What is Fecal Occult Blood Testing, and When Should You Have This Done?

Colorectal cancer is an important public health issue. While death rates from colorectal cancer in older adults have been decreasing over the past several decades, it remains the second leading cause of cancer death in the United States when men and women are combined. 

Improved screening initiatives have contributed to these declining mortality rates; however, nearly 30% of American adults are not up-to-date with colorectal screening. Fecal occult blood testing is a noninvasive screening test that may help identify hidden blood in the stool, potentially supporting early detection efforts.

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Understanding Fecal Occult Blood Testing

Occult blood refers to hidden blood in the stool that is not visible to the naked eye. The fecal occult blood test (FOBT) is a diagnostic tool that measures trace blood levels in stool. The most common reason for ordering an FOBT is to screen for colorectal cancer; however, doctors may also request the test for patients who have certain signs or symptoms, such as abdominal pain, fatigue, or unexplained anemia. (14)

FOBT can only detect the presence or absence of blood in the stool; it can't determine what is causing the bleeding. (13) Follow-up testing is required for a positive FOBT to determine the source of the gastrointestinal bleed. Blood in the stool can be a sign of:

• Colon polyps
• Colorectal cancer
• Diverticulosis
• Hemorrhoids
• Inflammatory bowel disease (IBD)
• Ulcers (14)

Types of Fecal Occult Blood Testing

The guaiac-based fecal occult blood test (gFOBT) involves collecting small samples of stool and placing them on designated areas on a specialized card. The card is coated with guaiac, a plant-based chemical, which reacts with the heme component of hemoglobin, the protein found in red blood cells. When blood is in the stool samples, the guaiac undergoes a chemical reaction with the hemoglobin and causes the test card to change color. (31) 

The fecal immunochemical test (FIT or iFOBT) detects occult blood using antibodies (immune proteins) that target hemoglobin's globin chain. When blood is in the stool, the antibodies bind to the hemoglobin, producing a positive result. FIT is often recommended over gFOBT because it has a higher specificity and sensitivity (i.e., false positives and negatives are less likely to occur) and does not require dietary or medication restrictions for test collection. (12) 

The Significance of Early Detection

Early disease detection may improve treatment outcomes by enabling timely intervention, potentially reducing mortality rates, lowering healthcare costs associated with advanced disease management, and enhancing patients' quality of life. (26, 30) 

Detecting Colorectal Cancer: 

Early detection of colorectal cancer may improve prognosis and increase the likelihood of successful treatment outcomes. Colorectal cancer survival rates vary greatly depending on the stage at diagnosis. For instance, the five-year survival rate for localized colorectal cancer, when the disease is confined to the colon or rectum, is approximately 90%. However, this rate drops dramatically to around 13% for cases diagnosed at a distant stage, where cancer has spread to distant organs or lymph nodes. (32) 

Detecting Other Gastrointestinal Conditions: 

By pairing FOBT with complementary diagnostic tools, healthcare providers can enhance the accuracy of diagnosis, optimize treatment decisions, and improve diagnostic accuracy and patient outcomes in managing gastrointestinal conditions. 

For instance, when gFOBT and FIT are ordered together, they can help distinguish between upper and lower gastrointestinal (GI) bleeds. gFOBT is more sensitive to upper GI bleeds (such as those caused by ulcers), whereas FIT is more specific to lower GI bleeds. By combining these tests, healthcare providers can localize the source of bleeding more accurately, guiding further diagnostic evaluation and treatment decisions. (29)

Similarly, combining FOBT with other biomarkers such as calprotectin and B-cell activating factor (BAFF) can increase the likelihood of distinguishing between IBD and irritable bowel syndrome (IBS). Elevated levels of calprotectin and BAFF, along with positive FOBT results, suggest a higher probability of underlying IBD, whereas normal levels may support a diagnosis of IBS. (18, 27) 

Who Should Have Fecal Occult Blood Testing?

An estimated 10.5% of new colorectal cancer cases occur in people younger than 50 years old. Because of this, the U.S. Preventive Services Task Force (USPSTF) changed the general screening guidelines for colorectal cancer in 2021, lowering the screening age from 50 to 45. (17) 

Screening Guidelines: 

For average-risk individuals ages 45-75, the USPSTF recommends colorectal cancer screening with either a stool-based (FOBT) or direct-visualization screening test (17). 

