These tests look for tumor markers, which are sometimes called cancer markers. Tumor markers are substances that are made by cancer cells or by normal cells in response to cancer. For example, some cancer cells make certain proteins in larger amounts than normal cells do. These proteins can act as tumor markers because their presence may mean cancerous cells are present.

Tumor marker tests are most often used after you have a cancer diagnosis.

Source: MedlinePlus, U.S. National Library of Medicine