Checking for cancer cells through blood is not very accurate. Blood draws can only detect common tumor markers, and elevated tumor markers do not necessarily mean that cancer is present; some inflammatory diseases may also cause elevated tumor markers. In addition, many cancers do not have specific tumor markers. Drawing blood does not detect cancer cells, but only tumor markers secreted by cancer cells into body fluids. Commonly used markers include carcinoembryonic antigen, glycoconjugate antigen 19-9, glycoconjugate antigen 15-3, glycoconjugate antigen 12-5, and prostate-specific antigen, and so on. A blood test can detect changes in these tumor markers and can also give your doctor an indication of the likelihood of early detection of cancer. It is difficult to make a specific judgment about the accuracy of blood screening for cancer cells. Blood draws can only check for elevated levels of common tumor markers, and there are different screening methods for different cancers not just one blood test.