Understanding Test Accuracy
The fecal occult blood test (FOBT) or fecal immunochemical test (FIT) detects hidden blood in your stool. This hidden blood may signal various digestive conditions, including polyps or certain cancers. The test identifies minute amounts of blood not visible to the naked eye. Accurate preparation helps ensure reliable results.
Certain substances can interfere with test accuracy, potentially leading to misleading results. This interference can cause a false positive, where the test indicates blood is present when it is not, or a false negative, where actual bleeding is missed. Red meat contains heme, which can react with test chemicals, mimicking human blood. Some plant-based foods contain enzymes that can also cause a false positive reaction.
High doses of vitamin C can interfere with the chemical reaction used in some tests, potentially leading to a false negative result by inhibiting blood detection. Medications that irritate the gastrointestinal lining, such as non-steroidal anti-inflammatory drugs (NSAIDs), can cause minor bleeding. Understanding these interactions is important for proper test preparation.
Foods to Exclude
To prevent false positive results, specific foods should be avoided before an occult blood test. Red meats, including beef, lamb, and pork, should be excluded from your diet for several days leading up to and during the test. These meats contain heme, which can trigger a positive reaction in some test types even when no human blood is present. Processed meats like sausages, deli meats, and hot dogs also fall into this category and should be avoided.
Certain vegetables and fruits contain peroxidase, an enzyme that can react with the chemicals in the guaiac-based FOBT, leading to a false positive. Examples include turnips, horseradish, radishes, broccoli, cauliflower, artichokes, cantaloupe, and parsnips. These should be temporarily removed from your diet.
Medications and Supplements to Temporarily Stop
Certain medications and supplements can interfere with the accuracy of occult blood tests. Non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, naproxen, and aspirin can cause irritation or minor bleeding in the gastrointestinal tract. This bleeding could lead to a false positive test result.
Iron supplements can cause dark or black stools, which might be mistaken for blood. High doses of vitamin C (ascorbic acid), exceeding 250 milligrams daily, can interfere with the chemical reaction used in some fecal occult blood tests. This interference can lead to a false negative result. Always consult your healthcare provider before stopping any prescribed medication.
Key Steps for Test Preparation
Proper preparation for an occult blood test involves avoiding certain foods and medications for a specific period. A 2 to 3-day avoidance period before and during stool collection is recommended for dietary restrictions and medication adjustments. This timeframe allows your digestive system to clear any substances that could interfere with the test results.
Certain physiological conditions can also impact test results. Avoid collecting stool samples during your menstrual period or if you have active bleeding from hemorrhoids. Recent dental procedures that cause bleeding gums might introduce blood into your digestive system, so postpone the test if this occurs. Always follow the specific instructions provided by your healthcare provider.