FOBT
Meaning of FOBT: A screening test looking for bleeding too slight to see, on the logic that bowel tumors leak before they cause symptoms.
Definition of FOBT
FOBT stands for fecal occult blood test, a stool test that detects blood present in amounts too small to change the appearance of the stool. Occult here means hidden. A sample is collected at home and processed in a laboratory, and a positive result is a reason to examine the bowel directly rather than a diagnosis in its own right.
Why look for blood nobody can see?
A growth in the bowel wall has a fragile surface that catches against passing stool and bleeds a little, often long before it causes pain, a change in habit or visible blood. Polyps behave the same way, and removing a polyp prevents the cancer it might have become. The bleeding is intermittent, and that is the main weakness of the approach: a tumor that happened not to bleed on the day of collection produces a negative result. Testing is therefore repeated on a schedule rather than done once.
What are the two kinds?
| Version | Detects | Practical difference |
|---|---|---|
| Guaiac based | Blood from any species | Card samples, and results affected by diet and some medicines |
| Immunochemical | Human blood protein only | Fewer samples, no diet restriction, specific to the lower bowel |
The older guaiac card depends on a chemical reaction with the iron-carrying part of blood, a reaction that red meat and peroxidase-rich vegetables also trigger, so those produce false alarms. The newer immunochemical version uses an antibody against human blood protein, which is digested away in the upper gut, so it answers specifically to bleeding from the colon and rectum. Where a screening program says FOBT now, the immunochemical form is usually what is meant.
What does a positive result mean?
That blood was found, and nothing beyond that. Hemorrhoids, an anal fissure, inflammation, an ulcer, a swallowed nosebleed and menstrual contamination all produce the same signal as a tumor. The purpose is to sort a large population into those who need a look inside the bowel and those who do not, so the colonoscopy that follows is the step establishing what is actually there. Programs of this kind pick up a greater share of disease at stage I colorectal cancer, while it is still confined to the bowel wall, and that is the entire reason for screening for colorectal cancer.
Also known as
Used in a sentence
An FOBT was returned as part of the national bowel screening program.
Good to know
A negative result describes only the day the sample was taken, because bowel tumors bleed intermittently, and a positive result indicates blood from any source rather than cancer, so the examination that follows is what establishes the cause.