It is used to analyze the number of white blood cells, red blood cells and platelets in the blood to help diagnose infectious diseases, anemia or bleeding disorders. There are various manifestations of rectal cancer, and some advanced rectal cancers may show anemia, which is manifested by the decrease of red blood cells and hemoglobin in routine blood tests. Diagnostic items of rectal cancer include the following: First, imaging examination: such as CT or MRI of abdomen. Secondly, endoscopy: that is, colonoscopy, after which the mass is taken for pathological examination. Thirdly, tumor markers: some tumor markers need to be drawn and tested. The above are the complete diagnostic items of rectal cancer, so routine blood alone cannot detect or diagnose rectal cancer.