Tumor marker testing is a method of diagnosing cancer by detecting tumor markers such as specific proteins released by cancer cells in the bloodstream. Tumor markers can be used to determine the organ that has developed cancer. Tumor marker testing is useful for understanding the course of the disease, the effects of the treatment, and the condition of the relapse. However, there are times when patients with negative results from cancer tests and those from non-cancer patients have positive test results and are generally not advocated for early cancer. For example, when diagnosing early colorectal cancer by the tumor marker carcinoembryonic antigen (CEA), the positive rate is very low. Moreover, not only carcinoembryonic antigen is present in the body of patients with colorectal cancer but also carcinoembryonic antigen in patients suffering from gastric cancer, pancreatic cancer, liver cancer, lung cancer and breast cancer, and it is difficult to determine the site where cancer occurs.