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