Cancer screening through blood mainly refers to the examination of tumor markers in blood to detect early cancer. At present, there are clear tumor markers that can assist in the diagnosis of primary liver cancer, lung cancer, colorectal cancer, ovarian cancer and so on. Take primary liver cancer as an example, if there is a history of chronic hepatitis B, long-term heavy drinking and blood test for alpha-fetoprotein or AFP, if the level is >400ng/mL for 1 month or 200ng/mL for more than 2 months, combined with ultrasound and CT examination, the diagnosis of primary liver cancer can be confirmed if there is an occupying lesion in the liver. And if CEA is found to be significantly elevated >10ng/mL or more in blood sampling, it should be suspected to be colorectal cancer or lung adenocarcinoma. If CA125 is significantly elevated, women should have pelvic CT to suspect the possibility of ovarian cancer. Different markers have specificity for specific tumors, but tumor markers are affected by many inflammatory diseases and benign diseases such as polyps, in addition to tumors.