What Can Cause a False Positive FOBT?

According to the American Cancer Society, annual fecal occult blood tests (FOBT) can be an effective screening tool for colorectal cancer, but certain factors can lead to inaccurate readings. A positive FOBT result doesn't always indicate a serious condition; understanding what can cause a false positive FOBT is crucial for an accurate diagnosis and reducing unnecessary anxiety.

Quick Summary

False positive fecal occult blood test results can be triggered by dietary factors like red meat, medications such as NSAIDs, and certain medical conditions like hemorrhoids. Proper test preparation, including temporary dietary and medication adjustments, is essential to minimize inaccurate outcomes during colorectal cancer screening.

Key Points

• Dietary Restrictions: Consuming red meat or high-peroxidase vegetables like turnips and broccoli can cause false positive results, especially with gFOBT.

• Medications: NSAIDs (aspirin, ibuprofen) and blood-thinners can cause minor GI bleeding and interfere with test results.

• Medical Conditions: Conditions such as hemorrhoids, ulcers, and diverticulosis can cause bleeding unrelated to cancer, leading to a false positive.

• Test Type: Guaiac-based FOBT (gFOBT) is more sensitive to dietary interference and medications than the fecal immunochemical test (FIT).

• Follow-Up is Key: A positive FOBT requires a follow-up colonoscopy to determine the true source of bleeding and confirm or rule out a serious condition like cancer.

• Contamination: Swallowed blood from dental procedures, nosebleeds, or menstrual blood can also contaminate a sample and lead to inaccurate results.

Understanding the Fecal Occult Blood Test

How the FOBT Works

A fecal occult blood test (FOBT) is a non-invasive screening tool designed to detect microscopic amounts of blood in the stool, which could indicate a problem within the digestive tract. There are two primary types of FOBTs: the guaiac-based test (gFOBT) and the fecal immunochemical test (FIT or iFOBT). While both tests screen for blood, their mechanisms differ, affecting what can cause a false positive result.

• Guaiac-based test (gFOBT): This test uses a chemical called guaiac to detect the peroxidase activity of heme, a component of hemoglobin found in red blood cells. The chemical reaction causes the test card to change color, indicating the presence of blood. Because this test relies on a chemical reaction, it is highly sensitive to substances other than human blood.
• Fecal immunochemical test (FIT or iFOBT): The FIT uses antibodies to specifically detect human hemoglobin. This specificity for human blood makes it less susceptible to interference from dietary peroxidases found in certain foods, and it does not require the same strict dietary restrictions as the gFOBT.

Why a False Positive Occurs

A false positive result means the test indicates the presence of blood when no pathological bleeding (from a tumor or polyp, for example) is present. This can cause unnecessary stress and may lead to additional, potentially invasive, follow-up procedures like a colonoscopy. Minimizing the chances of a false positive starts with understanding the factors that can interfere with the test, especially for the more chemically-sensitive gFOBT.

Dietary Causes of False Positive FOBT

One of the most common causes of false positive results, especially with gFOBT, comes from a patient's diet. The peroxidase enzymes found in certain foods can mimic the peroxidase activity of blood, triggering a false reading. This is why specific dietary changes are often recommended in the days leading up to a gFOBT.

• Red Meat: Consuming red meat, especially if it is rare, can cause a false positive result because of the presence of animal blood and myoglobin. Patients are typically advised to avoid red meat for at least 48 to 72 hours before and during the collection period.
• High-Peroxidase Fruits and Vegetables: Certain raw vegetables and fruits contain peroxidase enzymes that can interfere with the chemical reaction of the gFOBT. This includes items such as turnips, horseradish, broccoli, radishes, cauliflower, and cantaloupe.

Medications That Can Lead to False Positive Results

Certain medications can either cause minor gastrointestinal bleeding or interfere with the chemical reaction of the test, leading to a false positive reading. Patients should always consult their doctor before discontinuing any prescribed medication.

• NSAIDs: Nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin (including low-dose), ibuprofen, and naproxen, can irritate the stomach and intestinal lining, causing minor bleeding that is not a sign of serious disease. It is often recommended to avoid these for up to seven days before testing.
• Blood-Thinning Medications: Oral anticoagulants and other blood-thinning agents can increase the risk of minor bleeding in the gastrointestinal tract.
• Corticosteroids: These drugs can also contribute to gastrointestinal bleeding and should be avoided before a gFOBT if possible.

