Cancer cells are usually not detected in the blood of rectal cancer. Cancer cells are mainly found near the rectal cancer foci or metastatic foci, and cannot be detected in the blood even if they are spread through the blood, which can be reflected in the tumor control by detecting tumor markers. Rectal cancer is a malignant tumor originated from epithelial cells of rectal mucosa. Cancer cells are confined to the cancerous foci in the early stage, but in the later stage, after the cancer invades the surrounding tissues or metastasizes to the distant organs, cancer cells can be detected in the cancerous foci of other tissues. Tumor cells can be seen microscopically by taking a small amount of diseased tissue for biopsy through endoscopy. Cancer cells usually do not appear in the blood, but blood tests can detect tumor markers, such as carcinoembryonic antigen, CA199 and other common tumor markers of rectal cancer, and can also be used to understand the infection, anemia and other conditions through blood routine. The specific tests needed depend on the condition, and it is recommended to consult a specialized doctor after seeking medical treatment.