Doctors check poop for DNA as part of the screening process for colorectal cancer. It helps doctors diagnose people with this disease during its early stages or even when the cells are precancerous.

Changes, or mutations, in DNA can be markers for cancer. According to the National Cancer Institute, scientists can recognize hundreds of DNA and genetic changes that cause cancers to grow or spread.

Cancer develops when atypical cells in the body start growing uncontrollably. Most colorectal cancers start as growths, called polyps, in a person’s colon or rectum.

The American Cancer Society (ACS) explains that polyp cells often pass into a person’s feces, or poop. Doctors check the feces for any cells with atypical DNA.

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DNA is a molecule that contains a person’s genetic code. The code contains all the information cells need to function and reproduce.

Each DNA molecule consists of two strands that twist around each other, making a shape called a double helix. Although some DNA is unique to each individual, most DNA follows patterns.

The National Human Genome Research Institute (NHGRI) explains that each DNA strand comprises nucleotides. These are building blocks that contain a phosphate group, a sugar group, and one of four different nitrogen bases.

Scientists know that the nitrogen bases pair up in recognizable patterns or sequences and what this looks like in a healthy cell. The NHGRI says that doctors can identify some cancers using DNA sequencing.

Tiny changes in a cell’s DNA can make it cancerous. Different types of cancer change the DNA in different ways, but they also follow a pattern. Changes, or mutations, in the DNA sequence alert doctors to cancerous cells.

Human poop is mostly water and the remains of food after digestion. It also contains some cells from the digestive tract, and these cells contain DNA.

Sometimes poop contains microscopic amounts of blood. Blood cells also contain DNA. The ACS explains that blood in a person’s poop may be a symptom of colorectal cancer.

Where else can DNA appear in the human body?

Almost every cell in a person’s body contains DNA.

DNA can appear in:

  • skin cells
  • blood cells
  • hair
  • semen
  • bones
  • saliva
  • mucus
  • muscles
  • organs
  • teeth
  • nails
  • feces

A stool DNA test looks for atypical DNA in a person’s feces. According to the ACS, it is also called a multitargeted stool DNA test or FIT-DNA test.

For the test, people collect their entire stool and send it to a laboratory for analysis.

Individuals receive a special kit to collect the sample, usually through the mail. The kit consists of a sample container, preservative liquid, a bracket for holding the collection container in the toilet, a tube, labels, a shipping box, and instructions.

After collecting the sample, a person mails it to a laboratory. For the test to work, the sample has to reach the laboratory within 72 hours, and people should mail the sample within 24 hours of taking it.

The Centers for Disease Control and Prevention (CDC) recommend every adult aged 45–75 years undergo regular screening for colon cancer. There are several screening tests available, all with different risks and benefits. Below is a list of screening tests and how often a person should have them.

  • High-sensitivity guaiac-based fecal occult blood test: Every year
  • FIT: Every year
  • sDNA-FIT: Every 1–3 years
  • Colonoscopy: Every 10 years
  • CT colonography Every 5 years
  • Fecal sigmoidoscopy: Every 5 years
  • Fecal sigmoidoscopy with FIT: Fecal sigmoidoscopy every 10 years plus FIT every year

Doctors test feces for DNA to check for any mutations in the DNA sequencing. Mutations may indicate that cells are cancerous or precancerous.

These tests can reveal cancerous cells before a person experiences any symptoms.

According to the ACS, most colorectal cancers start as growths, or polyps, on the inner lining of the rectum or colon. These polyps can shed cells into a person’s feces.

Treating cancers early, either before cells become cancerous or before the disease has spread, increases the likelihood of a person surviving. The ACS estimates that over 90% of people diagnosed with colorectal cancer before it has spread outside the colon or rectum will still be alive after 5 years.

Learn more about colon polyps.

Testing stool for DNA is noninvasive, meaning it does not require a medical procedure. People can collect their samples in the privacy of their own homes.

In 2014, the Food and Drug Administration (FDA) approved Cologuard as the first DNA-based stool testing kit, according to the Colorectal Cancer Alliance.

Most health insurers in the United States completely cover the cost of the tests. Even the postage to the laboratory is prepaid.

People taking Cologuard tests do not need to change their eating or drinking habits before the test.

Cologuard tests check for DNA and blood in a person’s feces. Information from the test’s information leaflet describes the results as positive, negative, or no result obtained.

If the DNA shows signs of mutations that could lead to colorectal cancer, the test result is positive.

Doctors advise people with a positive result to have a colonoscopy. This procedure involves the insertion of a flexible tube with a tiny camera into the rectum and colon. Doctors can use different instruments through the tube to take a biopsy of any polyps.

If the polyps are cancerous, doctors can remove them before the cancer spreads to other parts of the body.

A negative result means the person’s feces do not contain DNA or blood.

If there is no result, it means the test was not conclusive. People with this result may need to provide another stool sample for testing.

Although the testing is usually accurate, it is possible to have a false-positive or false-negative result. In these rare cases, the results may indicate that a person has precancerous cells when they do not or that it did not detect atypical DNA when it was actually present in the body.

Learn more about colonoscopy.

The ACS says that death rates from colorectal cancers are dropping, as screening can help doctors identify and treat cancerous polyps early.

Testing poop for DNA is a noninvasive test that people can do once every 3 years.

Poop contains DNA, which doctors can analyze and find abnormalities, or mutations, that might lead to cancer.

Colorectal cancers are cancers of the colon and rectum. The DNA from feces can reveal the presence of precancerous or cancerous cells before a person experiences any symptoms.

Doctors can remove any polyps or growths from someone’s intestines before cancer spreads to other parts of the body.

Health insurers in the U.S. usually cover the cost of a stool DNA test once every 3 years.