Stool sample

The most common reason for a stool sample at home is a fecal occult blood test — a search for blood in the stool that is invisible to the eye. There are two main types. The guaiac test (gFOBT) uses a chemical called guaiac to find blood and usually requires samples from two or three separate bowel movements. The fecal immunochemical test (FIT or iFOBT) uses an antibody that attaches to hemoglobin; it is considered better at finding blood in the stool, and depending on the brand it needs samples from one to three bowel movements.

Different kits have different instructions, and the instructions that come with the specific kit are the ones to follow. The typical process has two parts: collecting the bowel movement — the kit may include special paper to place over the toilet, and plastic wrap or a clean, dry container works too — and then taking a small sample from it with the tool the kit provides. For a guaiac test, no urine should mix with the stool.

Some tests ask for preparation days before the collection. Depending on the test, the provider may advise avoiding nonsteroidal anti-inflammatory drugs such as ibuprofen, naproxen, and aspirin — with a separate conversation before stopping aspirin taken for heart problems — as well as vitamin C in amounts over 250 mg a day from supplements, fruit juices, or fruit, red meat such as beef, lamb, and liver because traces of blood from these meats can show up in the stool, and certain raw fruits and vegetables. The provider specifies what to avoid and for how long.

There is no known risk to the test itself — the only challenge is doing the collection carefully and returning the kit as instructed.

How the sample is collected

1. Read the kit instructions fully before the first collection — gFOBT and FIT kits differ.
2. Check with the provider which foods, supplements, and medicines to avoid in the days before, and for how long.
3. Catch the bowel movement on the kit's special paper, on plastic wrap, or in a clean, dry container.
4. Keep urine from mixing with the stool — essential for a guaiac test.
5. Take the sample with the kit's tool and apply or store it exactly as the instructions show.
6. Repeat on the required number of separate bowel movements — one to three depending on the test — and return the kit as instructed.

What can spoil the sample

• Mixing urine into the sample during a guaiac test.
• Eating red meat or taking high-dose vitamin C before a test that asks to avoid them — traces of blood from beef, lamb, or liver can show up in the stool.
• Stopping aspirin taken for heart problems without asking the provider first.
• Collecting all samples from a single bowel movement when the kit requires separate ones.
• Letting the finished kit sit for days instead of returning it as instructed.

Frequently asked questions

Sources