This increases the chance of finding blood, since the bleeding may not happen every day. This test is commonly used for colorectal cancer. If you have certain symptoms, such as abdominal pain, fatigue, pain around your rectum or rectal bleeding, your healthcare provider may. Fecal occult blood testing (FOBT) remains an appropriate screening option for colorectal cancer (CRC). . Learn more. A fecal occult blood test looks for blood in the stool. , However, FOBT is limited by a lack of patient adherence. For your FOBT, you will collect samples of your stool 3 days in a row. Blood in stool may be a sign of hemorrhoids, colorectal cancer, or another condition. The fecal occult blood test (FOBT) is a diagnostic test to assess for hidden (occult) blood in the stool. The fecal occult blood test (FOBT) looks for the presence of blood in the feces, which may be a sign of a problem in your digestive system.