Medical Conditions and Contamination

Beyond diet and medication, other physical and medical factors can result in a false positive test. These are typically unrelated to colorectal cancer but still cause blood to be present in the stool sample.

• Hemorrhoids: Bleeding hemorrhoids are a common source of blood in the stool and a frequent cause of false positive FOBT results. A patient with known hemorrhoidal bleeding should postpone the test until the condition is resolved.
• Other Gastrointestinal Bleeding: Other benign conditions, such as peptic ulcers, diverticulosis, or gastritis, can cause bleeding that is detected by the FOBT.
• Menstrual Period or Swallowed Blood: Women should not perform the test during their menstrual period. Similarly, swallowed blood from a nosebleed, bleeding gums from dental work, or throat issues can cause a false positive result.

gFOBT vs. FIT: A Comparison of False Positive Risks

The choice of FOBT can significantly impact the likelihood of a false positive result due to the test's mechanism. The more specific FIT has fewer limitations compared to the chemically reactive gFOBT.

Feature	Guaiac-based FOBT (gFOBT)	Fecal Immunochemical Test (FIT/iFOBT)
Detection Method	Chemical reaction detecting heme peroxidase.	Antibody-based test specific for human globin.
Dietary Restrictions	Yes, strict restrictions on red meat, and high-peroxidase fruits and vegetables.	No, not affected by dietary intake due to human-specific detection.
Medication Interference	Higher risk of interference from NSAIDs, aspirin, etc., due to potential GI bleeding.	Lower risk, as it's less sensitive to minor bleeds from upper GI tract.
Ease of Preparation	More complex, requires adherence to dietary guidelines for days leading up to the test.	Simpler preparation, no dietary changes needed, improving patient compliance.
False Positive Risks	Higher risk from non-human heme sources and GI irritants.	Lower risk, primarily from lower GI bleeding or medical conditions.

Navigating a Positive FOBT Result

Regardless of the cause, a positive FOBT result should not be ignored. It serves as a prompt for further investigation. The first step is to contact a healthcare provider who will assess your individual risk factors and determine the appropriate next steps. This almost always involves a colonoscopy, which is considered the gold standard for investigating the source of bleeding in the lower GI tract. A colonoscopy allows a specialist to visually inspect the colon for polyps, tumors, or other sources of bleeding, providing a definitive diagnosis.

Conclusion

False positive FOBT results can stem from a variety of sources, including specific foods, certain medications, and other benign medical conditions. While the gFOBT is more prone to these inaccuracies due to its chemical nature, the more advanced FIT has significantly reduced these risks. Proper preparation is essential for both, but particularly for gFOBT. The most important takeaway for anyone receiving a positive result is not to panic, but to follow up with a healthcare professional. A positive screening test is not a diagnosis of cancer, but a signal for further, more detailed diagnostic procedures like a colonoscopy, which can reveal the true cause of the bleeding and lead to appropriate treatment. For more information on colorectal cancer screening guidelines, visit the American Cancer Society website.

Frequently Asked Questions

What is a false positive FOBT?

A false positive FOBT is a test result that indicates the presence of blood in the stool when there is no underlying pathology, such as cancer or significant polyps, causing the bleeding.

Can certain foods really affect my FOBT results?

Yes, especially with the guaiac-based FOBT (gFOBT). Consuming red meat or vegetables high in peroxidase enzymes, like turnips and broccoli, can mimic the chemical reaction of blood and cause a false positive result.

Which medications can cause a false positive?

Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, and naproxen can cause minor gastrointestinal bleeding. Additionally, some blood-thinners and corticosteroids can also lead to false positive results.

Does the type of FOBT matter for false positives?

Yes. The fecal immunochemical test (FIT or iFOBT) is less prone to false positives from dietary and upper GI bleeding sources because it specifically detects human blood, unlike the chemical reaction-based guaiac test (gFOBT).

What medical conditions cause false positive FOBT?

Benign medical conditions such as bleeding hemorrhoids, ulcers, diverticulosis, or inflammatory bowel disease can all lead to a positive FOBT result without indicating cancer.

What should I do if my FOBT is positive?

A positive FOBT means further investigation is needed. You should contact your healthcare provider, who will likely recommend a colonoscopy to determine the source of the bleeding.

Is there a way to avoid a false positive FOBT?

To minimize the risk, follow all pre-test instructions carefully. This may include avoiding certain foods (red meat, high-peroxidase vegetables) and medications (NSAIDs, blood-thinners) for several days before a guaiac test. For FIT, fewer restrictions are necessary.