional assays and is not affected by plant peroxidases or dietary red meat.[28] Although highly sensitive, with a lower detection limit of 0.01 mg/g feces, MALDI-TOF-MS has not been validated for routine screening, and its high cost limits practicality in most clinical laboratories.
Indications
The primary indications for FOBT include evaluation of anemia, assessment for gastrointestinal bleeding, and CRC screening. FOBT may also assist in distinguishing irritable bowel syndrome from inflammatory bowel disease, the latter being more likely to produce a positive result.
Potential Diagnosis
Occult fecal blood may arise from multiple etiologies. Neoplastic causes include adenocarcinoma, gastrointestinal metastases, lymphoma, and leiomyosarcoma.[29] Inflammatory sources encompass Crohn disease, ulcerative colitis, gastritis, peptic ulcer disease, and diverticular bleeding.[30] Vascular pathologies include angiodysplasia, venous ectasia, variceal bleeding, hemangioma, gastric antral vascular ectasia, and Dieulafoy lesions.[31] Infectious etiologies comprise Salmonella species, enteroinvasive and enterohemorrhagic Escherichia coli, Shigella species, Neisseria species, Yersinia species, Mycobacterium tuberculosis, Campylobacter species, and Strongyloides infections.[32]
Normal and Critical Findings
An FOBT card that does not develop a blue color is interpreted as negative. A blue color change indicates a positive result, necessitating further gastroenterological evaluation.
Normal fecal blood loss can reach up to 1.5 mL per day, as measured using radiochromium (51Cr-labeled red cells) techniques.[33] In patients with benign adenomas or colorectal carcinoma, bleeding is often microscopic but may exceed 1.5 mL per day.[34] Bleeding may be intermittent. Therefore, distinguishing normal from pathological blood loss solely by fecal quantification is not always feasible.[35] Additionally, the volume of blood loss from tumors may vary daily, and hemoglobin and its degradation products are distributed nonuniformly throughout the feces.[36][37]
Peptic ulcers, hemorrhoids, and diverticula also contribute to fecal occult blood loss, with variable bleeding intensity. Among available methods, heme-porphyrin testing demonstrates superior sensitivity for detecting small volumes of upper gastrointestinal tract bleeding, such as that induced by aspirin.[38]
FOBT is employed to detect gastrointestinal bleeding and is most commonly utilized as a screening modality for CRC, one of the most prevalent malignancies worldwide. Numerous studies have evaluated the efficacy of FOBT for early detection of CRC.
FOBT is also a critical diagnostic tool in patients with iron-deficiency anemia to exclude gastrointestinal bleeding as the underlying cause. Selection of the appropriate test is essential since bleeding from any segment of the gastrointestinal tract can induce iron deficiency.[39] Guaiac-based and immunological methods are less sensitive for detecting upper gastrointestinal tract bleeding. Heme-porphyrin testing demonstrates superior sensitivity in this context, detecting approximately 90% of upper gastrointestinal tract bleeding and identifying blood loss as low as 5 mL per day.[40]
Interfering Factors
FOBT requires medication and dietary restrictions prior to testing to reduce the risk of false-negative and false-positive results.[41] Numerous studies have assessed factors contributing to these erroneous results. A retrospective study evaluated medications associated with false-positive outcomes and recommended avoiding these agents, when possible, for 7 days before testing.[42] Medications identified include acetylsalicylic acid, unfractionated or low-molecular-weight heparin, warfarin, clopidogrel, nonsteroidal anti-inflammatory drugs, and selective serotonin reuptake inhibitors. The study reported that 10.9% of patients with positive FOBT results and no dietary or medication restrictions prior to testing demonstrated normal findings on follow-up endoscopic evaluation.[43]
Dietary avoidance for 3 days before FOBT further mitigates the risk of false results. Improper sample collection in patients with hematuria or menstruation may also generate false-positive results.
Guaiac-based methods are susceptible to interference from multiple sources, including plant peroxidases. Raw fruits and vegetables, such as turnips, broccoli, horseradish, cauliflower, cantaloupe, parsnip, and red radish, contain high concentrations of peroxidases.[44] These enzymes are heme proteins with a prosthetic group of ferri-protoporphyrin IX (hemin) and can induce false-positive results in guaiac-based tests. Cooking vegetables at 100 °C for 20 minutes has been shown to inactivate plant peroxidase activity.[45]
Gastric acid denatures peroxidases. Therefore, ingestion of raw vegetables is unlikely to affect test results in individuals with normal gastric acid secretion.[46] Consumption of 750 g of raw, peroxidase-rich fruits and vegetables daily can lead to false-positive results, though this amount of raw produce is unusually large for daily intake.[47] Delaying slide development by 48 hours can reduce interference from plant peroxidases. However, many manufacturers of guaiac-based tests recommend avoiding high-peroxidase-containing fruits and vegetables before and during sample collection.
