A physician sometimes orders blood tests to determine if a patient has colorectal cancer or to help monitor the disease after diagnosis, according to the American Cancer Society. If those tests show signs that the patient could have colorectal cancer, the patient undergoes testing that more accurately detects the condition.
The most common exam to test a patient for colorectal cancer is a colonoscopy, explains the American Cancer Society. During a colonoscopy, the doctor passes a small camera affixed to the end of a thin tube through the entirety of the colon and rectum to look closely at the inside of the colon. The doctor can then biopsy or remove any suspicious-looking areas within the colon as needed to test for cancer.