What is being tested?
The faecal occult blood test (FOB) checks for blood in your stool. Normally, there will not be enough blood lost through the gastrointestinal tract (stomach and intestines) to turn an FOB positive or for you to notice it by looking. Any significant amount of blood being passed (seen as bloody or dark, tarry stools) should be investigated.

A positive FOB will tell your doctor that you have bleeding occurring somewhere in your gastrointestinal tract. This blood loss could be due to ulcers, diverticulosis, bleeding polyps, inflammatory bowel disease, haemorrhoids (piles), from swallowed blood due to bleeding gums or nosebleeds, or it could be due to benign or malignant tumours. Anything that sticks out into the intestine, like a polyp or tumour, and is rubbed against by the faeces as it passes through, has the potential to bleed now and again. Often this small amount of blood is the first, and sometimes the only, symptom of early bowel cancer, making the FOB a valuable screening tool.

How is the sample collected for testing?
First, your doctor will probably give you some instructions about dietary restrictions that you should follow and drug treatment that you should avoid before collecting the FOB samples. For example, you should not eat red meat within three days before testing. Other substances that could cause a false positive test result include fish, turnips, horseradish, and drugs such as colchicines and oxidizing drugs (for example, iodine and boric acid). Be sure to carefully follow your doctor's instructions.

The main method for faecal occult blood testing is the guaiac smear method, explained below.

Your doctor or laboratory will give you one or more test 'cards.' You collect a separate sample from three different stools, usually one a day for three days. Each stool sample should be collected into a clean container and should not be contaminated with urine or water. The slide is labeled with your name and the date; then, with an applicator stick, you apply a thin smear of stool onto each filter paper square/window contained on the card, and allow it to dry. Usually you will collect all three samples, then return all of them to your doctor/laboratory at the same time, sometimes by posting them.