Ingestion of red meat can produce false-positive results in FOBT due to the peroxidase activity of heme.[48] The false-positive rate is increased when rehydration is performed before analysis. Residual peroxidase activity may persist even after cooking. Studies in healthy volunteers indicate that approximately 3 days are required for the risk of false-positive results due to meat consumption to subside.[49] Although current recommendations advise avoiding red meat for at least 3 days before testing, the duration required to eliminate the risk may vary depending on clinical circumstances and bowel habits. However, some studies have reported no significant effect of red meat on occult blood testing.[50]
A meta-analysis concluded that dietary restriction may not be necessary for guaiac-based FOBTs.[51] Ascorbic acid (vitamin C) can interfere with the oxidation of α-guaiaconic acid due to its reducing properties. Ingestion of 1 to 2 grams daily of this nutrient may produce false-negative results.[52] In vitro studies suggest that normal dietary vitamin C intake is unlikely to generate false-positive results.[53] A recent study demonstrated that consumption of 60 mg of vitamin C or 500 mL of orange juice (approximately 350 mg of vitamin C) generated variable results, with high-dose supplementation resulting in false-negative outcomes.[54]
Use of povidone-iodine antiseptic solutions is associated with false-positive results in guaiac-based FOBT.[55] In vitro studies demonstrated that as little as 0.005 mL of a 1:1000 dilution of this solution can yield a false-positive result. This interference is attributable to iodine, which oxidizes α-guaiaconic acid.[56] Application of povidone-iodine to the perianal area or, during urinary catheterization, the urinary tract should be avoided before performing guaiac-based FOBT.
Medications such as aspirin, nonsteroidal anti-inflammatory drugs (eg, ibuprofen, naproxen), corticosteroids, phenylbutazone, and cancer chemotherapeutic agents, as well as excessive alcohol consumption, can generate positive reactions secondary to gastric irritation and subsequent blood loss. Recent studies indicate that low-dose aspirin does not significantly affect FOBT outcomes.[57] Research shows no significant difference in FOBT positivity between patients on anti-inflammatory medications like aspirin and those not on such drugs, implying that stopping these agents before testing may not be necessary.[58]
Toilet sanitizers may also interfere with FOBT. Chlorine-generating sanitizers produce false-positive results in guaiac-based assays.[59] Nonchlorine-generating sanitizers can reduce the sensitivity of immunological detection of hemoglobin.[60]
Complications
FOBT is not associated with any major complications. Adverse effects are limited to transient discomfort related to stool handling or collection procedures.
Patient Safety and Education
FOBT results are primarily influenced by patient preparation, making adherence to instructions essential. Certain foods can interfere with test outcomes. Therefore, a restricted diet is often recommended for 48 to 72 hours before testing. Additionally, medications such as anticoagulants, aspirin, colchicine, nonsteroidal anti-inflammatory drugs, iron preparations, and corticosteroids should be avoided for at least 7 days before testing.
Clinical Significance
In the U.S., CRC is the 3rd leading cause of cancer-related deaths in men and the 4th in women. When both sexes are considered together, the condition becomes the 2nd most common cause of cancer mortality. The disease affects all populations, irrespective of race, ethnicity, gender, or socioeconomic status.[61]
CRC is most frequently diagnosed in adults aged 65 to 74. Inadequate screening contributes to delayed diagnosis and treatment.[62] FOBT constitutes 1 of several validated methods for CRC screening in asymptomatic patients. This diagnostic tool facilitates early detection by identifying individuals who require follow-up evaluation with procedures such as colonoscopy. FOBT is not indicated for high-risk or symptomatic patients who should receive prompt referral to a gastroenterologist for further evaluation and management.
Improper or indiscriminate use of FOBT can lead to unnecessary diagnostic procedures, increased healthcare costs, and prolonged hospitalizations. Consequently, FOBT should be performed only when clinically indicated, and multiple professional organizations emphasize the importance of educating healthcare providers regarding appropriate indications.
The U.S. Preventive Services Task Force concludes with high certainty that CRC screening in adults aged 50 to 75 provides a substantial net benefit, whereas screening in adults aged 45 to 49 provides a moderate net benefit.[63] The U.S. Preventive Services Task Force further concludes with moderate certainty that screening in adults aged 76 to 85 who have undergone prior screening offers a small net benefit. These assessments of net benefit apply to stool-based tests with high sensitivity, colonoscopy, computed tomography colonography, and flexible sigmoidoscopy.[64]
Current guidelines from the American College of Gastroenterology recommend initiating CRC screening at age 45 for average-risk individuals.[65] In patients with a 1st-degree relative diagnosed with advanced adenomas or CRC before age 60, screening should begin at age 40 or 10 years before the age at diagnosis of the youngest affected relative, whichever is earlier. For patients with 1st-degree relatives diagnosed after reaching age 60, screening may begin at the standard age of 50.[66]
Recommendations also favor replacing guaiac-based FOBT with FIT due to superior sensitivity and specificity.[67] FIT targets human globin, predominantly derived from lower gastrointestinal bleeding, and demonstrates improved CRC detection compared with FOBT. FIT does not require dietary modifications, thereby enhancing patient adherence.[68]
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