Cancer marker tests are critical for the early detection of cancer, even when the patient is not yet showing any symptoms. These tests detect specific substances, called cancer markers, that are released into the bloodstream by cancer cells. Elevated levels of these substances, such as carcinoembryonic proteins, may indicate the presence of cancer, but are not always sufficient to confirm the presence of malignancy, as certain elevations may be associated with other health problems, such as inflammatory or joint diseases.