"Average risk" is defined as having an absence of specific risk factors that would warrant earlier or more frequent screening:

• A personal history of colorectal cancer, certain types of colon polyps, IBD, or previous treatment with abdominal/pelvic radiation 
• A family history of colorectal cancer
• A confirmed or suspected hereditary colorectal cancer syndrome

Recommended screening intervals for colorectal cancer screening tests in average-risk individuals include: 

• gFOBT or FIT every year
• CT colonography or flexible sigmoidoscopy every five years
• Flexible sigmoidoscopy every ten years if FIT is performed annually
• Colonoscopy every ten years (17) 

Risk Factors: 

Colorectal cancer risk factors include:

• Older age
• Black race
• Personal or family history of colorectal cancer
• History of abdominal or pelvic radiation therapy to treat previous cancer
• IBD
• Hereditary syndromes, including familial adenomatous polyposis (FAP) or Lynch syndrome
• A diet low in fiber and high in fat and processed meats
• Lack of regular exercise
• Diabetes
• Obesity 
• Tobacco and alcohol use

People at increased or high risk of colorectal cancer are recommended to start screening before age 45 and at more frequent intervals (every 1-3 years) (1).

Preparing for and Conducting Fecal Occult Blood Testing

Following these steps to prepare and conduct a FOBT may help ensure a smooth and accurate screening process.

Preparation: 

A gFOBT requires avoidance of certain foods, medications, and supplements that can interfere with the test, potentially leading to false-positive or negative results. Before collecting a gFOBT, your doctor will recommend avoiding the following for at least 72 hours:

• Red meat
• High-peroxidase fruits and vegetables, including turnips, horseradish, and melon
• Vitamin C and iron supplements
• Non-steroidal anti-inflammatory drugs (NSAIDs): ibuprofen, naproxen, aspirin
• Blood thinning medications (15)

The FIT is slightly more user-friendly because it doesn't require the patient to avoid any foods or medications before testing (12). 

Procedure: 

The FOBT collection procedure is simple and noninvasive. It requires individuals to collect small samples of stool at home using a provided stick or brush and apply them to designated areas on the test card. Typically, two to three samples are needed to help ensure accuracy. Patients should always follow the specific instructions in the test kit to ensure proper collection and handling of samples. 

Understanding Results: 

A negative FOBT result means that occult blood was not detected in the stool sample. Unless a false-negative test is suspected, no further testing is required.

A positive FOBT result indicates the presence of GI bleeding. A positive test is not diagnostic for any specific GI condition; further testing is required to identify the cause of bleeding. 

Step 1: Rule out Iron Deficiency & Iron Deficiency Anemia

GI bleeding is a common cause of iron deficiency and iron deficiency anemia. These can be screened by ordering ferritin and a complete blood count (CBC).

Step 2: Determine the Source of the Gastrointestinal Bleed

Your doctor will refer you for colonoscopy and/or endoscopy to visualize the inside of the intestines and find the source of bleeding. Gastroenterologists perform these procedures. (2) 

Limitations and Considerations of Fecal Occult Blood Testing

FOBT is a valuable tool in colorectal cancer screening, but it does have limitations that must be considered. One significant limitation is the potential for both false-positive and false-negative results. False positives can occur due to various factors, such as dietary intake of red meat, certain medications, or gastrointestinal bleeding from sources other than colorectal cancer. Conversely, false negatives can occur if there is intermittent bleeding or if the blood is not distributed evenly in the stool sample. (11, 22) 

FOBT is designed to detect blood in the stool, which can be an early indicator of colorectal cancer. It does not directly visualize the colon to detect, sample, and definitively diagnose abnormalities. This is why FOBT has a lower sensitivity and specificity than colonoscopy, which remains the gold-standard tool for colon cancer screening. 

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Key Takeaways

• Fecal occult blood testing (FOBT) is a non-invasive and accessible option for colorectal cancer screening. Early detection of colorectal cancer through FOBT can lead to timely intervention and improved treatment outcomes, potentially saving lives.
• Doctors may also order FOBT as a tool to screen for gastrointestinal bleeding caused by other GI conditions, such as IBD or ulcers. 
• Patients ages 45-75 at average risk for colorectal cancer can take an annual FOBT to comply with colorectal cancer screening guidelines. By adhering to recommended screening protocols, we can improve our goals of early detection and successful management of colorectal cancer.