Sometimes the instructions for collecting stool samples can be very confusing. What foods should you eat and which should you avoid? The recommended diet before collecting a stool sample varies according to the test being performed. Some tests do not require any special preparation or dietary restrictions. Others call for avoidance of specific foods to ensure the test results are accurate.
Stool Tests for Blood
Stool tests for blood are commonly done to screen for colon cancer or other digestive system problems. Two types of tests are used: the fecal occult blood test, or FOBT, and the fecal immunochemical test, or FIT. The FOBT uses a chemical reaction to check for a protein found in red blood cells. Some foods, medicines and vitamins can interfere with the FOBT, causing an inaccurate result. For this reason, you will need to avoid certain foods for 2 to 3 days before the test, such as rare or red meat, broccoli, cabbage, cantaloupe, cauliflower, horseradish, mushrooms, parsnips, radishes, turnips and zucchini. Large amounts of citrus fruits and vitamin C — more than 250 mg per day — and certain medicines, such as aspirin and ibuprofen (Advil, Motrin), should also be avoided, as they can also interfere with the test. The FIT test is not affected by foods, so there are no dietary restrictions.
Fecal Fat Test
The fecal fat test checks for an excess amount of fat in the stool, which can indicate a problem with food digestion and absorption. Typically, your diet for 2 to 3 days before the test and while collecting the stool samples needs to include a specified amount of fat — but certain types of fats should be avoided. For example, butter, oily salad dressings and low-calorie mayonnaise can skew the test results and should be avoided. Likewise, it's recommended not to ingest castor oil and mineral oil before or during the test. Since a high amount of dietary fiber can interfere with the test, you may be advised to limit your intake of bran and other whole grains before and during the test.
Other Stool Tests
Although many stool tests do not require dietary restrictions, you might need to fast for some of them. People with watery diarrhea may have stool electrolyte and osmolality tests, which are generally not affected by specific foods but sometimes require fasting. Typically, there are no dietary restrictions on tests for gastrointestinal infections, such as a stool culture or a test for parasites. Other stool tests not affected by your diet include fecal trypsin and chymotrypsin — which are used to check pancreatic function in people with cystic fibrosis — and lactoferrin and calprotectin, which are used to test for inflammation in the bowel.
Follow Instructions Carefully
When preparing for any laboratory test, follow the specific instructions given to you by your health-care provider. Some stool tests require you to fast or avoid certain foods, so make sure you have clear directions about how to prepare. Call your health-care provider if you have any questions.
- Family Practice Notebook: Fecal Occult Blood
- Lab Tests Online: Fecal Occult Blood Test and Fecal Immunochemical Test
- Clinical Methods: The History, Physical, and Laboratory Examinations, Third Edition; H. Kenneth Walker, et al.
- Lab Tests Online: Fecal Fat
- Lab Tests Online: Clostridium Difficile and C. Difficile Toxin Testing
- Lab Tests Online: O&P
- Lab Tests Online: Shiga Toxin-producing Escherichia Coli
- Lab Tests Online: Stool Culture