Certain screening tests can be perfected by drawing blood to find out if there is a possibility of cancer, but not to confirm the diagnosis of cancer. Cancer, also known as malignant tumor, can occur in almost any tissue or organ of the human body. Most cancers are accompanied by abnormal blood biochemical indexes, such as liver cancer patients with markedly elevated serum alpha-fetoprotein, gastric cancer with markedly elevated levels of CA199, and acute or chronic lymphocytic leukemia, in which naïve primitive cells can be found in the blood. However, the definitive diagnosis of cancer requires the clinician to combine the patient’s clinical manifestations, blood biochemistry, imaging and pathology and other auxiliary examinations to make a comprehensive judgment. However, abnormalities in blood tests are not enough to clarify neoplastic diseases.